Axin2 controls bone remodeling through the beta-catenin-BMP signaling pathway in adult mice.

To investigate the role of Wnt-beta-catenin signaling in bone remodeling, we analyzed the bone phenotype of female Axin2-lacZ knockout (KO) mice. We found that trabecular bone mass was significantly increased in 6- and 12-month-old Axin2 KO mice and that bone formation rates were also significantly increased in 6-month-old Axin2 KO mice compared with wild-type (WT) littermates. In vitro studies were performed using bone marrow stromal (BMS) cells isolated from 6-month-old WT and Axin2 KO mice. Osteoblast proliferation and differentiation were significantly increased and osteoclast formation was significantly reduced in Axin2 KO mice. Nuclear beta-catenin protein levels were significantly increased in BMS cells derived from Axin2 KO mice. In vitro deletion of the beta-catenin gene under Axin2 KO background significantly reversed the increased alkaline phosphatase activity and the expression of osteoblast marker genes observed in Axin2 KO BMS cells. We also found that mRNA expression of Bmp2 and Bmp4 and phosphorylated Smad1/5 protein levels were significantly increased in BMS cells derived from Axin2 KO mice. The chemical compound BIO, an inhibitor of glycogen synthase kinase 3beta, was utilized for in vitro signaling studies in which upregulated Bmp2 and Bmp4 expression was measured in primary calvarial osteoblasts. Primary calvarial osteoblasts were isolated from Bmp2(fx/fx);Bmp4(fx/fx) mice and infected with adenovirus-expressing Cre recombinase. BIO induced Osx, Col1, Alp and Oc mRNA expression in WT cells and these effects were significantly inhibited in Bmp2/4-deleted osteoblasts, suggesting that BIO-induced Osx and marker gene expression were Bmp2/4-dependent. We further demonstrated that BIO-induced osteoblast marker gene expression was significantly inhibited by Osx siRNA. Taken together, our findings demonstrate that Axin2 is a key negative regulator in bone remodeling in adult mice and regulates osteoblast differentiation through the beta-catenin-BMP2/4-Osx signaling pathway in osteoblasts.


Introduction
The Wnt family consists of a number of small, cysteine-rich, secreted glycoproteins that are involved in the regulation of a variety of cellular activities that are crucial for development (Huelsken and Birchmeier, 2001;Moon et al., 2002;Westendorf et al., 2004). The canonical Wnt signaling pathway is mediated by a protein complex containing β-catenin, through regulation of β-catenin protein levels and its subcellular localization. In the absence of Wnts, β-catenin levels are kept in a steady state. Free β-catenin proteins are ubiquitylated and degraded by the 26S proteasome (Aberle et al., 1997). A multiprotein complex containing the kinases glycogen synthase kinase 3β (GSK-3β) and casein kinase 1 (CK1) along with scaffolding proteins Axis inhibition protein 1 (Axin1), Axin2, adenomatous polyposis coli (APC) and disheveled (Dsh) mediates the degradation of excess β-catenin. This complex phosphorylates specific amino acid residues on β-catenin, creating a docking site for the F-box protein/E3 ligase complex (Behrens et al., 1998;Jiang and Struhl, 1998). Therefore, inhibition of β-catenin phosphorylation prevents degradation of β-catenin, increases its cytoplasmic level, and facilitates its nuclear translocation. Signaling from Wnt proteins makes the complex release β-catenin, and allows it to move to the nucleus, where it interacts with TCF/LEF transcription factors to activate the expression of target genes (Staal and Clevers, 2000;Moon et al., 2002;Cliffe et al., 2003;Westendorf et al., 2004).
At the cell surface, Wnts interact with two classes of proteins: frizzled receptors and low-density-lipoprotein (LDL)-receptorrelated protein 5 or 6 (LRP5/6). There are many genes encoding frizzled proteins, including ten in the human genome. Individual frizzled proteins probably have different affinities for various types of Wnt proteins. Wnt proteins can form a complex with both the cysteine-rich domain (CRD) of frizzled and with LRP5/6 simultaneously, leading to the formation of a dual-receptor complex (Bejsovec, 2000;Huelsken and Birchmeier, 2001). The intracellular portion of the receptors communicates this binding information and turns on the pathways that act on β-catenin inside the cell. Following Wnt binding, the intracellular tail of LRP5/6 binds Axin1 (or Axin2) and causes dissociation of β-catenin from its protein complex and activation of β-catenin signaling (Mao et al., 2001).
To investigate the role of Wnt-β-catenin signaling in bone remodeling, we analyzed the bone phenotype of female Axin2-lacZ knockout (KO) mice. We found that trabecular bone mass was significantly increased in 6-and 12-month-old Axin2 KO mice and that bone formation rates were also significantly increased in 6-month-old Axin2 KO mice compared with wildtype (WT) littermates. In vitro studies were performed using bone marrow stromal (BMS) cells isolated from 6-month-old WT and Axin2 KO mice. Osteoblast proliferation and differentiation were significantly increased and osteoclast formation was significantly reduced in Axin2 KO mice. Nuclear β-catenin protein levels were significantly increased in BMS cells derived from Axin2 KO mice. In vitro deletion of the βcatenin gene under Axin2 KO background significantly reversed the increased alkaline phosphatase activity and the expression of osteoblast marker genes observed in Axin2 KO BMS cells. We also found that mRNA expression of Bmp2 and Bmp4 and phosphorylated Smad1/5 protein levels were significantly increased in BMS cells derived from Axin2 KO mice. The chemical compound BIO, an inhibitor of glycogen synthase kinase 3β, was utilized for in vitro signaling studies in which upregulated Bmp2 and Bmp4 expression was measured in primary calvarial osteoblasts. Primary calvarial osteoblasts were isolated from Bmp2 fx/fx ;Bmp4 fx/fx mice and infected with adenovirus-expressing Cre recombinase. BIO induced Osx, Col1, Alp and Oc mRNA expression in WT cells and these effects were significantly inhibited in Bmp2/4-deleted osteoblasts, suggesting that BIO-induced Osx and marker gene expression were Bmp2/4-dependent. We further demonstrated that BIO-induced osteoblast marker gene expression was significantly inhibited by Osx siRNA. Taken together, our findings demonstrate that Axin2 is a key negative regulator in bone remodeling in adult mice and regulates osteoblast differentiation through the β-catenin-BMP2/4-Osx signaling pathway in osteoblasts. Axin2 controls bone remodeling Axin1 and its homolog Axin2 are negative regulators of canonical Wnt signaling through promotion of β-catenin degradation (Behrens et al., 1998;Kikuchi, 1999;Peifer and Polakis, 2000). Axin1 has interaction sites for proteins involved in Wnt signal transduction, including β-catenin, GSK-3β, CK1, APC, Dsh and LRP5 (Luo and Lin, 2004). Significantly, Axin1 associates with β-catenin and protein kinases GSK-3β and CK1. In this complex, β-catenin is phosphorylated and degraded by the ubiquitin-proteasome pathway. Wnt binding recruits Axin1 to the receptor complex, where it binds the cytoplasmic domain of LRP5 (Cong et al., 2004;Hay et al., 2005;Mao et al., 2001). Axin1 is a central component of the canonical Wnt pathway and acts as a scaffold for the protein complex involved in β-catenin degradation.
Mouse Axin2 cDNA encodes a protein of 840 amino acids (Axin2, also known as conductin). Several important binding domains in Axin2 have been mapped. Axin2 has a β-cateninbinding domain that is located in the central region of Axin2 protein (amino acids 396-465). Axin2 contains a N-terminal RGS (regulator of G protein signaling) domain (amino acids 78-200), a GSK-3β binding domain (amino acids 343-396) and a C-terminal sequence related to the protein Dsh (amino acids 783-833) (Behrens et al., 1998). Axin2 has been shown to mediate APCinduced β-catenin degradation. The Axin2 RGS domain binds the SAMP repeats of APC. A double mutant of β-catenin (W383A/R386A) does not bind APC but this mutant is still degraded by exogenous wild-type (WT) APC, indicating that APC does not need to bind directly to β-catenin to induce β-catenin degradation. A triple mutant β-catenin (W383A/R386A/H260A) does not bind APC or Axin2 and this mutant β-catenin is fully stable in the presence of exogenous WT APC (von Kries et al., 2000). These results demonstrate that Axin2 can link APC to βcatenin and mediate APC-induced β-catenin degradation.
Axin2 has 44% homology with Axin1 and its function is similar (Behrens et al., 1998;Chia and Costantini, 2005), but they have different expression patterns. Axin1 is widely expressed, whereas Axin2 is differentially expressed in various cells and tissues, and during different stages of maturation (Chia and Costantini, 2005;Yu et al., 2005). The deletion of the Axin1 gene results in early embryonic mortality. Mice die at embryonic day 9.5 with forebrain truncation, neural tube defects and axis duplications (Zeng et al., 1997). Knock-in of the Axin2 gene into the deleted Axin1 gene rescues the phenotype of Axin1-deficient mice (Chia and Costantini, 2005). A lacZ knock-in allele that is a null allele of Axin2 (Axin2 -/-) was generated by insertion of the lacZ cDNA into exon2 of the Axin2 gene. The homozygous Axin2 knockout (KO) mice are viable and fertile, but have craniofacial defects and premature closure of the cranial sutures due to increased βcatenin signaling (Yu et al., 2005). Nevertheless, despite some overlapping functions, Axin1 and Axin2 do not have redundant functions, which probably reflects their differential expression patterns.
Although canonical Wnt/β-catenin signaling plays a crucial role in controlling bone development, its role in bone remodeling remains poorly understood because either deletion of the β-catenin gene in a conventional method or tissue-specific manner often leads to embryonic lethality. Using the Axin2 KO mice as a unique mouse model, we have investigated the role of the Axin2/β-catenin signaling pathway in bone remodeling. Our findings demonstrate that Axin2/β-catenin signaling targets Bmp2/4, regulates Osx expression and controls osteoblast differentiation in osteoblast progenitor or precursor cells.

