Runx2: of bone and stretch

Impaired osteogenic differentiation associated with connexin43/microRNA-206 in steroid-induced avascular necrosis of the femoral head

Connexin(Cx)43 and microRNA(miR)-206 play an important role in osteogenesis. However, their role in steroid-induced femoral head osteonecrosis (SANFH) is still ambiguous. The present study aimed to establish a rabbit model and investigate osteogenesis in steroid-induced femoral head osteonecrosis occurring via Cx43/miR-206 and the changes of Wnt/β-catenin signal pathway-related proteins. A total of 72 adult New Zealand white rabbits were divided randomly into a model group (Group A) and a control group (Group B) of 36 rabbits each. Group A was injected intravenously with lipopolysaccharide (10μg/kg body weight, once per day). After 48h, three injections of methylprednisolone (MPS; 20mg/kg body weight) were administered intramuscularly at 24-hour intervals. Group B were fed and housed under identical conditions but received saline injections. All animals were sacrificed at two, four, and eight weeks from the first MPS injection. Typical early osteonecrosis symptoms were observed in Group A. The expression of miR-206 in Group A was significantly higher than that of Group B. The mRNA and protein levels of Cx43, β-catenin, runt-related transcription factor 2, and alkaline phosphatase gradually decreased while Dickkopf-1 (Dkk-1) gradually increased in Group A compared with Group B. These findings indicated that Cx43/miR-206 is involved in the pathogenesis of early stage SANFH and may be associate with Wnt/β-catenin signal pathway.

Osteogenic stem cell selection for repair and regeneration

The first osteogenic cells to attach to a titanium (Ti) implant after placement are the multipotent stromal cells (MSCs) that circulate in the bloodstream and are recruited to the site of tissue damage. The reservoirs of these cells are heterogeneous in nature, consisting of a mixture of cells with varying differentiation abilities. In order to utilise these cells and to reduce the chance of unwanted events during regenerative therapies, the selection of a subset of cells that is truly multipotent is required. The behaviour of these cells has been shown to be altered by modifications to Ti implant surfaces, most notably rough, hydrophilic Ti. These changes in behaviour underpin the differences seen in clinical performance of these surfaces. In this study Human bone marrow derived stromal cells (hBMSCs) have been cultured on modified Ti surfaces in order to analyse these changes in cell behaviour. The results demonstrate the different effects of the surfaces and suggest that one surface selectively enriches the population with osteogenic adult ‘stem cells’ by inducing the cell death of the more differentiated cells. Combined with subsequent expansion in bioreactors before implantation, this may lead to a new source of cells for regenerative therapies.

•

Different titanium surfaces elicit differing responses from bone marrow derived stromal cells.

•

Hydrophilic rough titanium induces increased apoptosis and necrosis in MSCs in vitro.

•

Cells selected on rough hydrophilic titanium are more osteogenic than the parent population.

•

This may lead to a new source of osteogenic cells for regenerative therapies.

In vitro vitamin K(2) and 1α,25-dihydroxyvitamin D(3) combination enhances osteoblasts anabolism of diabetic mice

In this study, we evaluated the anabolic effect and the underlying cellular mechanisms involved of vitamin K2 (10 nM) and 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3) (10 nM), alone and in combination, on primary osteoblasts harvested from the iliac crests of C57BL/KsJ lean (+/+) and obese/diabetic (db/db) mice. A lower alkaline phosphatase (ALP) activity plus a reduced expression of bone anabolic markers and bone formation transcription factors (osteocalcin, Runx2, Dlx5, ATF4 and OSX) were consistently detected in osteoblasts of db/db mice compared to lean mice. A significantly higher calcium deposits formation in osteoblasts was observed in lean mice when compared to db/db mice. Co-administration of vitamin K2 (10 nM) and 1,25(OH)2D3 (10 nM) caused an enhancement of calcium deposits in osteoblasts in both strains of mice. Vitamins K2 and 1,25(OH)2D3 co-administration time-dependently (7, 14 and 21 days) increased the levels of bone anabolic markers and bone formation transcription factors, with a greater magnitude of increase observed in osteoblasts of db/db mice. Combined vitamins K2 plus 1,25(OH)2D3 treatment significantly enhanced migration and the re-appearance of surface microvilli and ruffles of osteoblasts of db/db mice. Thus, our results illustrate that vitamins K2 plus D3 combination could be a novel therapeutic strategy in treating diabetes-associated osteoporosis.

Involvement of Prolyl Hydroxylase Domain Protein in the Rosiglitazone-Induced Suppression of Osteoblast Differentiation

Rosiglitazone is a well-known anti-diabetic drug that increases insulin sensitivity via peroxisome proliferator-activated receptor γ (PPARγ) activation, but unfortunately it causes bone loss in animals and humans. A previous study showed that prolyl hydroxylase domain protein (PHD) plays a role in rosiglitazone-induced adipocyte differentiation. Based on the inverse relationship between adipocyte and osteoblast differentiation, we investigated whether PHD is involved in the effects of rosiglitazone on osteoblast differentiation. Rosiglitazone inhibited osteoblast differentiation in a concentration-dependent manner, and in parallel induced three PHD isoforms (PHD1, 2, and 3). PHD inhibitors and knockdown of each isoform prevented the inhibitory effects of rosiglitazone on osteoblast differentiation and increased the expression of Runx2, a transcription factor essential for osteoblastogenesis. MG-132, a proteasomal inhibitor also prevented the rosiglitazone-induced degradation of Runx2. Furthermore, both increased PHD isoform expressions and reduced osteoblast differentiation by rosiglitazone were prevented by PPARγ antagonists, indicating these effects were mediated via PPARγ activation. In vivo oral administration of rosiglitazone to female ICR mice for 8 weeks reduced bone mineral densities and plasma alkaline phosphatase (ALP) activity, and increased PHD expression in femoral primary bone marrow cells and the ubiquitination of Runx2. Together, this suggests that the rosiglitazone-induced suppression of osteoblast differentiation is at least partly induced via PPARγ-mediated PHD induction and subsequent promotion of the ubiquitination and degradation of Runx2.

