Transgenic Overexpression of Ephrin B1 in Bone Cells Promotes Bone Formation and an Anabolic Response to Mechanical Loading in Mice

To test if ephrin B1 overexpression enhances bone mass, we generated transgenic mice overexpressing ephrin B1 under the control of a 3.6 kb rat collagen 1A1 promoter (Col3.6-Tg^efnb1). Col3.6-Tg^efnb1 mice express 6-, 12- and 14-fold greater levels of full-length ephrin B1 protein in bone marrow stromal cells, calvarial osteoblasts, and osteoclasts, respectively. The long bones of both genders of Col3.6-Tg^efnb1 mice have increased trabecular bone volume, trabecular number, and trabecular thickness and decreased trabecular separation. Enhanced bone formation and decreased bone resorption contributed to this increase in trabecular bone mass in Col3.6-Tg^efnb1 mice. Consistent with these findings, our in vitro studies showed that overexpression of ephrin B1 increased osteoblast differentiation and mineralization, osterix and collagen 1A1 expression in bone marrow stromal cells. Interaction of ephrin B1 with soluble clustered EphB2-Fc decreased osteoclast precursor differentiation into multinucleated cells. Furthermore, we demonstrated that the mechanical loading-induced increase in EphB2 expression and newly formed bone were significantly greater in the Col3.6-Tg^efnb1 mice than in WT littermate controls. Our findings that overexpression of ephrin B1 in bone cells enhances bone mass and promotes a skeletal anabolic response to mechanical loading suggest that manipulation of ephrin B1 actions in bone may provide a means to sensitize the skeleton to mechanical strain to stimulate new bone formation.     

Role of collagen XII in skin homeostasis and repair

Skin integrity and function depends to a large extent on the composition of the extracellular matrix, which regulates tissue organization. Collagen XII is a homotrimer with short collagenous domains that confer binding to the surface of collagen I-containing fibrils and extended flexible arms, which bind to non-collagenous matrix components. Thereby, collagen XII helps to maintain collagen suprastructure and to absorb stress. Mutant or absent collagen XII leads to reduced muscle and bone strength and lax skin, whereas increased collagen XII amounts are observed in tumor stroma, scarring and fibrosis.This study aimed at uncovering in vivo mechanisms by which collagen XII may achieve these contrasting outcomes. We analyzed skin as a model tissue that contains abundant fibrils, composed of collagen I, III and V with collagen XII decorating their surface, and which is subject to mechanical stress. The impact of different collagen XII levels was investigated in collagen XII-deficient (Col12-KO) mice and in mice with collagen XII overexpression in the dermis (Col12-OE). Unchallenged skin of these mice was histologically inconspicuous, but at the ultrastructural level revealed distinct aberrations in collagen network suprastructure. Repair of excisional wounds deviated from controls in both models by delayed healing kinetics, which was, however, caused by completely different mechanisms in the two mouse lines. The disorganized matrix in Col12-KO wounds failed to properly sequester TGFβ, resulting in elevated numbers of myofibroblasts. These are, however, unable to contract and remodel the collagen XII-deficient matrix. Excess of collagen XII, in contrast, promotes persistence of M1-like macrophages in the wound bed, thereby stalling the wounds in an early inflammatory stage of the repair process and delaying healing.Taken together, we demonstrate that collagen XII is a key component that assists in orchestrating proper skin matrix structure, controls growth factor availability and regulates cellular composition and function. Together, these functions are pivotal for re-establishing homeostasis after injury.     