Results
Age-related increase in bone mass in Axin2 KO mice To investigate changes in bone mass and bone microstructure, we analyzed the metaphyseal region of long bones (femora) of 2-, 6and 12-month-old WT and Axin2 KO mice using micro-CT imaging. No obvious changes in trabecular bone parameters were observed in long bones of 2-month-old Axin2 KO mice compared with WT controls (data not shown). By contrast, a significant increase in bone volume (BV) and bone mineral density (BMD) was observed in 6and 12-month-old Axin2 KO mice (Fig. 1). No significant difference in bone parameters was observed between heterozygous Axin2 KO mice and WT controls at these three time points (data not shown). These results suggest that Axin2 might play a dominant role in bone formation in older mice.
Increased bone mechanical strength in Axin2 KO mice Long-bone diaphyseal morphology was not found to be different between WT and Axin2 KO femurs nor between tibia, using micro-CT cross-sectional imaging. Diaphyseal cross-sectional area, bending moments of inertia, outer diameter and BMD were not significantly different (Table 1), with the exception that Axin2 KO femoral cortical thickness was 10% greater, whereas the outer medial-lateral diameter was 9% smaller (P<0.05). To determine the effect of Axin2 on bone mechanics, eight femurs from 12-month- Fig. 1. Bone mass is increased in 6-and 12-month-old female Axin2 KO mice. Three dimensional bone structure was analyzed in 6-and 12-month-old female Axin2 KO (Axin2 -/-) mice and WT controls (n=6) by micro-CT (A,J). Bone volume (BV) was increased 61 and 255% in 6-and 12-month-old Axin2 KO mice (B,K). Bone mineral density (BMD) was increased 30 and 73% in 6-and 12-month-old Axin2 KO mice when bone mineral content (BMC) was normalized to tissue volume (BMC/TV) (C,L). The structural model index (SMI) was significantly decreased (23 and 42%, respectively) in 6-and 12month-old Axin2 KO mice (D,M). The trabecular number (Tb.N.) was increased (16 and 39%, respectively) (E,N) and trabecular thickness (Tb.Th.) was increased 15 and 21%, respectively in 6-and 12month-old Axin2 KO mice (F,O). By contrast, trabecular separation (Tb.Sp.) was decreased (14 and 24%, respectively) in 6and 12-month-old Axin2 KO mice (G,P). *P<0.05, unpaired Student's t-test, n=6. Bone marrow stromal (BMS) cells were isolated from 2-and 6-month-old WT C57BL/6J mice and expression of Axin1 and Axin2 was examined by real-time PCR. Axin1 expression was reduced 47% and Axin2 expression was increased ninefold in BMS cells derived from 6month-old mice (H,I), suggesting that Axin2 might play a predominant role in older mice. *P<0.05, unpaired Student's t-test, n=3. All values are means ± s.e. Axin2 controls bone remodeling old WT and seven femurs from Axin2 KO mice underwent threepoint bending mechanical testing. Femurs from Axin2 KO animals were significantly stiffer by 31%, with a maximum bending moment that was 20% greater (P<0.05, Table 2) than for WT controls.
Increased osteoblast function in Axin2 KO mice Consistent with the phenotypic changes measured by micro-CT, histological analysis also showed a significant increase in bone volume in 6-and 12-month-old female Axin2 KO mice ( Fig. 2A,B). The mineral appositional rates (MAR) and bone formation rates (BFR) were increased 64 and 86%, respectively, in 6-month-old female Axin2 KO mice, as demonstrated by histomorphometric analysis (Fig. 2C,D). The micro-CT and histomorphometric results demonstrated a dramatic increase in bone mass in 6-and 12-monthold Axin2 KO mice and suggest that Axin2 is a negative regulator in the maintenance of bone mass in adult mice. Because BFR is increased in Axin2 KO mice, the data also suggest that osteoblast function is elevated in Axin2 KO mice.