Cyclic stretch enhances bone morphogenetic protein-2-induced osteoblastic differentiation through the inhibition of Hey1

Substantial evidence has indicated that osteoblastic differentiation may be regulated by mechanical loads or bone morphogenetic protein-2 (BMP-2). BMP-2-induced in vivo osteogenesis can be significantly enhanced in the presence of mechanical stimuli, revealing the therapeutic potential of the combined application of BMP-2 and mechanical loads in clinical bone diseases (e.g., bone fractures and osteoporosis); however, the underlying mechanisms remain elusive. In this study, we found that cyclic stretch or BMP-2 alone increased the expression of osteoblastic differentiation markers, including alkaline phosphatase (ALP) and runt-related transcription factor 2 (Runx2), as shown by RT-qPCR, western blot analysis and ALP activity test. Furthermore, our results revealed that cyclic mechanical stretch with 10% elongation at 0.1 Hz significantly enhanced the BMP-2-induced upregulation of ALP and Runx2 expression in osteoblast-like MC3T3-E1 cells. Cyclic stretch also inhibited the BMP-2-induced upregulation of Hes-related family bHLH transcription factor with YRPW motif 1 (Hey1, measured by RT-qPCR and immunofluorescence staining), a potent negative regulator of osteogenesis. Moreover, the transient transfection of a Hey1 expression plasmid (pcDNA3.1-Hey1) significantly reversed the effects of cyclic stretch on the BMP-2-induced upregulation of differentiation markers in the MC3T3-E1 cells. This revealed the importance of Hey1 in modulating BMP-2-induced osteoblastic differentiation in response to cyclic stretch. Taken together, our results demonstrated that cyclic stretch enhanced the BMP-2-induced osteoblastic differentiation through the inhibition of Hey1. The present study broadens our fundamental knowledge of osteoblastic mechanotransduction and also sheds new insight into the mechanisms through which the combined application of BMP-2 and mechanical load promotes osteogenesis.

Gene expression of human osteoblasts cells on chemically treated surfaces of Ti-6Al-4V-ELI

Surface modifications of titanium alloys are useful methods to enhance the biological stability of intraosseous implants and to promote a well succeeded osseointegration in the early stages of implantation. This work aims to investigate the influence of chemically modified surfaces of Ti-6Al-4V-ELI (extra-low interstitial) on the gene expression of human osteoblastic (HOb) cells. The surface treatments by acid etching or acid etching plus alkaline treatment were carried out to modify the topography, effective area, contact angle and chemical composition of the samples. The surface morphology was investigated using: scanning electron microscopy (SEM) and confocal laser-scanning microscope (CLSM). Roughness measurements and effective surface area were obtained using the CLSM. Surface composition was analysed by energy dispersive X-ray spectroscopy (EDX) and by X-Ray Diffraction (XRD). The expression levels of some bone related genes (ALPL, COL1A1, COL3A1, SPP1, RUNX2, and SPARC) were analysed using real-time Reverse Transcription Polymerase Chain Reaction (real-time RT-PCR). The results showed that all the chemical modifications studied in this work influenced the surface morphology, wettability, roughness, effective area and gene expression of human osteoblasts. Acid phosphoric combined to alkaline treatment presented a more accelerated gene expression after 7days while the only phosphoric etching or chloride etching combined to alkaline treatment presented more effective responses after 15days.

JMJD3 promotes chondrocyte proliferation and hypertrophy during endochondral bone formation in mice

JMJD3 (KDM6B) is an H3K27me3 demethylase and counteracts polycomb-mediated transcription repression. However, the function of JMJD3 in vivo is not well understood. Here we show that JMJD3 is highly expressed in cells of the chondrocyte lineage, especially in prehypertrophic and hypertrophic chondrocytes, during endochondral ossification. Homozygous deletion of Jmjd3 results in severely decreased proliferation and delayed hypertrophy of chondrocytes, and thereby marked retardation of endochondral ossification in mice. Genetically, JMJD3 associates with RUNX2 to promote proliferation and hypertrophy of chondrocytes. Biochemically, JMJD3 associates with and enhances RUNX2 activity by derepression of Runx2 and Ihh transcription through its H3K27me3 demethylase activity. These results demonstrate that JMJD3 is a key epigenetic regulator in the process of cartilage maturation during endochondral bone formation.

Transcription factor Runx2 in the low bone mineral density of girls with adolescent idiopathic scoliosis

OBJECTIVE

The molecular mechanism of low bone mass in girls with adolescent idiopathic scoliosis (AIS) has not been ascertained. Runx2 is a critical transcription factor regulating osteoblast differentiation and maturation. The present study aimed to explore the possible relationship between Runx2 expression in osteoblasts and bone mineral density (BMD) in subjects with AIS.

METHODS

Twenty-two girls with AIS scheduled to corrective surgery with iliac crest as donor site of autograft for spinal fusion were recruited. The BMD of lumbar spine and femoral neck were assessed by dual-energy X-ray absorptiometry, then patients were divided into two groups with either normal or reduced BMD. Cancellous bone was harvested from their iliac crests for primary culture of osteoblasts. mRNA and protein expression of Runx2 were assayed by reverse transcription-polymerase chain reaction and western blotting, respectively. Results were compared between the two groups and correlated with BMD.

RESULTS

AIS patients with normal BMD showed comparable maturity and body mass index but significant lower Cobb angle of main curve than those of patients with reduced BMD. The mean BMD of lumbar spine and femoral neck were 0.993 g/m(2) and 0.911 g/m(2) in patients with normal BMD, and were 0.757 g/m(2) and 0.733 g/m(2) in those with reduced BMD, respectively. The differences were significant between two groups (P < 0.05). The relative mean mRNA and protein expression of Runx2 were 0.49 ± 0.12 and 0.062 ± 0.020 in AIS with normal BMD, 0.35 ± 0.12 and 0.042 ± 0.006 in AIS with reduced BMD, respectively. Significantly lower Runx2 mRNA and protein expression were found in patients with AIS patients with reduced BMD than in those with normal BMD (P < 0.05). After controlling for age, weight and body mass index, positive correlations were found between Runx2 expression of both mRNA and protein and BMD of lumbar spine and femoral neck.

CONCLUSION

The abnormal expression of Runx2 in patients with AIS and reduced BMD indicates abnormal regulation of differentiation of their osteoblasts. Runx2 may play an important role in the pathogenesis of reduced BMD in patients with AIS.