Dynamic cell stretching increases human osteoblast proliferation and CICP synthesis but decreases osteocalcin synthesis and alkaline phosphatase activity

The cell activity of human-bone-derived cell cultures was studied after mechanical stimulation by cyclic strain at a magnitude occurring in physiologically loaded bone tissue. Monolayers of subconfluently grown human-bone-derived cells were stretched in rectangular silicone dishes with cyclic predominantly uniaxial movement along their longitudinal axes. Strain was applied over two days for 30 min per day with a frequency of 1 Hz and a strain magnitude of 1000 microstrain. Cyclic stretching of the cells resulted in an increased proliferation (10-48%) and carboxyterminal collagen type I propeptide release (7-49%) of human-cancellous bone-derived osteoblasts while alkaline phosphatase activity and osteocalcin release were significantly reduced by 9-25 and 5-32%, respectively. These results demonstrate that cyclic strain at physiologic magnitude leads to an increase of osteoblast activities related to matrix production while those activities which are characteristic for the differentiated osteoblast and relevant for matrix mineralization are decreased.     

Confocal imaging and timing of secretion of matrix proteins by osteoblasts derived from avian long bone

Primary osteoblasts derived from avian long bone have been evaluated in terms of spatial and temporal expression of known osteoblastic marker proteins during the early phases of cell culture. Confocal imaging of matrix proteins revealed that osteocalcin, bone sialoprotein, osteopontin, and osteonectin were restricted to the cell interior at day 4 of culture; secretion and deposition into the extra-cellular matrix of bone sialoprotein and osteopontin was evident at 8 and 12 days of culture. Osteocalcin and osteonectin were not deposited in the matrix within the timeframe of the study. Total collagen levels produced and alkaline phosphatase activity were substantial by day 4 of culture, and increased from that point 4.0- and 5.5-fold, respectively, by culture day 12. The expression of type I collagen, PTHrP receptor, osteopontin, bone sialoprotein and osteocalcin was followed by Northern blot analysis. Type I collagen and osteopontin mRNA were expressed at constant levels throughout the culture period. Over the 12 days of culture both PTH/PTHrP receptor and bone sialoprotein mRNA expression were found to increase by 2.3- and 2.5-fold, respectively. In contrast, the expression of osteocalcin message decreased by 2.5-fold by day 8 of culture.     

The bone marrow microenvironment contributes to type I diabetes induced osteoblast death

Type I diabetes increases an individual's risk for bone loss and fracture, predominantly through suppression of osteoblast activity (bone formation). During diabetes onset, levels of blood glucose and pro‐inflammatory cytokines (including tumor necrosis factor α (TNFα)) increased. At the same time, levels of osteoblast markers are rapidly decreased and stay decreased chronically (i.e., 40 days later) at which point bone loss is clearly evident. We hypothesized that early bone marrow inflammation can promote osteoblast death and hence reduced osteoblast markers. Indeed, examination of type I diabetic mouse bones demonstrates a greater than twofold increase in osteoblast TUNEL staining and increased expression of pro‐apoptotic factors. Osteoblast death was amplified in both pharmacologic and spontaneous diabetic mouse models. Given the known signaling and inter‐relationships between marrow cells and osteoblasts, we examined the role of diabetic marrow in causing the osteoblast death. Co‐culture studies demonstrate that compared to control marrow cells, diabetic bone marrow cells increase osteoblast (MC3T3 and bone marrow derived) caspase 3 activity and the ratio of Bax/Bcl‐2 expression. Mouse blood glucose levels positively correlated with bone marrow induced osteoblast death and negatively correlated with osteocalcin expression in bone, suggesting a relationship between type I diabetes, bone marrow and osteoblast death. TNF expression was elevated in diabetic marrow (but not co‐cultured osteoblasts); therefore, we treated co‐cultures with TNFα neutralizing antibodies. The antibody protected osteoblasts from bone marrow induced death. Taken together, our findings implicate the bone marrow microenvironment and TNFα in mediating osteoblast death and contributing to type I diabetic bone loss. J. Cell. Physiol. 226: 477–483, 2011. © 2010 Wiley‐Liss, Inc.     

