Cbfa1 in bone development

Physical stress by magnetic force accelerates differentiation of human osteoblasts

We examined the effect of magnetic force on differentiation of cultured human osteoblasts. Magnetic microparticles (MPs) were introduced into the cytoplasm of a human osteoblast cell line and the cells were cultured in a magnetic field (MF) in group MP-MF. Three groups of controls were used: cells without MPs were cultured out of MF (group C), cells without MPs were cultured in MF (group MF), and cells with MPs were cultured out of MF (group MP). The cells in group MP-MF became larger and were elongated along the axis of the magnetic poles. Appearance of alkaline phosphatase (AlPase) activity, formation of bone nodules, and calcium deposition were accelerated depending on the intensity of the magnetic field. It takes longer culture in the other three groups to exhibit these changes. Core-binding factor A1 (Cbfa1: transcription factor for osteoblast differentiation) and osteocalcin (a bone-matrix protein involved in controlling osteogenesis) were expressed earlier or stronger in group MP-MF than the other groups. Then we compared phosphorylation of mitogen-activated protein kinase (MAPK) between group MP-MF and group C. Phosphorylation of p38(MAPK) (p38) was increased in group MP-MF, while total p38 as well as total and phosphorylated forms of MAPK/ERK 1/2 and SAPK/JNK were not changed between the two groups. When a p38 inhibitor, SB 203580, was added to the culture medium in group C, AlPase activity, formation of bone nodules, and calcium deposits were completely inhibited. On the other hand, they were inhibited only partially by a MAPK/ERK 1/2 inhibitor, U-0126. Based on these results, it is concluded that (1) osteoblast differentiation is accelerated by a magnetic force, (2) this acceleration is mainly attributed to the activation of p38 phosphorylation, and (3) the stimulus induced by a magnetic field offers a new approach to osteoblast differentiation.

Transcriptional regulation of the human Runx2/Cbfa1 gene promoter by bone morphogenetic protein-7

It is well established that core binding factor Runx2/Cbfa1 is required for osteoblast recruitment and differentiation from mesenchymal stem cells. Transcriptional regulation of the Runx2/Cbfa1 gene by osteogenic factors such as bone morphogenetic proteins (BMPs) plays an important role in the stimulation of bone formation by these cytokines. BMP7 (also termed OP-1) is a member of the transforming growth factor beta (TGF-beta) superfamily and induces osteoblast differentiation from mesenchymal precursor stem cells in vitro as well as bone formation in vivo. This study examines the effects of BMP7 on markers of osteoblast differentiation and specifically on human Runx2/Cbfa1 gene transcription in a mouse C2C12 myoblast cell line where it induces expression of both alkaline phosphatase (ALP) and endogenous Runx2/Cbfa1. To further understand the mechanisms of human Runx2/Cbfa1 transcriptional regulation by BMP7, we cloned 3.0 kb of the human Runx2/Cbfa1 gene 5'-upstream flanking region and created a series of promoter deletions cloned into luciferase-based reporter vectors (Runx2/Cbfa1/Luc). Sequence data revealed six copies of the osteoblastic cis-acting element (OSE2) in the proximal promoter region. In C2C12 cells transiently transfected with Runx2/Cbfa1/Luc deletion constructs, transcriptional activity of Runx2/Cbfa1 was upregulated up to 2-fold after 24 h of BMP7 treatment. Mutational analysis demonstrated that the minimal responsive promoter region for BMP7-regulated transcription maps to a proximal -74 OSE2 site. Electromobility shift assays with C2C12 cellular extracts indicate that BMP7 increases binding of OSE2 promoter sequences, and supershift assays with anti-Runx2/Cbfa1 antibodies demonstrate that Runx2/Cbfa1 is part of the nucleoprotein complex binding OSE2. Together, these data indicate BMP7 can upregulate Runx2/Cbfa1 gene expression in C2C12 myoblast cells, and suggest that Runx2/Cbfa1 may bind to OSE2 elements within its own promoter to autoregulate gene transcription in differentiating osteoblasts.

Induction of osteoclast differentiation by Runx2 through receptor activator of nuclear factor-kappa B ligand (RANKL) and osteoprotegerin regulation and partial rescue of osteoclastogenesis in Runx2-/- mice by RANKL transgene

Receptor activator of nuclear factor-kappaB ligand (RANKL), osteoprotegerin (OPG), and macrophage-colony stimulating factor play essential roles in the regulation of osteoclastogenesis. Runx2-deficient (Runx2-/-) mice showed a complete lack of bone formation because of maturational arrest of osteoblasts and disturbed chondrocyte maturation. Further, osteoclasts were absent in these mice, in which OPG and macrophage-colony stimulating factor were normally expressed, but RANKL expression was severely diminished. We investigated the function of Runx2 in osteoclast differentiation. A Runx2-/- calvaria-derived cell line (CA120-4), which expressed OPG strongly but RANKL barely, severely suppressed osteoclast differentiation from normal bone marrow cells in co-cultures. Adenoviral introduction of Runx2 into CA120-4 cells induced RANKL expression, suppressed OPG expression, and restored osteoclast differentiation from normal bone marrow cells, whereas the addition of OPG abolished the osteoclast differentiation induced by Runx2. Addition of soluble RANKL (sRANKL) also restored osteoclast differentiation in co-cultures. Forced expression of sRANKL in Runx2-/- livers increased the number and size of osteoclast-like cells around calcified cartilage, although vascular invasion into the cartilage was superficial because of incomplete osteoclast differentiation. These findings indicate that Runx2 promotes osteoclast differentiation by inducing RANKL and inhibiting OPG. As the introduction of sRANKL was insufficient for osteoclast differentiation in Runx2-/- mice, however, our findings also suggest that additional factor(s) or matrix protein(s), which are induced in terminally differentiated chondrocytes or osteoblasts by Runx2, are required for osteoclastogenesis in early skeletal development.