Expression of core-binding factor α1 and osteocalcin in fluoride-treated fibroblasts and osteoblasts

To study the effects and importance of fluoride on FBs in the development of extraperiosteal calcification and the ossification of skeletal fluorosis, the presence of the osteogenic phenotype, which is indicated by the expression of core-binding factor α1 (Cbfa1) and osteocalcin (OCN), in an FB cell line (L929) and in osteoblasts (OBs) exposed to fluoride was determined. Fibroblasts and osteoblasts were exposed to different concentrations of fluoride (0, 0.0001, 0.001, 0.1, 1.0, 10.0 and 20.0 mg/L F(-)). By using RT-PCR and ELISA, the mRNA levels of Cbfa1 and OCN were measured at 48 h, and the protein levels of Cbfa1 and OCN were measured at 2, 4, 24, 48 and 72 h. The data demonstrated the following: (1) The Cbfa1 protein level in fluoride-treated fibroblasts clearly increased at 48 h in the groups treated with 0.0001, 0.001, 0.1, 1.0 and 20.0 mg/L F(-). The Cbfa1 protein level of the group treated with 10 mg/L F(-) at 72 h was higher than that of the control group. The level of Cbfa1 mRNA in the fibroblasts was much higher at 48 h in the group treated with 10.0 mg/L F(-) than in the control group. (2) The OCN protein level in fluoride-treated fibroblasts was significantly higher than that of the control group in the 0.0001, 0.1, 1.0, 10.0 and 20.0 mg/L F(-) groups at 2 h, and in the 0.001 and 0.1 F(-) groups at 4 h. A slightly higher level of OCN mRNA in fluoride-treated fibroblasts was also found in the 1.0 and 20.0 mg/L F(-) groups compared to the control group. (3) The expressions of Cbfa1 and OCN in osteoblasts treated with the same experimental conditions as the fibroblasts were up-regulated by fluoride following the same trend as in the fibroblasts. Our results showed an increase in the expression of Cbfa1 and OCN in fibroblasts and osteoblasts exposed to fluoride and suggested that the osteogenic function of fibroblasts induced by fluoride could play an important role in the development of extraperiosteal ossification during skeletal fluorosis.

Dynamic fluid flow stimulation on cortical bone and alterations of the gene expressions of osteogenic growth factors and transcription factors in a rat functional disuse model

Recently we have developed a dynamic hydraulic stimulation (DHS) as a loading modality to induce anabolic responses in bone. To further study the functional process of DHS regulated bone metabolism, the objective of this study was to evaluate the effects of DHS on cortical bone and its alterations on gene expressions of osteogenic growth factors and transcription factors as a function of time. Using a model system of 5-month-old hindlimb suspended (HLS) female Sprague-Dawley rats, DHS was applied to the right tibiae of the stimulated rats with a loading frequency of 2Hz with 30mmHg (p-p) dynamic pressure, 5days/week, for a total of 28days. Midshafts of the tibiae were analyzed using μCT and histology. Total RNA was analyzed using RT-PCR on selected osteogenic genes (RUNX2, β-catenin, osteopontin, VEGF, BMP2, IGF-1, and TGF-β) on 3-, 7-, 14- , and 21-day. Results showed increased Cort.Th and Ct.BV/TV as well as a time-dependent fashion of gradual changes in mRNA levels upon DHS. While DHS-driven fold changes of the mRNA levels remained low before Day-7, its fold changes started to elevate by Day-14 and then dropped by Day-21. This study further delineates the underlying molecular mechanism of DHS-derived mechanical signals, and its time-dependent optimization.

Mechanotransduction in bone: Intervening in health and disease

Mechanotransduction has been proven to be one of the most significant variables in bone remodeling and its alterations have been shown to result in a variety of bone diseases. Osteoporosis, Paget’s disease, orthopedic disorders, osteopetrosis as well as hyperparathyroidism and hyperthyroidism all comprise conditions which have been linked with deregulated bone remodeling. Although the significance of mechanotransduction for bone health and disease is unquestionable, the mechanisms behind this important process have not been fully understood. This review will discuss the molecules that have been found to be implicated in mechanotransduction, as well as the mechanisms underlying bone health and disease, emphasizing on what is already known as well as new molecules potentially taking part in conveying mechanical signals from the cell surface towards the nucleus under physiological or pathologic conditions. It will also focus on the model systems currently used in mechanotransduction studies, like osteoblast-like cells as well as three-dimensional constructs and their applications among others. It will also examine the role of mechanostimulatory techniques in preventing and treating bone degenerative diseases and consider their applications in osteoporosis, craniofacial development, skeletal deregulations, fracture treatment, neurologic injuries following stroke or spinal cord injury, dentistry, hearing problems and bone implant integration in the near future.

Senescent human periodontal ligament fibroblasts after replicative exhaustion or ionizing radiation have a decreased capacity towards osteoblastic differentiation

Loss of teeth increases with age or after genotoxic treatments, like head and neck radiotherapy, due to periodontium breakdown. Periodontal ligament fibroblasts represent the main cell type in this tissue and are crucial for the maintenance of homeodynamics and for its regeneration. Here, we have studied the characteristics of human periodontal ligament fibroblasts (hPDLF) that became senescent after replicative exhaustion or after exposure to ionizing radiation, as well as their ability for osteoblastic differentiation. We found that senescent hPDLF express classical markers of senescence, as well as a catabolic phenotype, as shown by the decrease in collagen type I and the increase of MMP-2 expression. In addition, we observed a considerably decreased expression of the major transcription factor for osteoblastic differentiation, i.e. Runx2, a down-regulation which was found to be p53-dependent. In accordance to the above, senescent cells have a significantly decreased alkaline phosphatase gene expression and activity, as well as a reduced ability for osteoblastic differentiation, as found by Alizarin Red staining. Interestingly, cells from both type of senescence express similar characteristics, implying analogous functions in vivo. In conclusion, senescent hPDLF express a catabolic phenotype and express a significantly decreased ability towards an osteoblastic differentiation, thus probably affecting tissue development and integrity.

Titanium nanotubes activate genes related to bone formation in vitro

Background:

Titanium is used worldwide to make osseointegrable devices, thanks to its favorable characteristics as mechanical proprieties and biocompatibility, demonstrated by in vivo studies with animal models and clinical trials over a forty-year period. However, the exact genetic effect of the titanium layer on cells is still not well characterized.

Materials and Methods:

To investigate how titanium nanotubes stimulate osteoblasts differentiation and proliferation, some osteoblast genes (SP7, RUNX2, COL3A1, COL1A1, ALPL, SPP1 and FOSL1) were analyzed by quantitative Real Time RT- PCR.