Acidic fibroblast growth factor inhibits osteoblast differentiation in vitro: Altered expression of collagenase,cell growth-related,and mineralization-associated genes

Fibroblast growth factors (FGF) are osteoblast mitogens, but their effects on bone formation are not clearly understood. Most in vitro studies examining the effects of FGFs on osteoblasts have been performed only during the initial proliferative stage of osteoblast culture. In these studies, we examined the consequential effect of acidic FGF in cultures of rat fetal diploid osteoblasts that undergo a developmental differentiation program producing a mineralized bone-like matrix. During the initial growth period (days 1–10), addition of acidic FGF (100 μg/ml) to actively proliferating cells increased (P < 0.05) ³H-thymidine uptake (2,515 ± 137, mean ± SEM vs. 5,884 ± 818 cpm/10⁴ cells). During the second stage of maturation (days 10–15), osteoblasts form multilayered nodules of cells and accumulate matrix, followed by mineralization (stage 3, days 16–29). Addition of acidic FGF to the osteoblast cultures from days 7 to 15 completely blocked nodule formation. Furthermore, addition of acidic FGF after nodule formation (days 14–29) inhibited matrix mineralization, which was associated with a marked increase in collagenase gene expression, and resulted in a progressive change in the morphology of the nodules, with only a few remnants of nonmineralized nodules present by day 29. Histochemical and biochemical analyses revealed a decrease in alkaline phosphatase and mineral content, confirming the acidic FGF-induced inhibition of nodule and matrix formation. To identify mechanisms contributing to these changes, we examined expression of cell growth and bone phenotypic markers. Addition of acidic FGF during the proliferative phase (days 7–8) enhanced histone H4, osteopontin, type 1 collagen, and TGF-β mRNA levels, which are coupled to proliferating osteoblasts, and blocked the normal developmental increase in alkaline phosphatase and osteocalcin gene expression and calcium accumulation. Addition of acidic FGF to the cultures during matrix maturation (days 14–15) reactivated H4, osteopontin, type I collagen, and TGF-β gene expression, and decreased alkaline phosphatase and osteocalcin gene expression. In an in vivo experiment, rats were treated with up to 60 μg/kg/day acidic FGF intravenously for 30 days. Proliferation of osteoblasts and deposition of bone occurred in the marrow space of the diaphysis of the femur in a dose-related fashion. The metaphyseal areas were unaffected by treatment. In conclusion, our data suggest that acidic FGF is a potent mitogen for early stage osteoblasts which leads to modifications in the formation of the extracellular matrix; increases in TGF-β and collagenase are functionally implicated in abrogating competency for nodule formation. Persistence of proliferation prevented expression of alkaline phosphatase and osteocalcin, also contributing to the block in the progression of the osteoblast developmental sequence. © 1996 Wiley-Liss, Inc.     

The organization of the osteocyte network mirrors the extracellular matrix orientation in bone

Bone is a dynamic tissue that is continually undergoing a process of remodeling - an effect due to the interplay between bone resorption by osteoclasts and bone formation by osteoblasts. When new bone is deposited, some of the osteoblasts are embedded in the mineralizing collagen matrix and differentiate to osteocytes, forming a dense network throughout the whole bone tissue. Here, we investigate the extent to which the organization of the osteocyte network controls the collagen matrix arrangement found in various bone tissues. Several tissue types from equine, ovine and murine bone have been examined using confocal laser scanning microscopy as well as polarized light microscopy and back-scattered electron imaging. From comparing the spatial arrangements of unorganized and organized bone, we propose that the formation of a highly oriented collagen matrix requires an alignment of osteoblasts whereby a substrate layer provides a surface such that osteoblasts can align and, collectively, build new matrix. Without such a substrate, osteoblasts act isolated and only form matrices without long range order. Hence, we conclude that osteoblasts synthesize and utilize scaffold-like primary tissue as a guide for the deposition of highly ordered and mechanically competent bone tissue by a collective action of many cells.     