Runx2, a multifunctional transcription factor in skeletal development

The identification of Runx2 (runt-related protein 2) function has greatly advanced the understanding of skeletal development over the last 5 years. Runx2 is regulated transcriptionally and post-translationally through the activity of many identified factors, although, the physiological significance of each remains to be demonstrated. The interaction of Runx2 with other transcription factors and cofactors has been shown to be important in Runx2-dependent gene regulation. Runx2 plays important roles in multiple steps of skeletal development. Runx2 determines the lineage of osteoblasts from multipotent mesenchymal cells, enhances osteoblast differentiation at an early stage, and inhibits osteoblast differentiation at a late stage. Runx2 plays crucial roles in chondrocyte maturation and in the specification of cartilage phenotypes. Furthermore, Runx2 is involved in vascular invasion into cartilage and osteoclastogenesis. Therefore, the determination of Runx2 function and the investigation of the cascades of Runx2-dependent gene regulation are important in the elucidation of skeletal biology.

Runx2 expression and action in chondrocytes are regulated by retinoid signaling and parathyroid hormone-related peptide (PTHrP)

OBJECTIVE

Runx2 (also known as Cbfa1) is a transcription factor required for chondrocyte maturation and osteoblast differentiation. While there is information on the regulation of its expression during osteogenesis, much less is known about it during cartilage maturation. Here we asked whether Runx2 expression and function are affected by retinoic acid (RA) and parathyroid hormone-related peptide (PTHrP), which represent an important stimulator and inhibitor of chondrocyte maturation, respectively.

DESIGN

We first cloned and characterized Runx2 expressed by chick chondrocytes (cRunx2). We then constructed expression vectors of cRunx2 and a dominant-negative form (DN-cRunx2) and determined their effects on chondrocyte maturation in culture before and during retinoid and PTHrP treatment.

RESULTS

cRunx2 showed similar transactivation activity to that of its mammalian counterparts although it has a very short QA domain and lacks a small portion of the PST domain. cRunx2 over-expression stimulated chondrocyte maturation, as indicated by increases in alkaline phosphatase activity (APase), mineralization, and type X collagen and MMP-13 expression, and by maintenance of Indian hedgehog (Ihh) expression. RA treatment stimulated cRunx2 gene expression and boosted its pro-maturation effects. PTHrP treatment blocked Runx2 expression and its pro-maturation effects. Over-expression of DN-cRunx2 inhibited maturation and even prevented RA from exerting its pro-maturation role.

CONCLUSIONS

As previously indicated by mammalian studies, cRunx2 has chondrocyte pro-maturation activity. Its expression and roles are favorably modulated by retinoid signaling but are completely inhibited by PTHrP. A model integrating cRunx2 with PTHrP, Ihh and retinoid signaling and operating during skeletogenesis is proposed.

Differentiation plasticity of chondrocytes derived from mouse embryonic stem cells

Evidence exists that cells of mesenchymal origin show a differentiation plasticity that depends on their differentiation state. We used in vitro differentiation of embryonic stem cells through embryoid bodies as a model to analyze chondrogenic and osteogenic differentiation because embryonic stem cells recapitulate early embryonic developmental phases during in vitro differentiation. Here, we show that embryonic stem cells differentiate into chondrocytes, which progressively develop into hypertrophic and calcifying cells. At a terminal differentiation stage, cells expressing an osteoblast-like phenotype appeared either by transdifferentiation from hypertrophic chondrocytes or directly from osteoblast precursor cells. Chondrocytes isolated from embryoid bodies initially dedifferentiated in culture but later re-expressed characteristics of mature chondrocytes. The process of redifferentiation was completely inhibited by transforming growth factor beta3. In clonal cultures of chondrocytes isolated from embryoid bodies, additional mesenchymal cell types expressing adipogenic properties were observed, which suggests that the subcultured chondrocytes indeed exhibit a certain differentiation plasticity. The clonal analysis confirmed that the chondrogenic cells change their developmental fate at least into the adipogenic lineage. In conclusion, we show that chondrocytic cells are able to transdifferentiate into other mesenchymal cells such as osteogenic and adipogenic cell types. These findings further strengthen the view that standardized selection strategies will be necessary to obtain defined cell populations for therapeutic applications.

DNA recognition by the RUNX1 transcription factor is mediated by an allosteric transition in the RUNT domain and by DNA bending

The Runt domain proteins are transcription regulators of major developmental pathways. Here we present the crystal structures of the Runt domain (RD) of the human protein RUNX1 and its DNA binding site in their free states and compare them with the published crystal structures of RD bound to DNA and to the partner protein CBFbeta. We demonstrate that (1) RD undergoes an allosteric transition upon DNA binding, which is further stabilized by CBFbeta, and that (2) the free DNA target adopts a bent-helical conformation compatible with that of the complex. These findings elucidate the mechanism by which CBFbeta enhances RD binding to DNA as well as the role of the intrinsic conformation of the DNA target in the recognition process.