Results:

After 15 days, osteoblasts cultivated on titanium naotube showed the up-regulation of bone related genes SP7, ENG, FOSL1 and SPP1 and the down-regulation of RUNX2, COL3A1, COL1A1, and ALPL. After 30 days of treatment, the bone related genes SP7, ENG, FOSL1 and RUNX2 were up-regulated while COL3A1, COL1A1, ALPL and SPP1 were down-regulated.

Conclusions:

Our results, demonstrates that titanium nanotubes can lead to osteoblast differentiation and extracellular matrix deposition and mineralization in dental pulp stem cells by the activation of osteoblast related genes SPP1, FOSL1 and RUNX2.

Titanium nanotubes stimulate osteoblast differentiation of stem cells from pulp and adipose tissue

Background:

Titanium is the gold standard among materials used for prosthetic devices because of its good mechanical and chemical properties. When exposed to oxygen, titanium becomes an oxide, anatase that is biocompatible and able to induce osseointegration.

Materials and Methods:

In this study we compared the expression profiling of stem cells cultivated on two types of surface: Pure titanium disk and nanotube titanium disk in order to detect if nanotube titanium instead (NTD) surface stimulates stem cells towards osteoblast differentiation.

Results:

Stem cells cultivated on nanotube titanium disks showed the upregulation of bone-related genes RUNX2, FOSL1 and SPP1.

Conclusions:

Results demonstrated that nanotube titanium disk surface is more osteo-induced surface compared to titanium disk, promoting the differentiation of mesenchymal stem cells in osteoblasts.

Calcium sulfate stimulates pulp stem cells towards osteoblasts differentiation

Calcium sulfate (CaS) is a highly biocompatible material and enhances bone formation in vivo. However, how CaS alters osteoblast activity to promote bone formation is poorly understood. To study how CaS can induce osteoblast differentiation in mesenchymal stem cells, the expression levels of bone related genes and mesenchymal stem cells marker were compared in normal osteoblasts and dental pulp stem cells, using real time Reverse Transcription-Polymerase Chain Reaction. Gene differentially expressed between the two cells type were the trascriptional factor RUNX2, osteopontin (SPP1), COL1A1 (collagen type 1α1) and alkaline phosphatase (ALPL). The obtained results demonstrated that CaS strongly influences the behavior of DPSCs in vitro enhancing proliferation, differentiation and deposition of matrix.

The effect of the cleidocranial dysplasia-related novel 1116_1119insC mutation in the RUNX2 gene on the biological function of mesenchymal cells

Bone extracellular matrix deposition or bone formation by differentiated osteoblasts begins at late stage during bone formation and lasts throughout life. Human mesenchymal stem cells (MSCs) from bone marrow or dental pulp can respectively differentiate into osteoblasts and odontoblasts in vitro. However, the relationship between MSCs and bone/tooth development in cleidocranial dysplasia (CCD) patient is still unclear. In this study, we investigated a patient with CCD, which is an autosomal-dominant, heritable skeletal disease caused by runt-related transcription factor 2 gene (RUNX2) mutation and is characterized by bone and dental anomalies. We found that the mutation is localized at c. 1116_1119insC, p. Q374fsX384 and the proliferative ability and osteogenic potential of the MSCs isolated from the bone marrow and dental pulp of the patient (RUNX2(+/m)) were decreased compared to normal individuals (RUNX2(+/+)). Furthermore, we were unable to recover the differentiation potential of RUNX2(+/m) MSCs isolated from the bone marrow (BMMSCs) upon manipulation of the Wnt/β-catenin pathway, which plays a critical role in stem/progenitor cell self-renewal and adult human MSCs differentiation. In conclusion, we identified a novel insertion/frameshift mutation in the RUNX2 gene that caused a typical CCD phenotype and altered the biological function of RUNX2(+/m) MSCs. The reduced ability of MSCs to differentiate into osteoblasts might provide an explanation for the defects of bone and teeth in the CCD patient. Finally, we demonstrated that manipulation of the Wnt/β-catenin signaling pathway could not overcome this absence.

Runx-2 gene expression is associated with age-related changes of bone mineral density in the healthy young-adult population

Bone mineral density (BMD) and peak bone mass (PBM) are important determinants of skeletal resistance. The development of bone densitometry improved the possibility of studying BMD and the influence of genetic and environmental factors on bone. Heredity factors are important for BMD, and Runx-2 is accepted as a regulator of osteoblasts and bone formation. The aim of our study was to evaluate the behavior of Runx-2 during skeletal maturity in the healthy young-adult population. We analyzed spine and hip BMD in 153 volunteers, 98 women and 55 men, using dual-energy X-ray absorptiometry. In a subgroup of these volunteers, a sample of peripheral blood was taken to perform gene expression analysis of Runx-2 both in peripheral mesenchymal stem cells (MSCs; 28 subjects) and in peripheral mononuclear cells (PBMCs; 140 subjects). In our work BMD was comparable in both genders after puberty, then became higher in men than women during the third and fourth decades. PBM was achieved in the third decade in women and in the fourth in men. More interestingly, Runx-2 gene expression highly correlated with BMD in both genders. MSCs and PBMCs showed the same gene expression profile of Runx-2. In conclusion, PBM is reached earlier in females, BMD becomes higher in males later in life, and BMD and PBM are strictly associated with Runx-2. In addition, PBMC should be considered an important source for gene expression analysis in bone diseases.

Mechanical stimulation of polycystin-1 induces human osteoblastic gene expression via potentiation of the calcineurin/NFAT signaling axis

Mechanical forces trigger biological responses in bone cells that ultimately control osteoblastogenesis and bone program. Although several mechanosensors have been postulated, the precise mechanotransduction pathway remains obscure as the initial mechanosensing event has not yet been identified. Studies in kidney cells have shown that polycystin-1 (PC1), via its extracellular N-terminal part, may function as an "antenna-like" protein providing a linkage between environmental cues and their conversion into biochemical responses that regulate various cellular processes via the calcineurin/NFAT pathway. Here we explored the involvement of PC1 in mechanical load (stretching)-induced signaling cascades that control osteoblastogenesis/bone formation. FACS and TransAM Transcription Factor ELISA analyses employing extracts from primary human osteoblast-like, PC1 expressing cells subjected to mechanical stretching (0-6 h) revealed that unphosphorylated/DNA-binding competent NFATc1 increased at 0.5-1 h and returned to normal at 6 h. In accordance with the activation mechanism of NFATc1, stretching of cultured cells pre-treated with cyclosporin A (CsA, a specific inhibitor of the calcineurin/NFAT pathway) abrogated the observed decrease in the abundance of the cytoplasmic pNFATc1 (phosphorylated/inactive) species. Furthermore, stretching of osteoblastic cells pre-treated with an antibody against the mechanosensing N-terminal part of PC1 also abrogated the observed decrease in the cytoplasmic levels of the inactive pNFATc1 species. Importantly, under similar conditions (pre-incubation of stretched cells with the inhibitory anti-PC1 antibody), the expression of the key osteoblastic, NFATc1-target gene runx2 decreased compared to untreated cells. Therefore, PC1 acts as chief mechanosensing molecule that modulates osteoblastic gene transcription and hence bone-cell differentiation through the calcineurin/NFAT signaling cascade.