Reduced osteoclast formation in Axin2 KO mice
To determine whether increased bone mass in Axin2 KO mice is also due to a decrease in osteoclast formation, we also analyzed changes in osteoclast numbers. We found that osteoclast surfaces and osteoclast numbers were significantly reduced in 6-monthold Axin2 KO mice (Fig. 2E,F). We also examined the ability of BMS cells to form osteoclasts in response to M-CSF (macrophage-colony stimulating factor) and RANKL (receptor activator for nuclear factor-kB ligand). The results showed that osteoclast formation was significantly reduced (n=4) in the cells derived from 6-month-old Axin2 KO mice compared with formation in the cells obtained from WT controls under the same culture conditions (Fig. 2G,H). In addition, we also found that osteoclastic bone resorption was also significantly reduced in bone marrow cells derived from Axin2 KO mice (Fig. 2I). The mRNA expression of osteoprotegerin (Opg) in BMS cells was significantly increased in 6-month-old Axin2 KO mice (Fig. 2J). By contrast, Rankl expression was slightly reduced in Axin2 KO mice (Fig. 2K). In addition, the expression of cathepsin K was significantly reduced in Axin2 KO mice (Fig. 2L). Consistent with the increased Opg mRNA expression, serum OPG protein levels were also significantly increased in Axin2 KO mice (Fig. 2M). These results suggest that reduction in osteoclast formation and bone resorption might also contribute to the high-bone-mass phenotype observed in Axin2 KO mice.
To determine the role of Axin2 in bone loss induced by estrogen deficiency, we performed an ovariectomy experiment in 6-monthold female Axin2 KO mice and WT controls. The changes in bone volume were monitored by micro-CT analysis. There was 48% bone loss (BV/TV) in WT mice 4 weeks after ovariectomy compared with sham-operated mice (n=10). By contrast, only 36% bone loss was found in Axin2 KO mice 4 weeks after ovariectomy (n=10) (supplementary material Fig. S1). These findings demonstrated that Axin2 KO mice are partially resistant to overiectomy-induced bone loss and that this might be related to the increased Opg expression in Axin2 KO mice.

