M. Douglas Benson

MAPK Pathways Activate and Phosphorylate the Osteoblast-specific Transcription Factor, Cbfa1

The bone-specific transcription factor, Cbfa1, regulates expression of the osteocalcin (OCN) gene and is essential for bone formation. However, little is known about the mechanisms regulating Cbfa1 activity. This work examines the role of the MAPK pathway in regulating Cbfa1-dependent transcription. Stimulation of MAPK by transfecting a constitutively active form of MEK1, MEK(SP), into MC3T3-E1 preosteoblast cells increased endogenous OCN mRNA, while a dominant negative mutant, MEK(DN), was inhibitory. MEK(SP) also stimulated activity of a 147-base pair minimal OCN promoter, and this stimulation required an intact copy of OSE2, the DNA binding site for Cbfa1. Effects of MEK(SP) were specific to Cbfa1-positive osteoblast-like cells. A purified His-tagged Cbfa1 fusion protein was directly phosphorylated by activated recombinant MAPK in vitro. Furthermore, 32P metabolic labeling studies demonstrated that MEK(SP) clearly enhanced phosphorylation of Cbfa1 in intact cells, while MEK(DN) decreased phosphorylation. The specific MEK1/MEK2 inhibitor, PD98059, inhibited extracellular matrix-dependent up-regulation of the OCN promoter, indicating that the MAPK pathway and, presumably, Cbfa1 phosphorylation are also required for responsiveness of osteoblasts to extracellular matrix signals. This study is the first demonstration that Cbfa1 is controlled by MAPKs and suggests that this pathway has an important role in the control of osteoblast-specific gene expression.

Bone morphogenetic proteins, extracellular matrix, and mitogen-activated protein kinase signaling pathways are required for osteoblast-specific gene expression and differentiation in MC3T3-E1 cells

Osteoblasts secrete a complex extracellular matrix (ECM) containing collagenous and noncollagenous proteins, bone morphogenetic proteins (BMPs), and growth factors. Osteoblast‐specific gene expression requires ascorbic acid (AA)‐dependent assembly of a collagenous ECM. Matrix responsiveness requires an α2β1 integrin‐collagen interaction and mitogen‐activated protein kinase (MAPK) activity, which phosphorylates and activates the osteoblast‐specific transcription factor Cbfa1. This study examines interactions between this integrin/MAPK‐mediated pathway and signals initiated by BMPs contained in the osteoblast matrix. MC3T3‐E1 cells were shown to constitutively express BMP‐2, BMP‐4, and BMP‐7. Noggin, a specific BMP inhibitor, reversibly blocked AA‐induced gene expression, indicating that BMP production by MC3T3‐E1 cells was necessary for differentiation. The ability of exogenously added BMP‐2, BMP‐4, or BMP‐7 to stimulate osteocalcin (OCN) and bone sialoprotein (BSP) mRNAs or OCN promoter activity was synergistically increased in cells that were actively synthesizing an ECM (i.e., were grown in the presence of AA). A minimum of 4 days of ECM accumulation was required for this synergistic response to be observed. Neither BMP‐7, AA, nor a combination of these two treatments had major effects on Cbfa1 messenger RNA (mRNA) or protein levels, as would be expected if regulation was mainly at the posttranscriptional level. U0126, a specific inhibitor of MAPK/extracellular signal‐regulated kinase (MEK), blocked AA‐ or BMP‐7/AA‐dependent gene expression in a time‐ and dose‐dependent manner that was closely correlated with inhibition of extracellular signal‐regulated kinase (ERK) phosphorylation. This work establishes that autocrine BMP production as well as integrin‐mediated cell‐collagen interactions are both required for osteoblast differentiation, and both these pathways require MAP kinase activity.

