New Aspect of Bone Morphogenetic Protein Signaling and Its Relationship with Wnt Signaling in Bone

Bone morphogenetic proteins (BMPs) were discovered and named in 1965 by Marshall Urist, who initially identified the ability of an unknown factor in bone to induce ectopic bones in muscle 1. In the last 45 years, the osteogenic function of BMPs has been extensively examined, mainly using osteoblasts in culture with exogenous treatments of BMPs 2. Based on their potent osteogenic abilities, clinical trials have been initiated to use BMP2 and BMP7 to improve fracture repair 2. The FDA (Food and Drug Administration) has approved BMP2 and BMP7 for clinical use in long bone open-fractures, non-union fractures and spinal fusion. However, recent clinical/pre-clinical studies have shown a negative impact of BMPs on bone formation under certain physiological conditions 3-7, challenging the current dogma. This book chapter will focus on the recent findings of roles of BMP signaling in bone including its relationship with Wnt signaling through Wnt (Wngless, Int-1) receptor SOST (Sclerostin) and DKK1 (Dickkopf1). This new molecular interaction would explain the negative outcomes of BMP’s therapy in orthopaedics.

pathways through p38 MAPK (mitogen-actiated protein kinase) and TAK1 (Transforming growth factor β-activated kinase 1) are also involved in the BMP signaling 15 .There are three type I receptors [BMPRIA (BMP receptor type IA, ALK3), BMPRIB (BMP receptor type IB, ALK6) and ACVRI (Activin receptor type I, ALK2) and three type II receptors [BMPRII (BMP receptor type II), ACVRIIA (Activin receptor type IIA) and ACVRIIB (Activin receptor type IIB)], and approximately 30 ligands are identified 16 .Type I receptor ACVRI was originally described as an activin receptor, but it is now believed to be a receptor for BMPs.In osteoblasts, BMP2, BMP4, BMP6 and BMP7 and their receptors BMPRIA and ACVRI are abundantly expressed 17 .BMPRIA is a potent receptor of BMP2 and BMP4 18,19 , as is ACVRI for BMP7 20 .In addition, BMP antagonists Noggin, Chordin, and Gremlin were identified in osteoblasts 21 .These antagonists fine-tune BMP signaling in osteoblasts, as BMPs upregulate expression levels of antagonists while inducing BMP signaling 22 (Table 1, Figure 1).Fig. 1.Potential molecular interaction of BMP signaling in osteoblasts.BMP2, BMP4, BMP6, and BMP7 are osteoinductive and are expressed by osteoblasts.BMP2 and BMP4 are potent ligands for BMPRIA as are BMP6 and BMP7 for ACVRI.Canonical BMP signaling is through the Smad pathway via Smad1, Smad5, and Smad8 (i.e.Smad1/5/8-Smad4 complex), while non-canonical BMP signaling is through non-Smad pathways including TAK1 and p38 MAPK.Target genes are activated by these two pathways in osteoblasts.Co-Smad Smad4 Non-Smad Pathways p38 MAPK, TAK1 Table 1.Osteogenic BMPs and their signaling cascades in osteoblasts

Molecular interaction of BMP and Wnt
In addition to BMP signaling, Wnt signaling has been examined for a decade because of its role in bone formation and bone mass [23][24][25][26][27] .The physiological impact of Wnt signaling on bone mass was first reported in 2001, by showing that loss-of-function mutations in the coreceptor LRP5 (Low-density lipoprotein receptor-related protein 5) cause the autosomal recessive disorder osteoporosis-pseudoglioma syndrome (OPPG), a low bone mass phenotype in humans 28 .The importance of other Wnt ligands and receptors as bone mass effectors has been documented using genetic approaches for DKK1 29 , DKK2 30 , sFRPs (secreted frizzled-related proteins) 31 , Sost/sclerostin 32 , Lrp5 33,34 and Lrp6, all of which are expressed in osteoblasts.However, changes in BMP signaling in bone had not been reported in Wnt-related mutations in mice.