Core-binding factor alpha 1 (Cbfa1) induces osteoblastic differentiation of C2C12 cells without interactions with Smad1 and Smad5

Core-binding factor alpha(1) (Cbfa1) is an essential transcription factor for osteoblastic differentiation and osteogenesis. Bone morphogenetic protein (BMP) is also a powerful inducer of differentiation of pluripotent mesenchymal cells to osteoblast lineage and bone formation. Recent studies suggest that Cbfa1 plays a critical role during BMP-induced osteoblastic differentiation through association with cytoplasmic BMP signaling molecules, Smads. However, other studies have suggested that Cbfa1 may exhibit its osteogenic function without interaction with Smads. Therefore, it remains unclear whether association with Smad is essential for Cbfa1 function. In this study we examine the effects of Cbfa1 on osteoblastic differentiation in the presence or absence of interactions with Smad1 or Smad5 using C2C12 undifferentiated mesenchymal cells. Cbfa1 expression was induced upon stimulation with BMP-2 in C2C12 cells. Introduction of Cbfa1 into C2C12 cells induced osteoblastic differentiation and promoted transactivation of osteocalcin gene promoter without forming the complex with Smad1 or Smad5. Furthermore, in C2C12 cells in which the association of Cbfa1 with Smad1/Smad5 was prevented by the overexpression of the natural antagonist, Smad6, Cbfa1 still induced osteoblastic differentiation and transactivated osteocalcin gene promoter, regardless of BMP-2 stimulation. These results suggest that the interactions with Smad1 or Smad5 are not essential for Cbfa1 to demonstrate its osteogenic actions. However, interactions with Smad1/Smad5 enhance these osteogenic actions of Cbfa1. Of note, BMP-2-induced or Smad-induced osteoblastic differentiation was inhibited by dominant-negative Cbfa1, suggesting that the function of Cbfa1 is critical for BMP-2-induced osteoblastic differentiation. Our results suggest that Cbfa1 is essential and also sufficient to induce osteoblastic differentiation in undifferentiated mesenchymal cells, and establishment of an association with Smad1/Smad5 enhances the osteogenic actions of Cbfa1. On the other hand, Cbfa1 expression requires the activation of Smad1/Smad5 by BMP-2.

The Runx3 transcription factor regulates development and survival of TrkC dorsal root ganglia neurons

The RUNX transcription factors are important regulators of linage-specific gene expression in major developmental pathways. Recently, we demonstrated that Runx3 is highly expressed in developing cranial and dorsal root ganglia (DRGs). Here we report that within the DRGs, Runx3 is specifically expressed in a subset of neurons, the tyrosine kinase receptor C (TrkC) proprioceptive neurons. We show that Runx3-deficient mice develop severe limb ataxia due to disruption of monosynaptic connectivity between intra spinal afferents and motoneurons. We demonstrate that the underlying cause of the defect is a loss of DRG proprioceptive neurons, reflected by a decreased number of TrkC-, parvalbumin- and beta-galactosidase-positive cells. Thus, Runx3 is a neurogenic TrkC neuron-specific transcription factor. In its absence, TrkC neurons in the DRG do not survive long enough to extend their axons toward target cells, resulting in lack of connectivity and ataxia. The data provide new genetic insights into the neurogenesis of DRGs and may help elucidate the molecular mechanisms underlying somatosensory-related ataxia in humans.

Impaired vascular invasion of Cbfa1-deficient cartilage engrafted in the spleen

Chondrocyte maturation and vascular invasion of cartilage are essential in the process of endochondral ossification. Cbfal-deficient (Cbfa1-/-) mice displayed a complete absence of osteoblast and osteoclast maturation as well as severely inhibited chondrocyte maturation in most parts of the skeleton. Although chondrocyte maturation and mineralization were observed in restricted areas of Cbfa1-/- mouse skeleton, vascular invasion of calcified cartilage was never noted. To investigate the possibility of chondrocyte maturation and vascular invasion in Cbfal-/- cartilage and the role of the hematopoietic system in the process of vascular invasion, we transplanted embryonic day 18.5 (E18.5) Cbfa1-/- femurs, which are composed of immature chondrocytes, into spleens of normal mice. One week later, the transplanted femurs contained terminally differentiated chondrocytes expressing osteopontin, bone sialoprotein (BSP), and matrix metalloproteinase (MMP) 13. In the diaphyses of the transplants, the cartilage matrix was mineralized and the cartilage was invaded by vascular vessels and osteoclasts. However, chondrocyte maturation and vascular invasion were severely retarded in comparison with transplants of E14.5 wild-type femurs, in which the cartilage was rapidly replaced by bone, and neither mature osteoblasts nor bone formation were observed. In primary culture of Cbfa1-/- chondrocytes, transforming growth factor (TGF) beta1, platelet-derived growth factor (PDGF), interleukin (IL)-1beta, and thyroid hormone (T3) induced osteopontin and MMP-13 expression. These findings indicated that factors in the hematopoietic system are able to support vascular invasion of cartilage independent of Cbfal but are less effective without it, suggesting that Cbfal functions in cooperation with factors from bone marrow in the process of growth plate vascularization.

Mutations of the AML1 gene in acute myeloid leukemia of FAB types M0 and M7

The AML1 gene encodes a transcription factor that, together with its heterodimeric partner CBFB, regulates a number of target genes that are essential for normal hemopoiesis. In acute myeloid leukemia (AML), AML1 is disrupted not only by chromosomal translocations but also by mutations in the runt domain, which binds both DNA and CBFB. Acquired mutations have been described predominantly in the AML FAB type M0. To date, most patients appear to have biallelic disease, suggesting a complete lack of normal AML1 function. Inherited loss of function mutations thought to lead to haploinsufficiency also have been described in patients who have a familial disorder with predisposition to AML (FPD/AML), indicating the role of AML1 in megakaryopoiesis. Using single-strand conformation polymorphism analysis, we studied the AML1 runt domain in 41 patients with M0 AML and identified potentially pathologic mutations in five (12%). Biallelic disease could be confirmed in only one patient, using loss of heterozygosity studies. At least three of the mutations would lead to truncated proteins similar to those reported in FPD/AML, suggesting that haploinsufficiency plays a role in the pathogenesis of this minimally differentiated type of leukemia. The incidence of acquired mutations in AML patients with acute megakaryoblastic leukemia (FAB type M7) was the same as that reported in other non-M0 patients, with only one mutation detected in 20 (5%) patients studied.