Cloning of a 2.5 kb Murine Bone Sialoprotein Promoter Fragment and Functional Analysis of Putative Osf2 Binding Sites

Bone sialoprotein (BSP) is an extracellular matrix protein that is intimately associated with the process of biomineralization. Osf2, a member of the Cbf/runt family of transcription factors, is required for the development of osteoblasts in vivo and has been reported to stimulate the transcription of BSP when overexpressed in mesenchymal cell lines. To investigate the role of Osf2 in BSP expression, we cloned a 2.5 kb fragment of a 5′ untranscribed sequence from the murine BSP gene and evaluated it for putative Osf2 binding sites. This promoter, which was able to direct 5‐ to 10‐fold higher levels of luciferase reporter expression in osteoblastic cells than in nonbone cell lines, contains two consensus core binding sites for members of the Cbf/runt family. One, at –61 relative to the start of transcription, is within a region having 75% overall sequence identity with the rat and human BSP promoters. The other is located at −1335, outside this highly conserved region. Neither site is completely conserved in the rat or human sequences. Only the −1335 site was able to bind a protein in nuclear extracts of osteoblastic cells, and this protein was identified as Osf2. Despite this in vitro binding ability, we detected no significant enhancer activity in the −1335 element when placed in front of a minimal osteocalcin promoter driving a luciferase reporter gene in osteoblastic cells nor any loss in transcriptional activity of a 5′ promoter deletion which eliminated this element as compared with the full‐length 2.5 kb promoter. These results suggest that Osf2 binding to the BSP promoter is not essential for its osteoblast‐selective expression.

NF1 tumor suppressor protein and mRNA in skeletal tissues of developing and adult normal mouse and NF1-deficient embryos

NF1 is a heritable disease with multiple osseous lesions. The expression of the NF1 gene was studied in embryonic and adult rodent skeleton and in NF1‐deficient embryos. The NF1 gene was expressed intensely in the cartilage and the periosteum. Impaired NF1 expression may lead to inappropriate development and dynamics of bones and ultimately to the osseous manifestations of the disease.

Ephrin reverse signaling controls palate fusion via a PI3 kinase-dependent mechanism.

Secondary palate fusion requires adhesion and epithelial‐to‐mesenchymal transition (EMT) of the epithelial layers on opposing palatal shelves. This EMT requires transforming growth factor β3 (TGFβ3), and its failure results in cleft palate. Ephrins, and their receptors, the Ephs, are responsible for migration, adhesion, and midline closure events throughout development. Ephrins can also act as signal‐transducing receptors in these processes, with the Ephs serving as ligands (termed “reverse” signaling). We found that activation of ephrin reverse signaling in chicken palates induced fusion in the absence of TGFβ3, and that PI3K inhibition abrogated this effect. Further, blockage of reverse signaling inhibited TGFβ3‐induced fusion in the chicken and natural fusion in the mouse. Thus, ephrin reverse signaling is necessary and sufficient to induce palate fusion independent of TGFβ3. These data describe both a novel role for ephrins in palate morphogenesis, and a previously unknown mechanism of ephrin signaling. Developmental Dynamics 240:357–364, 2011.

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.

A Homeodomain Protein Binding Element in the Bone Sialoprotein Promoter is Critical for Tissue-Specific Expression in Bone

Bone sialoprotein (BSP) expression is restricted to cells associated with the mineralization of bones and teeth. We previously identified a homeodomain binding element in a 2.5 kb fragment of the murine Bsp promoter that is required for osteoblast-selective expression in cell culture. To examine the role of this element (called OSHE1; osteoblast-specific homeodomain element 1) in the tissue-specific expression of Bsp in vivo, we generated transgenic mice using the wild-type 2472 bp promoter or the same promoter containing a 2 bp mutation in OSHE1. Promoter constructs driving both luciferase and lacZ reporter genes were microinjected into fertilized eggs from (C57BL/6 X SJL)F1 mice. Four lines containing the wild-type promoter and 5 lines containing the mutated promoter were established, and the tissue specificity of g -galactosidase staining and luciferase expression was examined. g -gal staining was observed in osteoblasts of calvaria and trabecular regions of tibia and femur in 12-day-old mice while chondrocytes, kidney, heart, muscle, spleen, liver, skin, stomach, and lung were negative. Whole tissue luciferase activity was also much higher in mineralized tissues although some soft tissue expression was detected. In contrast, analysis of OSHE1 mutant lines revealed expression of luciferase and g -gal in kidney, skin, liver, and lung. g -gal expression in these tissues was restricted to specific cell populations. Trabecular regions were devoid of g -gal staining in the tibia and femur of the mutant mice, while staining was seen in the chondrocytes. We therefore hypothesize that the OSHE1 site is involved in both the expression of Bsp in mineralizing tissues and the suppression of transcription in nonmineralizing tissues.