Increases in osteoblast proliferation and differentiation in Axin2 KO mice
To investigate changes in cellular function, we isolated BMS cells from 6-month-old Axin2 KO mice and WT controls and cultured these cells for different periods of time. We first examined cell proliferation by immunostaining using an anti-Ki-67 antibody and found a significant increase in Ki-67-positive cells (5.7-fold increase) in BMS cells from Axin2 KO mice ( Fig. 3A) versus WT controls. It has been reported that β-catenin directly activates cyclin D1 gene transcription in tumor cells (Tetsu and McCormick, 1999). We analyzed changes in cyclin D1 expression and found that the expression of cyclin D1 mRNA and protein was significantly increased in BMS cells derived from Axin2 KO mice compared with that in WT BMS cells (Fig. 3B,C).
We determined changes in osteoblast differentiation by examining a panel of osteoblast maker genes expressed at various developmental stages using enzyme activity assay and real-time PCR assays. The alkaline phosphatase (ALP) activity ( Fig. 3D,E) and the expression of osteoblast-specific transcription factors, Runx2 and osterix (Osx), were significantly increased in BMS cells derived from 6-month-old Axin2 KO mice (Fig. 3F,G). As a consequence, the expression of early osteoblast differentiation markers, type I collagen (Col1) and osteopontin (Opn) was also significantly increased in Axin2 KO BMS cells (Fig. 3H,I). The expression levels of later-stage marker genes such as bone sialoprotein (Bsp) and osteocalcin (Oc) were also significantly increased in Axin2 KO cells (Fig. 3J,K). These data suggest that osteoblast differentiation was largely advanced in the Axin2 KO   Histomorphometric analyses demonstrated the 64% increase in mineral appositional rates (MAR) and 86% increase in bone formation rates (BFR) in 6-month-old Axin2 KO mice (n=9) (C,D). Histomorphometric analyses also showed that osteoclast surface and osteoclast numbers were significantly reduced (37 and 42% reduction, respectively) in 6-month-old Axin2 KO mice (n=9) (E,F). In vitro osteoclast formation assay was performed using bone marrow cells, which were cultured in the presence or absence of M-CSF (30 ng/ml) for 3 days and M-CSF (30 ng/ml) and RANKL (20 ng/ml) for an additional 4 days. TRAP staining and quantification of TRAP-positive multi-nucleus osteoclast numbers were performed. Osteoclast formation was significantly reduced in bone marrow cells derived from 6-month-old Axin2 KO mice (n=3) (G,H). The cells were also cultured on the bone slices. After cell culture, cells were removed by brushing and bone slices were stained with toludine blue. The pit areas were counted. Bone resorption was significantly reduced in Axin2 KO mice (n=3) (I). We also measured Opg, Rankl and cathepsin K expression and found that the expression of Opg but not Rankl was significantly increased and expression of cathepsin K was significantly reduced in bone marrow cells derived from 6-month-old Axin2 KO mice (n=3) (J-L). Serum OPG protein levels were measured by Elisa assay. A significant increase in OPG protein level (>threefold) was found in Axin2 KO mice (n=3) (M). *P<0.05, unpaired Student's t-test. All values are means ± s.e.
BMS cells. In contrast to the findings observed in BMS cells derived from 6-month-old Axin2 KO mice, no significant changes in cell proliferation, ALP activity or the expression of osteoblast marker genes were found in BMS cells derived from 2-month-old Axin2 KO mice (data not shown). Osteoblast apoptosis was examined by TUNEL staining and no changes in cell apoptosis were found between the Axin2 KO and WT BMS cells from either 2-or 6month-old mice (data not shown). These in vitro studies demonstrate that the process of osteoblast proliferation and differentiation is accelerated in the Axin2 KO mice.
Activation of β-catenin signaling in Axin2 KO mice It has been reported that β-catenin promotes mesenchymal progenitor cells and osteoblast precursor cells to differentiate into osteoblasts (Hill et al., 2005;Day et al., 2005;Rodda and McMahon, 2006) and that it stimulates Opg expression in mature osteoblasts (Glass et al., 2005;Holmen et al., 2005). Because the phenotype found in Axin2 KO mice is consistent with the phenotypes observed in mice with β-catenin gene deletions at the different stages of osteoblast lineages, we examined cytoplasmic and nuclear βcatenin protein levels to determine changes in β-catenin signaling in Axin2 KO mice. Although the protein levels of β-catenin in cytoplasm were slightly increased, the protein levels of β-catenin (total and non-phosphorylated active form) in the nucleus were significantly increased in BMS cells derived from 6-month-old Axin2 KO mice (Fig. 4A,B). These results suggest that the bone phenotype observed in Axin2 KO mice could be due to the activation of β-catenin in osteoblasts.
To confirm the role of β-catenin activation in the bone phenotype of Axin2 KO mice, we crossed Axin2 KO mice with β-catenin fx/fx mice and generated Axin2 -/-;β-catenin fx/fx mice. BMS cells were then isolated from 6-month-old WT, Axin2 -/-, β- Fig. 3. Osteoblast proliferation and differentiation are increased in bone marrow stromal (BMS) cells of Axin2 KO mice. BMS cells were isolated from 6-month-old WT and Axin2 KO mice. Cells were cultured for 6 days and Ki-67 staining was performed. Ki-67-positive cells were increased 5.7-fold in BMS cells derived from Axin2 KO mice (A). Consistent with this finding, cyclin D1 mRNA and protein expression were also increased in BMS cells derived from Axin2 KO mice (B,C). *P<0.05, unpaired Student's t-test, n=4. BMS cells were cultured for 10 days for ALP staining and activity assay. The intensity of ALP staining and ALP activity (1.8fold increase) were significantly increased in BMS cells derived from 6-month-old Axin2 KO mice (D,E). BMS cells were also cultured for 10 days and total RNA was extracted from the cells and expression of osteoblast marker genes was measured by real-time PCR assay. The expression of Runx2, osterix (Osx), type I collagen (Col-1), osteopontin (Opn), bone sialoprotein (Bsp) (G) and osteocalcin (Oc) (H) was significantly increased in BMS cells derived from Axin2 KO mice (F-K). *P<0.05, unpaired Student's t-test, n=3. All values are means ± s.e. catenin fx/fx and Axin2 -/-;β-catenin fx/fx mice and infected with lentivirus expressing Cre recombinase (Lenti-Cre). ALP activity and the expression of osteoblast marker genes were examined in these cells. The results showed that the increased ALP activity and increased Bmp4 and Oc expression observed in Axin2 KO mice were completely inhibited in β-catenin-deleted BMS cells under the Axin2 KO background (Fig. 4C-E). In addition, the increased Bmp2 expression found in Axin2 KO mice was also significantly inhibited in β-catenin-deleted BMS cells (Fig. 4F). In vitro deletion of the β-catenin gene in BMS cells derived from 6-month-old Axin2 -/-;β-catenin fx/fx mice was confirmed by western blotting (Fig. 4G). These results strongly suggest that increased osteoblast function in Axin2 KO mice is due to the activation of β-catenin in BMS cells.

Activation of BMP signaling in Axin2 KO mice
The synergistic effect between β-catenin and bone morphogenetic protein (BMP) signaling in bone cells has been reported (Fischer et al., 2002;Mbalaviele et al., 2005;Nakashima et al., 2005;Chen et al., 2007). However, the mechanism remains unknown. To Journal of Cell Science 122 (19) investigate the interaction between BMPs and Axin2 and/or βcatenin, we examined changes in expression levels of Bmp. We analyzed the mRNA levels of Bmp in BMS cells by real-time PCR. The results demonstrated that mRNA levels of Bmp2 and Bmp4 were significantly increased in Axin2 KO BMS cells (Fig. 5A,B). By contrast, the mRNA levels of Bmp6 and Bmp7 was not significantly changed in Axin2 KO mice (data not shown). To determine whether signaling proteins downstream of BMP are activated in Axin2 KO mice we measured changes in protein levels of phosphorylated Smad1/5, the key signaling molecules in BMP signaling. The basal levels of phosphorylated Smad1/5 (two BMP signaling Smads expressed in osteoblasts) were dramatically increased in the cells derived from 6-month-old Axin2 KO mice. There was no change in the protein levels of total Smad1 (Fig. 5C), suggesting that the increased phosphorylated Smad1/5 level is not due to changes in protein degradation. In addition, BMP-2-induced formation of mineralized bone nodules was significantly enhanced in BMS cells derived from Axin2 KO mice (13-fold increase for Axin2 KO cells versus sixfold increase for WT cells) (Fig. 5D,E). Similar results were also obtained when BMS cells were incubated Fig. 4. Increased β-catenin levels are responsible for the bone phenotype observed in Axin2 KO mice. Cell lysates (cytosolic and nuclear fractions) were collected from BMS cells isolated from 6-month-old WT and Axin2 KO mice. Western blot assays were performed to examine changes in total and non-phosphorylated active form of β-catenin protein. Cytoplasm β-catenin levels (active and total) were slightly increased (A). By contrast, the nuclear β-catenin levels (active and total) were significantly increased in BMS cells derived from 6-month-old Axin2 KO mice (B). Axin2 KO mice were bred with β-catenin fx/fx mice to produce Axin2 -/-;β-catenin fx/fx mice. BMS cells were isolated from 6-month-old WT, Axin2 -/-, β-catenin fx/fx and Axin2 -/-;β-catenin fx/fx mice and were infected with lentivirus expressing Cre recombinase (Lenti-Cre). ALP activity and expression of osteocalcin (Oc) and Bmp2 and Bmp4 were examined. Deletion of the β-catenin gene by Lenti-Cre completely inhibited increased ALP activity (C) and Oc and Bmp4 expression (D,E) and significantly inhibited Bmp2 expression (F) observed in Axin2 KO mice. Deletion of the β-catenin gene was determined by measuring levels of β-catenin protein using western blot analysis (G). *P<0.05, unpaired Student's t-test, n=3. All values are means ± s.e. with BMP4 (data not shown). In contrast to the changes in BMP signaling, PTH-induced ALP activity was similar in the BMS cells from Axin2 KO and WT mice (data not shown).