Mechanosensitive mechanisms in transcriptional regulation

Transcriptional regulation contributes to the maintenance of pluripotency, self-renewal and differentiation in embryonic cells and in stem cells. Therefore, control of gene expression at the level of transcription is crucial for embryonic development, as well as for organogenesis, functional adaptation, and regeneration in adult tissues and organs. In the past, most work has focused on how transcriptional regulation results from the complex interplay between chemical cues, adhesion signals, transcription factors and their co-regulators during development. However, chemical signaling alone is not sufficient to explain how three-dimensional (3D) tissues and organs are constructed and maintained through the spatiotemporal control of transcriptional activities. Accumulated evidence indicates that mechanical cues, which include physical forces (e.g. tension, compression or shear stress), alterations in extracellular matrix (ECM) mechanics and changes in cell shape, are transmitted to the nucleus directly or indirectly to orchestrate transcriptional activities that are crucial for embryogenesis and organogenesis. In this Commentary, we review how the mechanical control of gene transcription contributes to the maintenance of pluripotency, determination of cell fate, pattern formation and organogenesis, as well as how it is involved in the control of cell and tissue function throughout embryogenesis and adult life. A deeper understanding of these mechanosensitive transcriptional control mechanisms should lead to new approaches to tissue engineering and regenerative medicine.

Influence of unilateral tooth loss in the temporomandibular joint and masseter muscle of rabbits

OBJECTIVE

The purpose of this study was to evaluate the influence of the masticatory system in patients with missing teeth.

STUDY DESIGN

The influence of tooth loss on the masticatory system was analyzed with the use of bone scintigraphy ((99m)Tc-MDP) and histochemistry. Eight white rabbits (New Zealand, 12 weeks old) were used. The rabbits were divided into 2 groups: 6 weeks and 12 weeks. Teeth were extracted unilaterally in each rabbit under general anesthesia. Six and 12 weeks after extraction, scintigraphy was conducted, and the rabbits were killed and their masseter muscles removed for histochemical analysis.

RESULTS

The results of bone metabolism (relative ratio) measured by bone scintigraphy were 48.27% at extraction sites and 51.73% at nonextraction sites at 6 weeks and 39.96% at extraction sites and 60.04% at nonextraction sites at 12 weeks. There was a significant difference at 12 weeks (P < .05). Tissue calcium contents and osteoclast counts showed different results between the extraction and nonextraction sites, but these differences did not reach statistical significance.

CONCLUSIONS

The bone metabolism of temporomandibular joints and histochemical aspects of masticatory muscles may be associated with occlusal alterations following tooth loss.

RUNX2 tandem repeats and the evolution of facial length in placental mammals

Background

When simple sequence repeats are integrated into functional genes, they can potentially act as evolutionary ‘tuning knobs’, supplying abundant genetic variation with minimal risk of pleiotropic deleterious effects. The genetic basis of variation in facial shape and length represents a possible example of this phenomenon. Runt-related transcription factor 2 (RUNX2), which is involved in osteoblast differentiation, contains a functionally-important tandem repeat of glutamine and alanine amino acids. The ratio of glutamines to alanines (the QA ratio) in this protein seemingly influences the regulation of bone development. Notably, in domestic breeds of dog, and in carnivorans in general, the ratio of glutamines to alanines is strongly correlated with facial length.

Results

In this study we examine whether this correlation holds true across placental mammals, particularly those mammals for which facial length is highly variable and related to adaptive behavior and lifestyle (e.g., primates, afrotherians, xenarthrans). We obtained relative facial length measurements and RUNX2 sequences for 41 mammalian species representing 12 orders. Using both a phylogenetic generalized least squares model and a recently-developed Bayesian comparative method, we tested for a correlation between genetic and morphometric data while controlling for phylogeny, evolutionary rates, and divergence times. Non-carnivoran taxa generally had substantially lower glutamine-alanine ratios than carnivorans (primates and xenarthrans with means of 1.34 and 1.25, respectively, compared to a mean of 3.1 for carnivorans), and we found no correlation between RUNX2 sequence and face length across placental mammals.

Conclusions

Results of our diverse comparative phylogenetic analyses indicate that QA ratio does not consistently correlate with face length across the 41 mammalian taxa considered. Thus, although RUNX2 might function as a ‘tuning knob’ modifying face length in carnivorans, this relationship is not conserved across mammals in general.

Directed osteogenic differentiation of human mesenchymal stem/precursor cells on silicate substituted calcium phosphate

Insufficient, underactive, or inappropriate osteoblast function results in serious clinical conditions such as osteoporosis, osteogenesis imperfecta and fracture nonunion and therefore the control of osteogenesis is a medical priority. In vitro mesenchymal stem cells (MSCs) can be directed to form osteoblasts through the addition of soluble factors such as β-glycerophosphate, ascorbic acid, and dexamethasone; however this is unlikely to be practical in the clinical setting. An alternative approach would be to use a scaffold or matrix engineered to provide cues for differentiation without the need for soluble factors. Here we describe studies using Silicate-substituted calcium phosphate (Si-CaP) and unmodified hydroxyapatite (HA) to test whether these materials are capable of promoting osteogenic differentiation of MSCs in the absence of soluble factors. Si-CaP supported attachment and proliferation of MSCs and induced osteogenesis to a greater extent than HA, as evidenced through upregulation of the osteoblast-related genes: Runx2 (1.2 fold), Col1a1 (2 fold), Pth1r (1.5 fold), and Bglap (1.7 fold) Dmp1 (1.1 fold), respectively. Osteogenic-associated proteins, alkaline phosphatase (1.4 fold), RUNX2, COL1A1, and BGLAP, were also upregulated and there was an increased production of mineralized bone matrix (1.75 fold), as detected by the Von Kossa Assay. These data indicate that inorganic substrates are capable of directing the differentiation programme of stem cells in the absence of known chemical drivers and therefore may provide the basis for bone repair in the clinical setting.