Ephrin-B stimulation of calvarial bone formation

Introduction: Ephrin‐B2 on osteoclasts was reported to promote bone formation as part of homeostasis by activating the EphB4 tyrosine kinase receptor on osteoblasts. Little is known about the role of ephrin‐B signaling to EphBs in developmental bone formation. Results: We observed expression of an ephrin‐B2 LacZ chimeric allele in the periosteum, sutural bone fronts, and dura mater of embryonic and neonatal mice. Expression in the adult skull was confined to sutures, but was heavily upregulated at sites of bone injury. Culture of embryonic calvariae with soluble recombinant ephrin‐B2/Fc doubled their bone content without altering suture width or overall skull morphology. Ephrin‐B2/Fc also stimulated osteoblast marker gene expression in cultured MC3T3 preosteoblastic cells without the need for type 1 collagen‐induced differentiation. EphB4 was absent in embryonic and adult skulls. However, EphB1 and EphB2, both physiological receptors for ephrin‐Bs, were expressed at sites of osteogenesis, and EphB1 knockout mice displayed a reduction in calvarial bone content compared to controls. Conclusions: These data support a role for ephrin‐B2 in the development and healing of bone through activation of osteoblast‐specific gene expression. EphB1 and EphB2 are likely candidates receptors for the ephrin‐B2 in bone. Developmental Dynamics, 2012.

Ephrin reverse signaling mediates palatal fusion and epithelial-to-mesenchymal transition independently of tgfß3

The mammalian secondary palate forms from shelves of epithelia‐covered mesenchyme that meet at midline and fuse. The midline epithelial seam (MES) is thought to degrade by apoptosis, epithelial‐to‐mesenchymal transition (EMT), or both. Failure to degrade the MES blocks fusion and causes cleft palate. It was previously thought that transforming growth factor ß3 (Tgfß3) is required to initiate fusion. Members of the Eph tyrosine kinase receptor family and their membrane‐bound ephrin ligands are expressed on the MES. We demonstrated that treatment of mouse palates with recombinant EphB2/Fc to activate ephrin reverse signaling (where the ephrin acts as a receptor and transduces signals from its cytodomain) was sufficient to cause mouse palatal fusion when Tgfß3 signaling was blocked by an antibody against Tgfß3 or by an inhibitor of the TgfßrI serine/threonine receptor kinase. Cultured palatal epithelial cells traded their expression of epithelial cell markers for that of mesenchymal cells and became motile after treatment with EphB2/Fc. They concurrently increased their expression of the EMT‐associated transcription factors Snail, Sip1, and Twist1. EphB2/Fc did not cause apoptosis in these cells. These data reveal that ephrin reverse signaling directs palatal fusion in mammals through a mechanism that involves EMT but not apoptosis and activates a gene expression program not previously associated with ephrin reverse signaling. J. Cell. Physiol. 9999: 1–12, 2015.

The role of bone morphogenetic proteins 2 and 4 in mouse dentinogenesis

OBJECTIVE The bone morphogenetic proteins (BMPs) play crucial roles in tooth development. However, several BMPs retain expression in the dentin of the fully patterned and differentiated tooth. We hypothesized that BMP signaling therefore plays a role in the function of the differentiated odontoblast, the job of which is to lay down and mineralize the dentin matrix. DESIGN We generated mice deficient in Bmp2 and 4 using a dentin matrix protein 1 (Dmp1) promoter-driven cre recombinase that was expressed in differentiated odontoblasts. RESULTS The first and second molars of these Bmp2 and Bmp4 double conditional knockout (DcKO) mice displayed reduced dentin and enlarged pulp chambers compared to cre-negative littermate controls. DcKO mouse dentin in first molars was characterized by small, disorganized dentinal fibers, a wider predentin layer, and reduced expression of dentin sialophosphoprotein (DSPP), dentin matrix protein 1 (DMP1), and bone sialoprotein (BSP). DcKO mouse odontoblasts demonstrated increased type I collagen mRNA production, indicating that the loss of BMP signaling altered the rate of collagen gene expression in these cells. Bmp2 and Bmp4 single Dmp1-cre knockout mice displayed no discernable dentin phenotype. CONCLUSIONS These data demonstrate that BMP signaling in differentiated odontoblasts is necessary for proper dentin production in mature teeth.