BIO-induced osteoblast differentiation is mediated by osterix
To further determine the regulatory mechanism of Bmp expression by Axin2 and β-catenin, we utilized the chemical compound BIO (6-bromoindirubin-3Ј-oxime), a GSK-3β inhibitor. BIO activated β-catenin signaling, stimulated ALP activity (Fig. 6A-D) and increased new bone formation in the periosteal bone formation assay in WT mice (Fig. 6E,F). BIO stimulated the expression of Bmp2 and Bmp4 genes over a 24-hour time period in primary osteoblasts (Fig. 7A,B). To determine whether BIO-induced osteoblast differentiation is dependent on Bmp2/4, primary calvarial osteoblasts were isolated from Bmp2 fx/fx ;Bmp4 fx/fx mice and infected with adenovirus-expressing Cre recombinase (Ad-Cre). Infection of Ad-Cre in osteoblasts efficiently deleted Bmp2 and Bmp4 genes in these cells (Fig. 7C,D). BIO stimulated Runx2 expression in both WT and Bmp2/4-deleted cells (Fig. 7E), suggesting that BIO-induced Runx2 expression is Bmp2/4independent. By contrast, BIO-induced Osx expression was significantly inhibited and BIO-induced Col1, Alp and Oc expression was completely abolished in Bmp2/4-deleted osteoblasts (Fig. 7F-I), suggesting that BIO-induced Osx and subsequent osteoblast marker gene expression are Bmp2/4dependent.
To further determine the role of Osx in BIO-induced osteoblast differentiation, Osx RNAi was transfected into primary osteoblasts, which were subsequently treated with BIO. The results showed that BIO-induced expression of Alp, Col1 and Oc was significantly or completely inhibited when Osx gene expression was knocked down by Osx siRNA (Fig. 7J-M). These results demonstrated that BIO (or activation of β-catenin signaling) activates Osx expression and subsequently regulates the expression of other osteoblast marker genes.

Axin2 is a key regulator of bone remodeling
Osteoporosis is a major health threat for the elderly, particularly for postmenopausal women. It is characterized by low bone mass, induced by ineffective bone remodeling, which leads to an increased incidence of fractures and to higher morbidity and mortality in affected individuals. Bone remodeling, a dynamic process involving bone resorption followed by bone formation, takes place throughout life to maintain bone homeostasis. In osteoporotic patients, progressive bone loss with age could be due to increased bone resorption or inadequate bone formation, or to a combination of both. A better understanding of the mechanisms through which bone formation and resorption are controlled is therefore crucial in determining the pathogenesis of osteoporosis and identifying potential molecular targets for the development of new therapies for this debilitating disease.  5. BMP signaling is activated in Axin2 KO mice. The mRNA expression of Bmp2 and Bmp4 was examined by real-time PCR using BMS cells derived from WT and Axin2 KO mice. The expression of Bmp2 and Bmp4 was significantly increased in BMS cells derived from 6-month-old Axin2 KO mice (A,B). Western blot assay was performed to analyze changes in phosphorylated Smad1/5 protein levels. The phosphorylated Smad1/5 was significantly increased in BMS cells derived from 6month-old Axin2 KO mice (C). In nodule formation assay, the responsiveness of the BMS cells to BMP-2 was higher in Axin2 KO cells than WT cells (13-fold increase in Axin2 KO cells versus sixfold increase in WT cells) (D,E). *P<0.05, unpaired Student's t-test, n=3. All values are means ± s.e.
In the present studies, we discovered that bone mass is significantly increased in 6-and 12-month-old adult Axin2 KO mice in which lacZ cDNA was inserted into exon2 of the Axin2 gene and thereby effectively prevented Axin2 expression in all cells of the mice (Yu et al., 2005). The present studies demonstrate for the first time that Axin2 plays a crucial role in the maintenance of bone mass in adult mice. Our in vivo results are complemented by cell-culture studies that show that osteoblast function is increased in vitro in cells from Axin2 KO mice. We also found that mRNA expression of Bmp2 and Bmp4 and phosphorylated Smad1/5 protein levels are increased in Axin2 KO BMS cells. These findings suggest an important cross-talk between the Axin2/βcatenin and BMP signaling pathways.