The T-box transcription factor Brachyury mediates cartilage development in mesenchymal stem cell line C3H10T1/2

The BMP2-dependent onset of osteo/chondrogenic differentiation in the acknowledged pluripotent murine mesenchymal stem cell line (C3H10T1/2) is accompanied by the immediate upregulation of Fibroblast Growth Factor Receptor 3 (FGFR3) and a delayed response by FGFR2. Forced expression of FGFR3 in C3H10T1/2 is sufficient for chondrogenic differentiation, indicating an important role for FGF-signaling during the manifestation of the chondrogenic lineage in this cell line. Screening for transcription factors exhibiting a chondrogenic capacity in C3H10T1/2 indentified that the T-box containing transcription factor Brachyury is upregulated by FGFR3-mediated signaling. Forced expression of Brachyury in C3H10T1/2 was sufficient for differentiation into the chondrogenic lineage in vitro and in vivo after transplantation into muscle. A dominant-negative variant of Brachyury, consisting of its DNA-binding domain (T-box), interferes with BMP2-mediated cartilage formation. These studies indicate that BMP-initiated FGF-signaling induces a novel type of transcription factor for the onset of chondrogenesis in a mesenchymal stem cell line. A potential role for this T-box factor in skeletogenesis is further delineated from its expression profile in various skeletal elements such as intervertebral disks and the limb bud at late stages (18.5 d.p.c.) of murine embryonic development.

Expression of the osteoblast differentiation factor RUNX2 (Cbfa1/AML3/Pebp2alpha A) is inhibited by tumor necrosis factor-alpha

The transcription factor RUNX2 (Cbfa1/AML3/Pebp2alphaA) is a critical regulator of osteoblast differentiation. We investigated the effect of the inflammatory cytokine tumor necrosis factor alpha (TNF) on the expression of RUNX2 because TNF is known to inhibit differentiation of osteoblasts from pluripotent progenitor cells. TNF treatment of fetal calvaria precursor cells or MC3T3-E1 clonal pre-osteoblastic cells caused a dose-dependent suppression of RUNX2 steady state mRNA as measured by reverse transcription-PCR. The IC(50) for TNF inhibition was 0.6 ng/ml. TNF suppression of RUNX2 mRNA was confirmed using Northern analysis. The effect of TNF was studied using isoform-specific primers that flanked unique regions of two major RUNX2 isoforms. TNF suppressed expression of the mRNA coding for the shorter MRIPV isoform by >90% while inhibiting expression of the mRNA for the longer MASNS isoform by 50%. RUNX2 nuclear content was evaluated by electrophoretic mobility shift assay using a rat osteocalcin promoter binding sequence as probe and by Western analysis. TNF reduced nuclear RUNX2 protein. Inhibition of new protein synthesis with cycloheximide failed to prevent TNF inhibition of RUNX2 mRNA, suggesting that a newly translated protein did not mediate the TNF effect. RUNX2 mRNA half-life was 1.8 h and reduced to 0.9 h by TNF. The effect of TNF on RUNX2 gene transcription was evaluated using a 0.6-kb RUNX2 promoter-luciferase reporter in MC3T3-E1 cells. TNF caused a dose-dependent inhibition of transcription to 50% of control values. The inhibitory effect of TNF was preserved with deletions to nucleotide -108 upstream of the translational start site; however, localization downstream of nucleotide -108 was obscured by loss of basal activity. Our results indicate that TNF regulates RUNX2 expression at multiple levels including destabilization of mRNA and suppression of transcription. The disproportionate inhibition of RUNX2 nuclear protein suggests that additional post-transcriptional mechanisms may be occurring. Suppression of RUNX2 by TNF may decrease osteoblast differentiation and inhibit bone formation in TNF excess states.

Participation of Cbfa1 in regulation of chondrocyte maturation

OBJECTIVE

Cbfa1 is a transcription factor, which is classified into the runt family. The mice lacking this gene display complete loss of bone formation, indicating that Cbfa1 is an essential factor for osteoblast differentiation. The Cbfa1-deficient mice also show an abnormality in cartilage development. Although cartilage anlagens are well formed in these mice, endochondral ossification is blocked, and most of chondrocytes fail to differentiate into their maturation form as characterized by the absence of type X collagen and low levels of alkaline phosphatase activity. It is suggested that Cbfa1 may participate in chondrocyte differentiation. In this study, we have investigated the role of Cbfa1 in chondrocytes during their cytodifferentiation in vitro.

DESIGN

To investigate the role of Cbfa1 in regulation of chondrocyte differentiation, we over-expressed Cbfa1 or its dominant negative form in cultured chick chondrocytes using a retrovirus (RCAS)system and examined changes in chondrocyte behaviour induced by the introduced genes.

RESULTS

Mature chondrocytes isolated form the cephalic portion of sterna seemed to express Cbfa1 more prominently than immature chondrocytes isolated from the one-third caudal portion of sterna. Over-expression of Cbfa1 in immature chondrocytes strongly stimulated alkaline phosphatase activity and matrix calcification. In contrast, expression of a dominant negative form of Cbfa1, which lacks the C-terminal PST domain, severely inhibited alkaline phosphatase activity and matrix calcification in mature chondrocytes.