Titanium Disk Surfaces Modulate Dental Implants Osseointegration

Titanium and titanium alloys are widely used as implant materials due to their excellent biocompatibility and mechanical properties. The aim of this work is to compare five different titanium layers in order to investigate which one had a greater osteoconductive power using Human Osteoblasts (HObs) culture for seven days on these surfaces. The expression levels of some bone-related genes (ALPL, COL1A1, COL3A1, SPP1, RUNX2 and SPARC) were analyzed using real time Reverse Transcription-Polymerase Chain Reaction (real time RT-PCR). Results obtained in this study demonstrate that titanium disks can lead to osteoblast differentiation and extracellular matrix deposition and mineralization by the activation of different osteoblast genes in relation to the specific type of surface treatment.

Transcriptomic comparison of osteopontin, osteocalcin and core binding factor 1 genes between human adipose derived differentiated osteoblasts and native osteoblasts

BACKGROUND

There are significant limitations in repair of irrecoverable bone defects. Stem-cell therapy is a promising approach for the construction of bone tissue. Mesenchymal stem cells (MSCs) have been introduced as basic tools for bone tissue generation. Through MSCs, adipose-derived stem cells (ADSCs) are more interesting. Since the similarity of native osteoblasts and differentiated osteoblasts from ADSCs in terms of gene expression pattern is unknown, this study was designed to compare gene expression patterns of some genes involved in osteogenesis between human native osteoblasts and adipose-derived differentiated osteoblasts.

MATERIALS AND METHODS

Realtime qRT-PCR was used for studying the gene expression of osteocalcin, osteopontin, and core binding factor alpha 1 (Cbfa1) in human native osteoblasts and adipose derived osteogenic osteoblasts at days 7, 14, 21, and 28 of differentiation.

RESULTS

This study demonstrated that native osteoblasts and differentiated osteoblasts, cultured in common osteogenic medium, have significant differences in gene expression levels for osteocalcin and osteopontin. Compared to native osteoblasts, these genes are expressed lower in all four groups of differentiated osteoblastic cells. We also found, there is a progressive increase in cbfa1 expression over the differentiation period of ADSCs from day 7 to day 28.

CONCLUSIONS

Our findings help for better assessment of adipose-derived differentiated cells as a source for cell-based therapy.

The enhanced modulation of key bone matrix components by modified Titanium implant surfaces

Modifications to Titanium (Ti) implant surfaces enhance osseointegration by promoting bone-implant contact and peri-implant bone accrual; which in vitro analyses of osteoblastic cells suggest is due to an enhancement in cellular phenotypic maturation and function. To evaluate these effects on uncommitted cells, this study examined the osteogenic mineralisation and phenotypic marker expression of human marrow derived stromal cells (hBMSCs) from three unrelated donors cultured on tissue culture plastic (TCP), polished (P), rough-hydrophobic (SLA) and rough-hydrophilic (modSLA) Ti surfaces over the course of 21 days. Transcriptional analyses indicated a significant early up-regulation of both Runx2 (p<0.05) and Osteopontin (OP) (p<0.05) but not Bone Sialoprotein 2 (BSP2) (p<0.05) by rough surfaces 1 day post seeding. The phenotypic analyses showed that whilst cellular proliferation was relatively restricted and slower on the rough substrates; osteogenic mineralisation, assessed by quantifying extracellular matrix calcium deposition, collagen formation and the ratio of collagen to mineral deposited were significantly higher (p<0.05); as was alkaline phosphatase (ALP) activity (p<0.05). The rough surfaces caused an increase of secreted osteoblastic markers Osteoprotegrin (OPG) (p<0.05), growth differentiation factor 15 (GDF-15) (p<0.05) and Osteocalcin (OC) (p<0.05). These findings suggest that modified Ti surfaces induce an enhancement in osteogenic commitment and differentiation, which likely underlie the deposition of more stable bone matrix early in the healing process in vivo.

Bone development: overview of bone cells and signaling

Vertebrates evolved elaborating a structure made up of more than 200 bones and cartilages articulated with one another to form the skeleton, through which locomotion, organ protection, lodging of hematopoiesis, and mineral homeostasis are allowed. Skeletogenesis starts at the fetal stage, along with marrow hematopoiesis, and evolves postnatally through modeling and remodeling processes that permit skeletal mass buildup. Preservation of skeletal mass is then implemented by balanced remodeling, which ensures continuous renovation of the tissue to allow its mechanical, structural, and metabolic properties to remain unaltered until ageing or diseases disrupt this equilibrium. Skeletal homeostasis is fulfilled by specialized bone cells in association with systemic and local regulators. Herein I review landmark discoveries that shed light on the intricate mesh connecting bone cells among themselves and with other systems, thus representing the cellular basis of normal and abnormal bone development and homeostasis.

Matrix protein biglycan induces osteoblast differentiation through extracellular signal-regulated kinase and Smad pathways

Biglycan (Bgn) is a member of the small leucine-rich proteoglycan (SLRP) family found in bone extracellular matrix (ECM), and hence involved in regulating bone formation and matrix mineralization. It has been reported that Bgn facilitates osteoblast differentiation, and extracellular signal-regulated kinase (Erk) and Smad are two important pathways in regulating osteoblast differentiation. However, the underlying mechanism for Bgn facilitating osteoblast differentiation has not been fully elucidated. The present study demonstrated that the matrix protein Bgn activates Erk signaling pathway and therefore increases Runx2 transcriptional activity, in which glycosaminoglycans (GAGs) chains play an essential role. Additionally, Bgn also activated Smad pathway, another signaling pathway related with osteoblast differentiation. The activation of these two signaling pathways induced by Bgn facilitated the mineralization deposition in vitro. These results demonstrated the mechanism of Bgn promoting osteoblast differentiation and matrix mineralization.