Journal of Cell Science 122 (19) Axin2 inhibits osteoblast proliferation and differentiation
Axin2 is a scaffolding protein and a negative regulator of β-catenin function (Behrens et al., 1998). It has been reported that the Nterminal RGS domain of Axin2 binds to APC and mediates APCinduced β-catenin degradation (von Kries et al., 2000). In the present studies, we found that activation of β-catenin signaling is the major event in BMS cells of Axin2 KO mice. β-catenin plays a crucial role in the commitment and differentiation of mesenchymal cells into the osteoblast lineage. Conditional deletion of the β-catenin gene in mesenchymal progenitor cells (targeted by Prx1-Cre or Dermo1-Cre mice) (Hill et al., 2005;Day et al., 2005) leads to the inhibition of osteoblast differentiation, suggesting that β-catenin promotes osteoblast differentiation in osteoblast progenitor cells. Fig. 6. BIO activates β-catenin signaling stimulates osteoblast differentiation and bone formation. Primary calvarial osteoblasts were isolated 3-day-old WT neonatal mice and cultured with or without BIO (1 μM) for different periods of time as indicated. The β-catenin protein levels and nuclear translocation were examined by western blot and immunostaining. BIO increased protein levels of active form of β-catenin and reached its maximal effect at 4 hours (A). BIO also induced β-catenin nuclear translocation (B). Primary calvarial osteoblasts, isolated from TOPGal transgenic mice, were treated with BIO (1 μM) for 24 hours and β-Gal activity was measured using cell lysates isolated from these cells. Wnt3a (100 ng/ml) was used as a positive control in this experiment. BIO significantly increased the β-Gal activity (C). *P<0.05, unpaired Student's t-test, n=3. Primary calvarial osteoblasts were cultured with 0.1 and 1 μM of BIO for 2 days and changes in ALP activity were examined by ALP staining. BIO (at both concentrations) significantly increased ALP activity in these cells (D). To further determine whether BIO induces new bone formation in vivo, 25 and 50 μg/mouse of BIO was injected into 1-month-old WT mice subcutaneously over the surface of calvariae for 5 days. Calcein labeling was performed at day 5 and 15. Mice were sacrificed 2 days after the second calcein labeling and periosteal new bone was evaluated and mineral appositional rates (MAR) were measured. FGF-1 (2 μg/mouse, 5 day injection) was used as a positive control. BIO significantly increased new bone formation and MAR in this assay (E,F). *P<0.05, one-way ANOVA followed by Dunnett's test (BIO versus control) and unpaired Student's t-test (FGF-1 versus control), n=5. All values are means ± s.e. Axin2 controls bone remodeling Fig. 7. BIO induces osteoblast differentiation in a Bmp2/4-Osx-dependent manner. Primary calvarial osteoblasts were isolated 3-day-old WT neonatal mice and cultured with or without BIO (1 μM) for 24 hours. BIO significantly upregulated Bmp2 (2.9-fold) and Bmp4 (1.8-fold) expression in these cells (A,B). To determine whether BIO-induced osteoblast differentiation is Bmp2/4-dependent, primary calvarial osteoblasts were isolated from Bmp2 fx/fx ;Bmp4 fx/fx mice and infected with Ad-GFP or Ad-Cre. Over 80% infection efficiency and Bmp2/Bmp4 gene deletion efficiency were observed (C,D). BIO induced Runx2 expression in Bmp2/4-deleted cells as well as in control cells (~1.5-fold increase), suggesting that BIO-induced Runx2 expression is not Bmp2/4-dependent (E). By contrast, BIO induced Osx, Col-1, Alp and Oc expression in control cells and these effects were significantly or completely inhibited in Bmp2/4-deleted osteoblasts, demonstrating that BIO stimulates Osx and subsequent osteoblast marker genes in a Bmp2/4-dependent manner (F-I). To further determine whether BIO-induced osteoblast marker genes in an Osx-dependent manner, Osx siRNA and control scramble siRNA were transfected into these cells. Transfection of Osx siRNA efficiently knocked down Osx expression (J) and significantly inhibited BIO-induced Alp expression (K) and completely inhibited BIO-induced Col-1 and Oc expression (L,M), suggesting that BIO-induced osteoblast marker gene expression is also Osxdependent. *P<0.05, unpaired Student's t-test, n=3. All values are means ± s.e. By contrast, continued activation of β-catenin signaling in Osx1 + cells (targeted by Osx1-Cre mice) promotes cell differentiation in Runx2 + /Osx1 + osteoblast precursor cells but arrests osteoblasts in a low Oc expressing stage (Rodda and McMahon, 2006). In the present studies, we found that nuclear β-catenin protein levels are increased and osteoblast proliferation and differentiation are accelerated in Axin2 KO mice. The results from the rescue experiments further demonstrate that activation of β-catenin signaling is the primary cause for the bone phenotype of the Axin2 KO mice. These results suggest that Axin2 regulates osteoblast function through β-catenin signaling in BMS cells, mainly through osteoblast precursor cells.

Axin2 promotes osteoclast formation
In the present studies, we found that Opg expression is increased in Axin2 KO mice. By contrast, the expression of Rankl is not significantly altered. As a consequence, osteoclast formation is reduced in Axin2 KO mice. Although it seems that changes in osteoblast function are more pronounced than those in osteoclast function, the decreased osteoclast formation might also contribute to the bone phenotype observed in Axin2 KO mice. In mature osteoblasts, β-catenin specifically activates Opg expression and inhibits osteoclast formation (Holmen et al., 2005;Glass et al., 2005). In the present studies, we observed a significant increase in Opg expression and a decrease in osteoclast formation in BMS cells of Axin2 KO mice. To further investigate whether Axin2 directly targets mature osteoblasts to regulate osteoclast formation and bone mass, we have recently generated 2.3Col1-Axin2 transgenic mice in which the expression of an Axin2 transgene is driven by the 2.3 kb mouse type I collagen promoter. The bone phenotype of these mice (two independent lines) was analyzed by micro-CT and histological methods. We found that there were no significant changes in bone mass, bone microstructure and bone mechanical properties in 3-month-old 2.3Col1-Axin2 transgenic mice compared with the WT littermates (supplementary material Fig. S2). These results suggest that Axin2 might be mainly targeting osteoblast progenitor and precursor cells, instead of mature osteoblasts.