Skeletal Unloading Induces Biphasic Changes in Insulin-Like Growth Factor-I mRNA Levels and Osteoblast Activity

To determine the local mechanisms involved in the effects of skeletal unloading on bone formation, we studied the temporal pattern of mRNA levels for insulin-like growth factor-I (IGF-I), IGF-I receptor type I (IGF-IR), and transforming growth factor beta receptor type II (TGF-betaRII) in relation to osteoblast phenotypic markers and osteoblast activity in hindlimb suspended rats. Skeletal unloading decreased bone volume and the mineralizing and osteoblastic surfaces at 4, 7, and 14 days in the tibial metaphysis, whereas the mineral appositional rate returned to normal at 14 days of suspension. RT-PCR analysis showed that skeletal unloading decreased type 1 collagen (Col 1) and osteocalcin (OC) mRNA levels in metaphyseal bone at days 4 and 7, and the levels returned to normal at 14 days of suspension. Unloading also decreased mRNA levels for IGF-I, IGF-IR, and TGF-betaRII at 4-7 days in the metaphyseal bone. However, IGF-I and IGF-IR levels rose above normal at 14 days of suspension. The biphasic changes in IGF-I mRNA levels were strongly correlated with Col 1 and OC mRNA levels. The associated biphasic pattern of IGF-I/IGF-IR expression, osteoblast markers, and osteoblast activity strongly suggests an important role for IGF-I signaling in the local effect of skeletal unloading on metaphyseal bone formation.     

Effects of Zinc Transporter on Differentiation of Bone Marrow Mesenchymal Stem Cells to Osteoblasts

The differentiation of bone marrow mesenchymal stem cells (MSCs) into osteoblasts is a crucial step during bone formation. However, the exact mechanisms regulating the early stages of osteogenic differentiation remain unknown. In the present study, we found that ZnT7, a member of the zinc transporter family SLC30A(ZnTs), was downregulated during dexamethasone-induced differentiation of rat MSCs into osteoblasts. Dexamethasone treatment resulted in significantly lower levels of ZnT7 compared with cocultured cells without dexamethasone. Differentiation was evaluated by measuring alkaline phosphatase (ALP) activity and staining for ALP, von Kossa, collagen type I, and osteocalcin. Overexpression of ZnT7 decreased the expression of the osteoblast alkaline phosphatase, type I collagen, as well as calcium deposition in mesenchymal cells. In contrast, knockdown of ZnT7 using siRNA promoted gene expression associated with osteoblast differentiation and matrix mineralization in vitro. Moreover, according to the ZnT7 inhibition or activation experiments, Wnt and ERK signaling pathways were found to be important signal transduction pathways in mediating the osteogenic effect of MSCs, and this effect is intensified by a decrease in the level of ZnT7 induced by dexamethasone. These findings suggest that ZnT7 is involved in the switch from the undifferentiated state of MSC to an osteogenic program, and marking the expression level of ZnT7 may be useful in the detection of early osteogenic differentiation.     

Chondrocytes Transdifferentiate into Osteoblasts in Endochondral Bone during Development,Postnatal Growth and Fracture Healing in Mice

One of the crucial steps in endochondral bone formation is the replacement of a cartilage matrix produced by chondrocytes with bone trabeculae made by osteoblasts. However, the precise sources of osteoblasts responsible for trabecular bone formation have not been fully defined. To investigate whether cells derived from hypertrophic chondrocytes contribute to the osteoblast pool in trabecular bones, we genetically labeled either hypertrophic chondrocytes by Col10a1-Cre or chondrocytes by tamoxifen-induced Agc1-CreERT2 using EGFP, LacZ or Tomato expression. Both Cre drivers were specifically active in chondrocytic cells and not in perichondrium, in periosteum or in any of the osteoblast lineage cells. These in vivo experiments allowed us to follow the fate of cells labeled in Col10a1-Cre or Agc1-CreERT2 -expressing chondrocytes. After the labeling of chondrocytes, both during prenatal development and after birth, abundant labeled non-chondrocytic cells were present in the primary spongiosa. These cells were distributed throughout trabeculae surfaces and later were present in the endosteum, and embedded within the bone matrix. Co-expression studies using osteoblast markers indicated that a proportion of the non-chondrocytic cells derived from chondrocytes labeled by Col10a1-Cre or by Agc1-CreERT2 were functional osteoblasts. Hence, our results show that both chondrocytes prior to initial ossification and growth plate chondrocytes before or after birth have the capacity to undergo transdifferentiation to become osteoblasts. The osteoblasts derived from Col10a1-expressing hypertrophic chondrocytes represent about sixty percent of all mature osteoblasts in endochondral bones of one month old mice. A similar process of chondrocyte to osteoblast transdifferentiation was involved during bone fracture healing in adult mice. Thus, in addition to cells in the periosteum chondrocytes represent a major source of osteoblasts contributing to endochondral bone formation in vivo.     