Biological effect of resorbable plates on normal osteoblasts and osteoblasts derived from Pfeiffer syndrome

Biodegradable fixation devices made of the polymers polylactide, polyglycolide and their copolymers are used routinely during maxillofacial, craniofacial, and orthopedic reconstructive surgical procedures, thanks to their property of biodegradation that avoid the need for implant removal. In particular, they are used in the treatment of craniosynostosis in pediatric patients affected by Pfeiffer syndrome, where the resorption time of 1 year or less does not interfere with the normal growth of the skull. To study the mechanism how polylactide-polyglycolide (PLPG) acid plates can induce osteoblast differentiation and proliferation in normal osteoblasts and in osteoblasts derived from a patient with Pfeiffer syndrome, the expression levels of bone-related genes were analyzed using real-time reverse transcription-polymerase chain reaction. Osteoblasts grown on the PLPG acid plates resulted in significant upregulation of mRNA expression of many genes related to osteodifferentiation during the treatment, indicating that polylactide, polyglycolide biopolymers enhance proliferation, differentiation, and deposition of matrix in osteoblasts. This study also revealed some differences in gene expression between normal osteoblasts and osteoblasts derived from patients with Pfeiffer syndrome, cultivated on PLPG acid plates.

Hypertrophic differentiation and calcification during intervertebral disc degeneration

BACKGROUND

In degenerative intervertebral discs (IVDs) collagen type X expression and calcifications have been demonstrated, resembling advanced osteoarthritis (OA), which is associated with hypertrophic differentiation, characterized by the production of collagen type X, Runt-related transcription factor 2 (Runx2), osteoprotegerin (OPG), alkaline phosphatase (ALP) and calcifications.

OBJECTIVE

The aim of this study was to determine if hypertrophic differentiation occurs during IVD degeneration.

METHODS

IVDs from all Thompson degeneration grades were prepared for histology, extraction of nucleus pulposus (NP) and annulus fibrosis (AF) tissue (N=50) and micro-CT (N=27). The presence of collagen type X, OPG and Runx2 was determined by immunohistochemistry, with OPG levels also determined by Enzyme-linked immunosorbent assay (ELISA). The presence of calcification was determined by micro-CT, von Kossa and Alizarin Red staining.

RESULTS

Immunohistochemical staining for collagen type X, OPG, Runx2 appeared more intense in the NP of degenerative compared to healthy IVD samples. OPG levels correlated significantly with degeneration grade (NP: P<0.000; AF: P=0.002) and the number of microscopic calcifications (NP: P=0.002; AF: P=0.008). The extent of calcifications on micro-CT also correlated with degeneration grade (NP: P<0.001, AF: P=0.001) as did von Kossa staining (NP: P=0.015, AF: P=0.016). ALP staining was only incidentally seen in the transition zone of grades IV and V degenerated IVDs.

CONCLUSION

This study for the first time demonstrates that hypertrophic differentiation occurs during IVD degeneration, as shown by an increase in OPG levels, the presence of ALP activity, increased immunopositivity of Runx2 and collagen type X.

The significance of RUNX2 in postnatal development of the mandibular condyle

OBJECTIVE

RUNX2, in the Runt gene family, is one of the most important transcription factors in the development of the skeletal system. Research in recent decades has shown that this factor plays a major role in the development, growth and maturation of bone and cartilage. It is also important in tooth development, mechanotransduction and angiogenesis, and plays a significant role in various pathological processes, i.e. tumor metastasization. Mutations in the RUNX2 gene correlate with the cleidocranial dysplasia (CCD) syndrome, important to dentistry, particularly orthodontics because of its dental and orofacial symptoms. Current research on experimentally-induced mouse mutants enables us to study the etiology and pathogenesis of these malformations at the cellular and molecular biological level. This study's aim is to provide an overview of the RUNX2 gene's function especially in skeletal development, and to summarize our research efforts to date, which has focused on investigating the influence of RUNX2 on mandibular growth, which is slightly or not at all altered in many CCD patients.

MATERIALS AND METHODS

Immunohistochemical analyses were conducted to reveal RUNX2 in the condylar cartilage of normal mice and of heterozygous RUNX2 knockout mice in early and late growth phases; we also performed radiographic and cephalometric analyses.

RESULTS

We observed that RUNX2 is involved in normal condylar growth in the mouse and probably plays a significant role in osteogenesis and angiogenesis. The RUNX2 also has a biomechanical correlation in relation to cartilage compartmentalization. At the protein level, we noted no differences in the occurrence and distribution of RUNX2 in the condyle, except for a short phase during the 4th and 6th postnatal weeks, so that one allele might suffice for largely normal growth; other biological factors may have compensatory effects. However, we did observe small changes in a few cephalometric parameters concerning the mandibles of heterozygous knockout animals. We discuss potential correlations to our findings by relating them to the most current knowledge about the RUNX2 biology.

Short-term exposure to low-carbohydrate, high-fat diets induces low bone mineral density and reduces bone formation in rats

Low-carbohydrate, high-fat (LC-HF) diets are popular for inducing weight loss in adults and are also used as part of a treatment for children with epilepsy. However, potential risks and side effects remain controversial. We investigated effects of LC-HF diets on growth, bone mineral density (BMD), and turnover in growing rats fed for 4 weeks either normal chow (CH, 9% fat, 33% protein, and 58% carbohydrates), LC-HF-1 (66% fat, 33% protein, and 1% carbohydrates), or LC-HF-2 (94.5% fat, 4.2% protein, and 1.3% carbohydrates). Rats fed LC-HF diets accumulated significantly more visceral and bone marrow fat and showed increased leptin but decreased insulin-like growth-factor 1 (IGF-1). Both LC-HF diets significantly decreased body length (nose to rump), but lengths of humerus, tibia, and femur were significantly reduced with LC-HF-2 only. Peripheral quantitative computed tomography (pQCT) and micro-CT (microCT) independently revealed significant reductions in BMD of tibiae in both LC-HF groups, and tibial maximum load was impaired. Bone-formation marker N-terminal propeptide of type I procollagen was reduced in sera of LC-HF groups, whereas bone resorption marker CrossLaps remained unchanged. Real-time PCR analysis revealed significant reductions by 70% to 80% of transcription factors influencing osteoblastogenesis (Runx2, osterix, and C/EBPbeta) in bone marrow of rats fed LC-HF diets. In conclusion, both LC-HF diets impaired longitudinal growth, BMD, and mechanical properties, possibly mediated by reductions in circulating IGF-1. Serum bone-formation markers as well as expression of transcription factors influencing osteoblastogenesis were reduced. This might indicate a lower rate of mesenchymal stem cells differentiating into osteoblasts, thus explaining reduced bone formation with LC-HF diets.