Axin2 inhibits Bmp2 and Bmp4 expression in osteoblasts
In these studies, we found that mRNA expression of Bmp2 and Bmp4 and levels of phosphorylated Smad1/5 proteins are significantly increased and addition of noggin, an antagonist of BMP-2 and BMP-4, completely blocks elevated phosphorylated Smad1/5 levels in Axin2 KO BMS cells. Our in vitro studies further demonstrate that the GSK-3β inhibitor BIO induces osteoblast-specific transcription factor Osx and osteoblast marker genes in a Bmp2/4-dependent manner. These findings suggest that Bmp2 and Bmp4 are downstream mediators of Axin2/β-catenin signaling in osteoblast precursor cells and that the accelerated osteoblast differentiation observed in Axin2 KO mice might be the result of the activation of BMP signaling in osteoblast precursor cells. Consistent with these findings, in a separate study, we found that Wnt3a induces the expression of hypertrophic marker gene type X collagen and this effect is completely blocked by addition of noggin to the cell cultures of primary chondrocytes , suggesting that the BMPs are downstream mediators of canonical Wnt signaling in chondrocytes. Previous studies have shown that Axin2 KO mice have cranial bone defects, a phenotype resembling craniosynostosis in humans (Yu et al., 2005). Further, BMP signaling is activated in calvarial osteoblasts of Axin2 KO mice (Liu et al., 2007). However, it remains to be determined whether osteoblast differentiation induced by Axin2 deletion (or activation of β-catenin) is totally dependent on the activation of BMP signaling. Our evidence that deletion of the Bmp2/4 genes abrogates the BIO-induced Osx and subsequent osteoblast marker gene expression suggests that Axin2 deletion might trigger the β-catenin-Bmp2/4-osterix signaling pathway in osteoblast precursor cells.
It has been reported that β-catenin activates the bone-specific transcription factor Runx2 in osteoblasts and chondrocytes (Gaur et al., 2005;Dong et al., 2006). In the present studies, we found that both Runx2 and Osx expression were increased in Axin2 KO mice and that treatment with BIO in calvarial osteoblasts enhanced the expression of Runx2, Osx and other osteoblast marker genes.
To determine whether BIO-induced osteoblast marker gene expression is Bmp2/4-dependent, primary calvarial osteoblasts were isolated from Bmp2 fx/fx ;Bmp4 fx/fx mice and infected with Ad-Cre and treated with BIO. We found that BIO-induced Runx2 expression was not affected by deletion of the Bmp2/4 genes. By contrast, deletion of the Bmp2/4 genes significantly inhibited the BIO-induced Osx expression and completely abolished BIO-induced expression of other osteoblast marker genes. We further demonstrated that Osx siRNA also inhibited BIO-induced osteoblast marker gene expression. These observations suggest that BIO (or activation of β-catenin signaling) might stimulate osteoblast differentiation through a Bmp2/4-Osx pathway.

Axin2 inhibits Smad1/5 phosphorylation in osteoblasts
In the present studies, we found a significant increase in the levels of phosphorylated Smad1/5 in BMS cells derived from Axin2 KO mice. A possible mechanism for this could be increased Smad1/5 phosphorylation or decreased Smad1/5 dephosphorylation. Three different types of phosphatases have recently been identified to specifically induce Smad1/5 dephosphorylation. These are pyruvate dehydrogenase phosphatase 1 and 2 (PDP1 and PDP2), small Cterminal domain phosphatase (SCP) and magnesium-dependent protein phosphatase 1α (PPM1α) (Chen et al., 2006;Knockaert et al., 2006;Duan et al., 2006). We have obtained expression plasmids of these phosphatases from different collaborators and tested the effects of Axin2 on Smad1/5 dephosphorylation induced by these phosphatases. We found that PPM1A is the most potent phosphatase for inducing Smad1/5 dephosphorylation in BMS cells. However, Axin2 (or Axin1) could not prevent or delay Smad1/5 dephosphorylation in BMS cells (supplementary material Fig. S3). It has been recently reported that GSK-3β can phosphorylate the linker domain of Smad1, leading to Smad1 polyubiquitylation (Fuentealba et al., 2007). We have looked at whether Axin2 regulates Smad1 levels through affecting the phosphorylation of the linker domain of Smad1 and found that Axin2 has no effect on Smad1 linker domain phosphorylation in 2T3 cells (supplementary material Fig. S4). In addition, the total Smad1 protein levels were not changed in Axin2 KO BMS cells. These results demonstrate that the increase in Smad1 phosphorylation level is due to increased Bmp expression but not to a decrease in Smad1 dephosphorylation or linker domain phosphorylation of Smad1.

Conclusions
In summary, our findings demonstrate for the first time that Axin2 is a crucial negative regulator of bone remodeling in adult mice. Deletion of the Axin2 gene causes an age-dependent high-bonemass phenotype. In Axin2 KO mice, β-catenin signaling is activated; osteoblast proliferation and differentiation are accelerated; and Opg expression is increased and leads to decreased osteoclast formation. Axin2 controls bone remodeling We also discovered that BMPs are downstream mediators of Axin2/β-catenin signaling in osteoblast precursor cells, suggesting an important cross-talk between β-catenin and BMP signaling in the regulation of osteoblast differentiation. Our findings suggest that Axin2 might be an important therapeutic target for modulation of bone homeostasis.

Micro-CT analysis
Quantitative analysis was performed in bone samples of femora from WT and Axin2 KO mice (6-and 12-month-old) at 10-μM resolution on a micro-CT Scanner (VivaCT 40; Scanco Medical AG, Bassersdorf, Switzerland) (Hildebrand et al., 1999). Briefly, scanning in the femora began approximately at the lower growth plate and extended proximally for 350 slices (each 10-μm thick). Morphometric analysis was performed on 150 slices extending proximally, beginning with the first slice in which the femoral condyles have fully merged. The trabecular bone is segmented from the cortical shell manually on key slices using a contouring tool, and the contours are morphed automatically to segment the trabecular bone on all slices. The three-dimensional structure and morphometry was constructed and analyzed. These parameters include BV/TV (%), BMD (mg HA/mm 3 ), Tb.N. (mm -1 ), Tb.Th. (mm) and Tb.Sp. (mm) and SMI. Tb.Th., Tb.N. and Tb.Sp., are computed using algorithms that do not rely on assumptions about the underlying trabecular structure. Micro CT imaging was also performed in the mid diaphysis of the femur. Midshaft evaluation of 100 slices (1.0 mm) was performed to quantify the cross-sectional area, maximum and minimum bending moments of inertia, cortical thickness and maximum and minimum external radii, and cortical bone mineral density.