CONCLUSION

Taken together with the observation that Cbfa1 transcripts dominantly localized in hypertrophic chondrocytes as well as in osteoblasts, it is suggested that Cbfa1 plays an important role in the progression of chondrocyte maturation.

Spatial and temporal expression pattern of Runx3 (Aml2) and Runx1 (Aml1) indicates non-redundant functions during mouse embryogenesis

The human RUNX3/AML2 gene belongs to the 'runt domain' family of transcription factors that act as gene expression regulators in major developmental pathways. Here, we describe the expression pattern of Runx3 during mouse embryogenesis compared to the expression pattern of Runx1. E10.5 and E14.5-E16.5 embryos were analyzed using both immunohistochemistry and beta-galactosidase activity of targeted Runx3 and Runx1 loci. We found that Runx3 expression overlapped with that of Runx1 in the hematopoietic system, whereas in sensory ganglia, epidermal appendages, and developing skeletal elements, their expression was confined to different compartments. These data provide new insights into the function of Runx3 and Runx1 in organogenesis and support the possibility that cross-regulation between them plays a role in embryogenesis.

Identification of novel CBFA1/RUNX2 mutations causing cleidocranial dysplasia

Core binding factor A1 (CBFA1/RUNX2) is a runt-like transcription factor essential for osteoblast differentiation. Haplotype insufficiency causes cleidocranial dysplasia (CCD), a syndrome featuring supernumerary tooth buds, delayed tooth eruption, patent fontanels, Wormian bones, short stature, dysplasia of the clavicles, growth retardation and hypoplasia of the distal phalanges. We identified novel CBFAI/RUNX2 mutations after PCR and direct sequencing of patient leukocyte DNA. In family 1 mother and son are affected by CCD. Both carry the missense mutation R190W (CGG > TGG). This nucleotide change introduced a BsmI restriction site, which was used to independently confirm the mutation. It was absent in healthy members of the family. Family 2, in which father and daughter are affected by CCD, shows a deletion of nucleotide C821. This deletion causes a frameshift mutation with premature stop after the insertion of 18 aberrant amino acids. Healthy family members did not have this mutation. The clavicular dysplasia was more pronounced with the R19OW mutation, while the bone density was markedly reduced in individuals with either mutation, suggesting a previously underemphasized increased risk for osteoporosis in CCD.

Regulation of human cbfa1 gene transcription in osteoblasts by selective estrogen receptor modulators (SERMs)

Cbfa1, a transcription factor critical for bone formation, plays a key role in osteoblast recruitment and differentiation. We have cloned and characterized a 3.0 KB 5'-flanking region of the human cbfa1 gene isolated from a P1 human genomic library. DNA sequencing revealed several known canonical nuclear transcription factor binding sites, including two AP1 and six OSE2 binding sites in the proximal promoter region. Although no estrogen (E2)-response element (ERE) binding sites were identified, E2 has been shown to regulate gene activity via AP1 promoter sites. We examined the effect of selective estrogen receptor modulators (SERMs) on human cbfa1 gene promoter activity using cell-based luciferase reporter transcriptional assays. Three characterized SERMs, tamoxifen, raloxifene, and ICI 178,180, all upregulated cbfa1-luciferase (cbfa1Luc) gene activity 5- to 10-fold in a dose-dependent manner. This effect was mediated by both ER alpha and ER beta. Mutational analysis demonstrated that the minimal promoter region for basal of SERM-activated transcription was mapped to adjacent AP1-like and OSE2 binding sites within -93 and +7 of the transcription start condon. Further, electrophoretic mobility shift assays (EMSAs) demonstrate that ICI 178,180 increased binding of AP1 and OSE2 site by ER alpha and cbfa1, respectively. These studies suggest that SERMs can modulate bone-specific cbfa1 gene expression in a human osteosarcoma cell line.

Overexpression of Cbfa1 in osteoblasts inhibits osteoblast maturation and causes osteopenia with multiple fractures

Targeted disruption of core binding factor alpha1 (Cbfa1) showed that Cbfa1 is an essential transcription factor in osteoblast differentiation and bone formation. Furthermore, both in vitro and in vivo studies showed that Cbfa1 plays important roles in matrix production and mineralization. However, it remains to be clarified how Cbfa1 controls osteoblast differentiation, bone formation, and bone remodelling. To understand fully the physiological functions of Cbfa1, we generated transgenic mice that overexpressed Cbfa1 in osteoblasts using type I collagen promoter. Unexpectedly, Cbfa1 transgenic mice showed osteopenia with multiple fractures. Cortical bone, which was thin, porous, and enriched with osteopontin, was invaded by osteoclasts, despite the absence of acceleration of osteoclastogenesis. Although the number of neonatal osteoblasts was increased, their function was impaired in matrix production and mineralization. Furthermore, terminally differentiated osteoblasts, which strongly express osteocalcin, and osteocytes were diminished greatly, whereas less mature osteoblasts expressing osteopontin accumulated in adult bone. These data indicate that immature organization of cortical bone, which was caused by the maturational blockage of osteoblasts, led to osteopenia and fragility in transgenic mice, demonstrating that Cbfa1 inhibits osteoblast differentiation at a late stage.