Hypoxia inhibits the growth, differentiation and bone-forming capacity of rat osteoblasts

We investigated the effect of hypoxia on rat osteoblast function in long-term primary cultures. Reduction of pO2 from 20% to 5% and 2% decreased formation of mineralized bone nodules 1.7-fold and 11-fold, respectively. When pO2 was reduced further to 0.2%, bone nodule formation was almost abolished. The inhibitory effect of hypoxia on bone formation was partly due to decreased osteoblast proliferation, as measured by 3H-thymidine incorporation. Hypoxia also sharply reduced osteoblast alkaline phosphatase (ALP) activity and expression of mRNAs for ALP and osteocalcin, suggesting inhibition of differentiation to the osteogenic phenotype. Hypoxia did not increase the apoptosis of osteoblasts but induced a reversible state of quiescence. Transmission electron microscopy revealed that collagen fibrils deposited by osteoblasts cultured in 2% O2 were less organized and much less abundant than in 20% O2 cultures. Furthermore, collagen produced by hypoxic osteoblasts contained a lower percentage of hydroxylysine residues and exhibited an increased sensitivity to pepsin degradation. These data demonstrate the absolute oxygen requirement of osteoblasts for successful bone formation and emphasize the importance of the vasculature in maintaining bone health. We recently showed that hypoxia also acts in a reciprocal manner as a powerful stimulator of osteoclast formation. Considered together, our results help to explain the bone loss that occurs at the sites of fracture, tumors, inflammation and infection, and in individuals with vascular disease or anemia.     

Neuropeptide Y is expressed by osteocytes and can inhibit osteoblastic activity

Osteocytes are the most abundant osteoblast lineage cells within the bone matrix. They respond to mechanical stimulation and can participate in the release of regulatory proteins that can modulate the activity of other bone cells. We hypothesize that neuropeptide Y (NPY), a neurotransmitter with regulatory functions in bone formation, is produced by osteocytes and can affect osteoblast activity. To study the expression of NPY by the osteoblast lineage cells, we utilized transgenic mouse models in which we can identify and isolate populations of osteoblasts and osteocytes. The Col2.3GFP transgene is active in osteoblasts and osteocytes, while the DMP1 promoter drives green fluorescent protein (GFP) expression in osteocytes. Real‐time PCR analysis of RNA from the isolated populations of cells derived from neonatal calvaria showed higher NPY mRNA in the preosteocytes/osteocytes fraction compared to osteoblasts. NPY immunostaining confirmed the strong expression of NPY in osteocytes (DMP1GFP⁺), and lower levels in osteoblasts. In addition, the presence of NPY receptor Y1 mRNA was detected in cavaria and long bone, as well as in primary calvarial osteoblast cultures, whereas Y2 mRNA was restricted to the brain. Furthermore, NPY expression was reduced by 30–40% in primary calvarial cultures when subjected to fluid shear stress. In addition, treatment of mouse calvarial osteoblasts with exogenous NPY showed a reduction in the levels of intracellular cAMP and markers of osteoblast differentiation (osteocalcin, BSP, and DMP1). These results highlight the potential regulation of osteoblast lineage differentiation by local NPY signaling. J. Cell. Biochem. 108: 621–630, 2009. © 2009 Wiley‐Liss, Inc.