Mechanical testing
Changes in mechanical properties of the femoral shaft were determined using threepoint bending mechanical testing. Testing was performed using an Instron Dynamite 8841 servo-hydraulic materials testing device equipped with a 1 kN load cell, with control and data collection using Bluehill software version 1.8 (Instron, Norwood, MA). Prior to testing, each femur was harvested and cleaned of soft-tissue and soaked in PBS for 3 hours to ensure hydration (Broz et al., 1993). The anterior surface of each femur was placed on the two support beams, which were separated by an 8mm span. The femora were bent about the medial-lateral axis at a rate of 5 mm/minute (Jamsa et al., 1998). Force versus deformation data were collected and analyzed to calculate the bending stiffness (N/mm), yield bending moment (N mm), maximum bending moment (N mm) and toughness to maximum bending moment (N mm).

Histological and histomorphometric analysis
Detail methods for histological and histomorphometric analyses have been described previously (Zhao et al., 2002;Zhao et al., 2004). Histomorphometric analyses were performed on proximal tibial metaphyses in 6-month-old mice. The trabecular bone volume was quantified in the entire area of the bone marrow cavity 0.4-1.4 mm underneath the growth plates and expressed as a percentage of tissue volume. Histomorphometric analysis was performed using the OsteoMeasure system (OsteoMetrics, Atlanta, GA).

In vitro osteoclast formation assay
Bone marrow cells were isolated from femur and tibia of 6-month-old Axin2 -/mice and WT littermates and cultured with minimum essential medium (α-MEM) supplemented with 10% fetal bovine serum (FBS) in six-well culture plates and treated with M-CSF (30 ng/ml) (R&D) for 3 days and then cultured in 96-well culture plates at the density of 3ϫ10 5 /ml (200 μl per well). Cells were then treated with M-CSF (30 ng/ml) and RANKL (30 ng/ml) (R&D) for additional 4 days. Tartrate-resistant acid phosphatase (TRAP) staining was performed and the numbers of TRAP-positive multi-nuclear (Ն3 nucleus) osteoclasts were counted.
Ki-67 staining BMS cells isolated from 6-month-old WT and Axin2 -/mice were plated in a fourwell chamber slide at a density of 1ϫ10 6 cells per well and cultured with α-MEM supplemented with 10% FBS at 37°C for 6 days. The cells were fixed with cold methanol for 5 minutes, endogenous peroxidase quenched with 3% hydrogen peroxide for 20 minutes to block the non-specific binding site and then blocked with 1:20 normal goat serum for 20 minutes. The slides were incubated with 1:20 dilution of rabbit anti-Ki-67 antibody (Lab Vision Corp.) overnight at 4°C and then with 1:200 goat anti-rabbit IgG (Cell Signaling) for 30 minutes, and with 1:250 horseradish peroxidase streptavidin for 30 minutes. The slides were treated with deionized water mixed with 1 drop of Tween 20, and the color reaction with Romulin AEC Chromagen detected for 3-5 minutes. The hematoxylin counterstain was then performed. The nuclei of Ki-67-positive cells showed a dark brown color.

ALP activity and nodule formation
BMS cells or primary calvarial osteoblasts were fixed in 10% formalin. ALP staining was performed by 0.1% fast red salt (Sigma-Aldrich) (buffered with 0.05% Napthol AS-MX phosphate disodium salt, 2.8% N,N-dimethyl formamide, 0.1 M Tris maleate buffer, pH 8.4) at 37°C for 30 minutes. The assays for ALP activity and nodule formation were described in previous studies (Zhao et al., 2002;Zhao et al., 2004).

Lenti-Cre infection and Zeocin selection
HFPW-Cre and lentivirus packaging vectors were kindly provided by Eric Brown (University of Pennsylvania, Philadelphia, PA). Lenti-Cre virus was produced in the HEK 293FT cell line with the Virapower Trex Lenti-viral Expression System (Invitrogen) according to the manufacturer's protocol. Lenti-Cre condition media were kept at -80°C until used. BMS cells were infected with Lenti-Cre condition media for 7 days and then cultured with α-MEM containing Zeocin (Invitrogen, 20 μg/ml). At the end of the cell culture, mRNA was extracted and expression of osteoblast marker genes was examined by real-time PCR.
In vivo periosteal bone formation assay BIO and fibroblast growth factor (FGF-1) were obtained from Calbiochem. FGF-1 was dissolved in PBS containing 0.1% BSA and was used as a positive control. BIO was dissolved in Corn Oil (Sigma) at 1 mg/ml. BIO was injected subcutaneously over the parietal bone of the calvariae of one-month-old C57BL/6J WT mice. Mice received either vehicle (corn oil alone) or BIO (25 and 50 μg/mouse) and FGF-1 (2 μg/mouse) daily for 5 days. Calcein (20 mg/kg i.p.) labeling was performed at days 7 and 14 and mice were sacrificed 2 days after the second label. At the end of the experiments, calvarial bones were removed, fixed in 75% ethanol, and embedded in plastic. Transverse sections were cut at 3-μm thickness and unstained sections were viewed under fluorescent microscope. Mineral appositional rates (MAR) were measured using the OsteoMeasure system (OsteoMetrics, Atlanta, GA).

Statistical analysis
For comparisons between two data groups the unpaired Student's t-test was used. For multiple comparisons between more than two groups, data was analyzed by oneway analysis of variance (ANOVA), followed by Dunnett's test. Differences were considered significant when a P value was less than 0.05.
This publication was made possible by Grant Numbers R01 AR051189, K02 AR052411, R01 AR054465 and R01 AR055915 from National Institute of Health (to D.C.) and Grant Number N08G-070 from New York State Department of Health and the Empire State Stem Cell Board (to D.C.). J.H.J. was sponsored by NIAMS T32-AR053459 training grant. We would like to thank Xinhua Feng (Baylor College of Medicine, Houston, TX) for providing the PPM1A expression plasmid and Edward De Robertis (University of California, Los Angeles, CA) for providing us the anti-pSmad1 GSK antibody. Authors have no conflict of interest. Deposited in PMC for release after 12 months.