Structural analyses of DNA recognition by the AML1/Runx-1 Runt domain and its allosteric control by CBFbeta

The core binding factor (CBF) heterodimeric transcription factors comprised of AML/CBFA/PEBP2alpha/Runx and CBFbeta/PEBP2beta subunits are essential for differentiation of hematopoietic and bone cells, and their mutation is intimately related to the development of acute leukemias and cleidocranial dysplasia. Here, we present the crystal structures of the AML1/Runx-1/CBFalpha(Runt domain)-CBFbeta(core domain)-C/EBPbeta(bZip)-DNA, AML1/Runx-1/CBFalpha(Runt domain)-C/EBPbeta(bZip)-DNA, and AML1/Runx-1/CBFalpha(Runt domain)-DNA complexes. The hydrogen bonding network formed among CBFalpha(Runt domain) and CBFbeta, and CBFalpha(Runt domain) and DNA revealed the allosteric regulation mechanism of CBFalpha(Runt domain)-DNA binding by CBFbeta. The point mutations of CBFalpha related to the aforementioned diseases were also mapped and their effect on DNA binding is discussed.

Negative regulation of BMP/Smad signaling by Tob in osteoblasts

Bone morphogenetic protein (BMP) controls osteoblast proliferation and differentiation through Smad proteins. Here we show that Tob, a member of the emerging family of antiproliferative proteins, is a negative regulator of BMP/Smad signaling in osteoblasts. Mice carrying a targeted deletion of the tob gene have a greater bone mass resulting from increased numbers of osteoblasts. Orthotopic bone formation in response to BMP2 is elevated in tob-deficient mice. Overproduction of Tob represses BMP2-induced, Smad-mediated transcriptional activation. Finally, Tob associates with receptor-regulated Smads (Smad1, 5, and 8) and colocalizes with these Smads in the nuclear bodies upon BMP2 stimulation. The results indicate that Tob negatively regulates osteoblast proliferation and differentiation by suppressing the activity of the receptor-regulated Smad proteins.

A fundamental transcription factor for bone and cartilage

Cbfa1-deficient mice were found to show a complete lack of bone formation owing to the maturational arrest of osteoblasts. Cbfa1 plays key roles in the determination of osteoblastic lineage from multipotential mesenchymal cells, their differentiation into mature osteoblasts, and transcriptional regulation of bone matrix-related genes. Cbfa1 positively regulates chondrocyte maturation and osteoclast differentiation and is required for vascular invasion into cartilage. Therefore, complete elucidation of the function of Cbfa1 and its signaling would be of great benefit in understanding skeletogenesis.

Multilineage differentiation of Cbfa1-deficient calvarial cells in vitro

We characterized calvaria-derived cells of Cbfa1-deficient mice to determine their stages of differentiation. In long-term culture, Cbfa1-deficient calvarial cells did not acquire osteoblastic phenotypes, but numerous adipocyte foci appeared with an increase in the expression of marker genes for adipocyte differentiation. In culture with BMP-2, Cbfa1-deficient calvarial cells still failed to generate bone nodules but differentiated into chondrocytes and further to terminal hypertrophic chondrocytes, and adipocyte foci were decreased. Cbfa1-deficient calvarial cells transplanted into the peritoneal cavity of athymic mice using BMP-2-coated diffusion chambers generated cartilage but not bone. These data indicate that Cbfa1-deficient calvarial cells completely lack the ability to differentiate into mature osteoblasts and Cbfa1 has an inhibitory function in adipocyte differentiation. As Cbfa1-deficient calvarial cells were enriched with immature mesenchymal cells, which can differentiate into adipocytes and chondrocytes, it is suggested that Cbfa1 plays an essential role in determining the lineage of multipotential mesenchymal precursor cells.

Is aplastic osteodystrophy a disease of malnutrition?

Adynamic bone disease is emerging as a major type of renal osteodystrophy in chronic dialysis patients. Relative hypoparathyroidism is one of the important abnormalities underlying this disease. Recently, several reports have suggested that hypoparathyroidism reflects, at least in part, a state of malnutrition and contributes to the poor prognosis of patients on hemodialysis and chronic ambulatory peritoneal dialysis. Such a risk of survival may result not only from the malnutritional state, but also from unknown mechanisms resulting from parathyroid hormone (PTH) deficiency, or from other abnormalities that suppress PTH secretion. Another major abnormality underlying adynamic bone disease is the skeletal resistance to PTH in patients with uremia. Owing to the recent research on bone turnover at the molecular level, several new mechanisms for this abnormality have been elucidated. Correction of this 'skeletal resistance to PTH' will lead to the optimal management of parathyroid function and bone turnover in the future.

Cbfa1 is a positive regulatory factor in chondrocyte maturation

Cbfa1 is a transcription factor that belongs to the runt domain gene family. Cbfa1-deficient mice showed a complete lack of bone formation due to the maturational arrest of osteoblasts, demonstrating that Cbfa1 is an essential factor for osteoblast differentiation. Further, chondrocyte maturation was severely disturbed in Cbfa1-deficient mice. In this study, we examined the possibility that Cbfa1 is also involved in the regulation of chondrocyte differentiation. mRNAs for both Cbfa1 isotypes, type I Cbfa1 (Pebp2alphaA/Cbfa1) and type II Cbfa1 (Osf2/Cbfa1 or til-1), which are different in N-terminal domain, were expressed in terminal hypertrophic chondrocytes as well as osteoblasts. In addition, mRNA for type I Cbfa1 was expressed in other hypertrophic chondrocytes and prehypertrophic chondropcytes. In a chondrogenic cell line, ATDC5, the expression of type I Cbfa1 was elevated prior to differentiation to the hypertrophic phenotype, which is characterized by type X collagen expression. Treatment with antisense oligonucleotides for type I Cbfa1 severely reduced type X collagen expression in ATDC5 cells. Retrovirally forced expression of either type I or type II Cbfa1 in chick immature chondrocytes induced type X collagen and MMP13 expression, alkaline phosphatase activity, and extensive cartilage-matrix mineralization. These results indicate that Cbfa1 is an important regulatory factor in chondrocyte maturation.