Profiling type I collagen gene expression in growing mandibular structures

We conducted a temporal gene expression analysis with type I collagen in the coronoid process, alveolar process and mandibular angle of the rat. We observed gene expression cross-sectionally across different important physiological time points in the rat postnatal life in order to observe in which developmental stage mandibular development mainly occur. This study indicates prominent type I collagen expression at day 10 postpartum in the mandibular ramus and at day 21 in the alveolar process. These findings correspond well with previously obtained data from proliferation studies in facial bone suggesting that craniofacial growth in the rat occurs mainly between days 10 and 21.

Murine preosteoblast differentiation induced by a peptide derived from bone morphogenetic proteins-9

Bone morphogenetic proteins (BMPs) increase the differentiation of osteoblasts implicated in bone formation and repair. In a previous study, we demonstrated that a peptide derived from BMP-9 (pBMP-9) at 400 ng/mL inhibited murine preosteoblasts MC3T3-E1 proliferation. Here, we compared the effects of equimolar concentrations of BMP-2 (50 ng/mL), BMP-9 (42.3 ng/mL), and pBMP-9 (4.52 ng/mL) on the differentiation of MC3T3-E1 in a serum-free medium. Like BMP-2, BMP-9 and pBMP-9 activated the Smad pathway. In contrary to BMP-2, the Smad phosphorylation induced by BMP-9 and pBMP-9 is not prevented by noggin, an extracellular antagonist of BMP-2. Further, BMP-9 and pBMP-9 increased, dose dependently, alkaline phosphatase activity, an early marker of osteoblast differentiation, after 1 day. Quantitative real-time polymerase chain reaction analysis demonstrated that BMP-2, BMP-9, and pBMP-9 (4.52 or 400 ng/mL) all activated the transcription of Runx2, Osterix, type I collagen alpha1 chain, and Osteocalcin genes within day 6. Alizarin red S quantification demonstrated that pBMP-9 (400 ng/mL) and pBMP-9 (4.52 ng/mL) allowed a slight deposition of Ca(2+) in the extracellular matrix of cells within 12 and 18 days, respectively. Therefore, pBMP-9 might be a promising replacement for costly BMP in tissue engineering applications that require a well-defined serum-free medium.

Selective signaling by Akt2 promotes bone morphogenetic protein 2-mediated osteoblast differentiation

Mesenchymal stem cells are essential for repair of bone and other supporting tissues. Bone morphogenetic proteins (BMPs) promote commitment of these progenitors toward an osteoblast fate via functional interactions with osteogenic transcription factors, including Dlx3, Dlx5, and Runx2, and also can direct their differentiation into bone-forming cells. BMP-2-stimulated osteoblast differentiation additionally requires continual signaling from insulin-like growth factor (IGF)-activated pathways. Here we identify Akt2 as a critical mediator of IGF-regulated osteogenesis. Targeted knockdown of Akt2 in mouse primary bone marrow stromal cells or in a mesenchymal stem cell line, or genetic knockout of Akt2, did not interfere with BMP-2-mediated signaling but resulted in inhibition of osteoblast differentiation at an early step that preceded production of Runx2. In contrast, Akt1-deficient cells differentiated normally. Complete biochemical and morphological osteoblast differentiation was restored in cells lacking Akt2 by adenoviral delivery of Runx2 or by a recombinant lentivirus encoding wild-type Akt2. In contrast, lentiviral Akt1 was ineffective. Taken together, these observations define a specific role for Akt2 as a gatekeeper of osteogenic differentiation through regulation of Runx2 gene expression and indicate that the closely related Akt1 and Akt2 exert distinct effects on the differentiation of mesenchymal precursors.

Signaling networks and transcription factors regulating mechanotransduction in bone

Mechanical stimulation has a critical role in the development and maintenance of the skeleton. This function requires the perception of extracellular stimuli as well as their conversion into intracellular biochemical responses. This process is called mechanotransduction and is mediated by a plethora of molecular events that regulate bone metabolism. Indeed, mechanoreceptors, such as integrins, G protein-coupled receptors, receptor protein tyrosine kinases, and stretch-activated Ca(2+) channels, together with their downstream effectors coordinate the transmission of load-induced signals to the nucleus and the expression of bone-related genes. During the past decade, scientists have gained increasing insight into the molecular networks implicated in bone mechanotransduction. In the present paper, we consider the major signaling cascades and transcription factors that control bone and cartilage mechanobiology and discuss the influence of the mechanical microenvironment on the determination of skeletal morphology.

Is Runx a linchpin for developmental signaling in metazoans?

The Runt domain (Runx) is a 128 amino acid sequence motif that defines a metazoan family of sequence-specific DNA binding proteins, which appears to have originated in concert with the intercellular signaling systems that coordinate multicellular development in animals. In the model organisms where they have been studied (fruit fly, mouse, sea urchin, and nematode) Runx genes are essential for normal development, and in humans they are causally associated with a variety of cancers, manifesting both oncogenic and tumor suppressive attributes. During development Runx proteins support both cell proliferation and differentiation, and function in both transcriptional activation and repression. Runx function is thus context-dependent, with the context provided genetically by cis-regulatory sequence architecture and epigenetically by development. This context dependency makes it difficult to formulate reductionistic generalizations concerning Runx function in normal and carcinogenic development. However, a growing body of literature links Runx function to each of the major intercellular signaling systems in animals, suggesting that the general function of Runx transcription factors may be to potentiate and govern genomic responsiveness to developmental signaling.

Mechanotransduction in osteoblast regulation and bone disease

Osteoblasts are key components of the bone multicellular unit and have a seminal role in bone remodeling, which is an essential function for the maintenance of the structural integrity and metabolic capacity of the skeleton. The coordinated function of skeletal cells is regulated by several hormones, growth factors and mechanical cues that act via interconnected signaling networks, resulting in the activation of specific transcription factors and, in turn, their target genes. Bone cells are responsive to mechanical stimuli and this is of pivotal importance in developing biomechanical strategies for the treatment of osteodegenerative diseases. Here, we review the molecular pathways and players activated by mechanical stimulation during osteoblastic growth, differentiation and activity in health, and consider the role of mechanostimulatory approaches in treating various bone pathophysiologies.