Gly369Cys mutation in mouse FGFR3 causes achondroplasia by affecting both chondrogenesis and osteogenesis

Missense mutations in fibroblast growth factor receptor 3 (FGFR3) result in several human skeletal dysplasias, including the most common form of dwarfism, achondroplasia. Here we show that a glycine-to-cysteine substitution at position 375 (Gly375Cys) in human FGFR3 causes ligand-independent dimerization and phosphorylation of FGFR3 and that the equivalent substitution at position 369 (Gly369Cys) in mouse FGFR3 causes dwarfism with features mimicking human achondroplasia. Accordingly, homozygous mice were more severely affected than heterozygotes. The resulting mutant mice exhibited macrocephaly and shortened limbs due to retarded endochondral bone growth and premature closure of cranial base synchondroses. Compared with their wild-type littermates, mutant mice growth plates shared an expanded resting zone and narrowed proliferating and hypertrophic zones, which is correlated with the activation of Stat proteins and upregulation of cell-cycle inhibitors. Reduced bone density is accompanied by increased activity of osteoclasts and upregulation of genes that are related to osteoblast differentiation, including osteopontin, osteonectin, and osteocalcin. These data reveal an essential role for FGF/FGFR3 signals in both chondrogenesis and osteogenesis during endochondral ossification.

CBFA1 mutation analysis and functional correlation with phenotypic variability in cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is a dominantly inherited skeletal dysplasia caused by mutations in the osteoblast-specific transcription factor CBFA1. To correlate CBFA1 mutations in different functional domains with the CCD clinical spectrum, we studied 26 independent cases of CCD and a total of 16 new mutations were identified in 17 families. The majority of mutations were de novo missense mutations that affected conserved residues in the runt domain and completely abolished both DNA binding and transactivation of a reporter gene. These, and mutations which result in premature termination in the runt domain, produced a classic CCD phenotype by abolishing transactivation of the mutant protein with consequent haploinsufficiency. We further identified three putative hypomorphic mutations (R391X, T200A and 90insC) which result in a clinical spectrum including classic and mild CCD, as well as an isolated dental phenotype characterized by delayed eruption of permanent teeth. Functional studies show that two of the three mutations were hypomorphic in nature and two were associated with significant intrafamilial variable expressivity, including isolated dental anomalies without the skeletal features of CCD. Together these data show that variable loss of function due to alterations in the runt and PST domains of CBFA1 may give rise to clinical variability, including classic CCD, mild CCD and isolated primary dental anomalies.

AML3/CBFalpha1 is required for androgen-specific activation of the enhancer of the mouse sex-limited protein (Slp) gene

A complex 120-base pair enhancer, derived from the mouse sex-limited protein (Slp) gene, is activated solely by the androgen receptor (AR) in specific tissues, although it contains a hormone response element recognized by several steroid receptors. The generation of this transcriptional specificity has been ascribed to the interactions of the receptor with tissue-specific nonreceptor factors bound to accessory sites within the enhancer. Protein-DNA interaction assays revealed two factors binding the 5' part of the enhancer that differ widely in abundance between cells showing AR-specific activation of the Slp element compared with those that also permit activation by glucocorticoid receptor (GR). The factor designated B formed a complex centered on the sequence TGTGGT, a core motif recognized by members of the AML/CBFalpha transcription factor family. This complex was competed by a high affinity binding site specific for AML/CBFalpha and was specifically supershifted by an antibody to AML3/CBFalpha1, placing factor B within the AML3/CBFalpha1 subclass. Interestingly, this factor was shown to bind to a second site in the 3' part of the enhancer, positioned between the two critical AR binding sites. Transfection studies revealed that AML1-ETO, a dominant-negative AML/CBFalpha construct, abrogated AR induction of the enhancer, but not of simple hormone response elements. Furthermore, overexpression of AML3/CBFalpha1 could rescue the AML1-ETO repression. Finally, glutathione S-transferase-AML/CBFalpha fusion proteins demonstrated direct interaction between AML/CBFalpha and steroid receptors. Although this interaction was equivalent between AML1/CBFalpha2 and AR or GR, AML3/CBFalpha1 showed stronger interaction with AR than with GR. These data demonstrate that AML3/CBFalpha1 is functionally required for hormonal induction of the Slp enhancer and that direct, preferential protein-protein interactions may contribute to AR-specific activation. These results demonstrate an intriguing role of AML3/CBFalpha1 in steroid- as well as tissue-specific activation of target genes.

Isolation and characterization of the distal promoter region of mouse Cbfa1

Cbfa1 is an essential transcription factor for bone formation, and as such little is known about the region responsible for the transcriptional regulation of this gene. Here we report the determination of the transcription start sites, isolation and partial characterization of distal promoter region of this gene. Three transcription start sites were identified by the 5'-Cap site method, recently invented for rapid examination of the 5'-end of genes of interest. A reporter construct containing 1.8 kb of 5' of transcription start sites had approximately 25-fold more luciferase activity than the promoter-less vector in osteoblastic cell lines. Deletion analysis of the reporter construct demonstrated that the minimal region to express promoter activity lies between bp -168 and -99, taking the most downstream transcription start site as +1. By Northern blot analysis, mRNA expression from the distal promoter was detected in the differentiated osteoblastic cell lines, UMR-106, ROS17/2.8 and MC3T3-E1, but not in cell lines of immature phenotype or originated from other organs. Luciferase activity was strongest in UMR-106 and ROS17/2.8, and weakest in COS-1 and HepG2, which are cell lines originating from other organs, corresponding to the level of mRNA expression. These results demonstrated that the distal promoter region examined here is important for tissue- and cell-type-specific gene expression of Cbfa1.

Maturational disturbance of chondrocytes in Cbfa1-deficient mice

Cbfa1, a transcription factor that belongs to the runt-domain gene family, plays an essential role in osteogenesis. Cbfa1-deficient mice completely lacked both intramembranous and endochondral ossification, owing to the maturational arrest of osteoblasts, indicating that Cbfa1 has a fundamental role in osteoblast differentiation. However, Cbfa1 was also expressed in chondrocytes, and its expression was increased according to the maturation of chondrocytes. Terminal hypertrophic chondrocytes expressed Cbfa1 extensively. The significant expression of Cbfa1 in hypertrophic chondrocytes was first detected at embryonic day 13.5 (E13.5), and its expression in hypertrophic chondrocytes was most prominent at E14.5-16.5. In Cbfa1-deficient mice, whose entire skeleton was composed of cartilage, the chondrocyte differentiation was disturbed. Calcification of cartilage occurred in the restricted parts of skeletons, including tibia, fibula, radius, and ulna. Type X collagen, BMP6, and Indian hedgehog were expressed in their hypertrophic chondrocytes. However, osteopontin, bone sialoprotein, and collagenase 3 were not expressed at all, indicating that they are directly regulated by Cbfa1 in the terminal hypertrophic chondrocytes. Chondrocyte differentiation was severely disturbed in the rest of the skeleton. The expression of PTH/PTHrP receptor, Indian hedgehog, type X collagen, and BMP6 was not detected in humerus and femur, indicating that chondrocyte differentiation was blocked before prehypertrophic chondrocytes. These findings demonstrate that Cbfa1 is an important factor for chondrocyte differentiation.

Cbfa1 isoforms exert functional differences in osteoblast differentiation

Cbfa1 is an essential transcription factor for osteoblast differentiation and bone formation. We investigated functional differences among three isoforms of Cbfa1: Type I (originally reported as Pebp2alphaA by Ogawa et al. (Ogawa, E., Maruyama, M., Kagoshima, H., Inuzuka, M., Lu, J., Satake, M., Shigesada, K., and Ito, Y. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 6859-6863), Type II (originally reported as til-1 by Stewart et al. (Stewart, M., Terry, A., Hu, M., O'Hara, M., Blyth, K., Baxter, E., Cameron, E., Onions, D. E., and Neil, J. C. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 8646-8651), and Type III (originally reported as Osf2/Cbfa1 by Ducy et al. (Ducy, P., Zhang, R., Geoffroy, V., Ridall, A. L., and Karsenty, G. (1997) Cell 89, 747-754). A reverse transcriptase-polymerase chain reaction analysis demonstrated that these isoforms were expressed in adult mouse bones. The transient transfection of Type I or Type II Cbfa1 in a mouse fibroblastic cell line, C3H10T1/2, induced the expression of alkaline phosphatase (ALP) activity. This induction was synergistically enhanced by the co-introduction of Xenopus BMP-4 cDNA. In contrast, the transient transfection of Type III cDNA induced no ALP activity. In C3H10T1/2 cells stably transfected with each isoform of Cbfa1, the gene expression of ALP was also strongly induced in cells transfected with Type I and Type II Cbfa1 but not in cells with Type III Cbfa1. Osteocalcin, osteopontin,and type I collagen gene expressions were induced or up-regulated in all of the cells stably transfected with each isoform of Cbfa1, and Type II transfected cells exhibited the highest expression level of osteocalcin gene. A luciferase reporter gene assay using a 6XOSE2-SV40 promoter (6 tandem binding elements for Cbfa1 ligated in front of the SV40 promoter sequence), a mouse osteocalcin promoter, and a mouse osteopontin promoter revealed the differences in the transcriptional induction of target genes by each Cbfa1 isoform with or without its beta-subunit. These results suggest that all three of the Cbfa1 isoforms used in the present study are involved in the stimulatory action of osteoblast differentiation, but they exert different functions in the process of osteoblast differentiation.

Excessive extramedullary hematopoiesis in Cbfa1-deficient mice with a congenital lack of bone marrow

Cbfa1, a transcriptional factor that belongs to the runt-domain gene family, plays a pivotal role in osteogenesis. Cbfa1-deficient mice completely lacked both intramembranous and endochondral bone formation owing to the maturational arrest of osteoblasts, resulting in a total lack of bone marrow throughout the entire skeleton. To investigate the development of hematopoiesis in Cbfa1-deficient mice, hematopoietic tissues and peripheral blood were examined. Colony-forming assays of yolk sac at embryonic day 10.5 (E10.5) and liver at E12.5 showed normal numbers of hematopoietic precursors in the mutant embryos. Further, histological examination and flow-cytometric analysis showed normal hematopoietic development of liver and spleen in the mutant embryos until E17.5. However, mutant embryos at E18.5 showed large hematopoietic foci in the periportal area of liver and mild splenomegaly with dilated vessels, and an increase of myeloid cells was observed in both organs. Flow-cytometric analysis also demonstrated a relative increase of granulocytes and a relative decrease of B-cells reciprocally in the livers and spleens of the mutant embryos. Colony-forming assays showed that the frequency of colony-forming unit-culture (CFU-C) was elevated in liver, and both the frequency and total number of CFU-C were increased in spleen of mutant embryos at E18.5. Moreover, leukoerythroblastosis was observed in peripheral blood of the mutant embryos. These data demonstrate that the congenital lack of bone marrow caused excessive levels of extramedullary hematopoiesis in both liver and spleen at the late embryonic stage.