In vitro relationship
In vitro experiments using pluripotent mesenchymal cell lines or primary osteoblasts to test the interaction between BMP and Wnt signaling in osteoblasts have yielded both synergistic and antagonistic results: the treatment of C2C12 cells and primary osteoblasts with BMP2 induced Wnt3a expression and stabilized Wnt/β-catenin signaling [35][36][37] .The treatment of C3H10T1/2 cells with Wnt3a induced the BMP4 expression levels 38 .These suggest a positive autocrine loop 37,39 .In contrast, inhibition of BMP signaling by treatment of primary osteoblasts with dorsomorphin, an inhibitor of BMP type I receptors, increased canonical Wnt signaling 40 .Treatment of C2C12 cells with Wnt3a repressed BMP2-dependent Id1 (Inhibitor of DNA binding 1) expression 41 .Similarly, treatment of cultured skull bone with BMP antagonist Noggin increased canonical Wnt signaling 42 .Moreover, one study investigated intracellular cross-talk between BMP and Wnt pathways using uncommitted bone marrow stromal cells and provided a potential mechanism whereby BMP-2 antagonizes Wnt3a-induced proliferation in osteoblast progenitors by promoting an interaction between Smad1 and Dvl-1 [i.e. the human homolog of the Drosophila dishevelled gene (dsh) 1] that restricts Wnt/β-catenin activation 43 .Another interaction via Pten (phosphatase and tensin homolog)-Akt pathway has been reported in hair follicle stem/progenitor cells 44 ; however, it is less likely in osteoblasts 45 .Taken together, there seems to be both positive and negative feedback loops between the two signaling pathways (Figure 2).
Fig. Further studies are desired to investigate the details of each condition.

In vivo relationship
In vivo, only a few studies have revealed a link between the two signaling pathways.We recently found that loss-of-function of BMP signaling in osteoblasts via BMPRIA upregulates canonical Wnt signaling during embryonic and postnatal bone development, suggesting a negative regulation of Wnt signaling by BMP 40,42 .In these studies, we found that upregulation of Wnt signaling is at least in part mediated by suppression of Wnt inhibitors Sost/sclerostin and Dkk1, and both Sost/sclerostin and Dkk1 are direct targets of BMP signaling.In addition, Sost expression was severely downregulated in Bmpr1a-deficient bones as assessed by microarray analysis 42 .Interestingly, both Smad-dependent and Smadindependent pathways appear to contribute to the Dkk1 expression, whereas Sost/sclerostin requires only Smad-dependent signaling, suggesting differential regulation of these genes by the BMP signaling via BMPRIA 40 .BMP and Wnt signaling regulate the development and remodeling of many tissues and interact synergistically or antagonistically in a context-and age-dependent manner in vivo 46,47 .It is possible that in bone, BMP signaling inhibits Wnt signaling by upregulating the Sost/sclerostin expression in osteoblasts (Figure 3).

SOST/Sclerostin and DKK1
Both SOST and DKK1 are inhibitors for canonical Wnt signaling and have been highlighted because neutralizing antibodies for SOST (AMG785) and DKK1 (BHQ880) have been developed as bone anabolic agents and these potential drugs are under clinical trial 48 .It is known that both Dkk1 and Sost/sclerostin inhibit Wnt/β-catenin signaling by binding to coreceptors.As both Dkk1 and Sost/sclerostin are secreted proteins expressed by osteoblasts, their role in regulating bone mass has been investigated using human and mouse genetic approaches.

BMP signal
Wnt signal  The BMP signaling upregulates the Sost expression primarily through the Smad-dependent signaling while it upregulates the Dkk1 expression through both the Smad and non-Smad signaling (p38 MAPK).As Dkk1 and sclerostin/Sost act as Wnt signaling inhibitors, BMP signaling in osteoblasts, in turn, leads to a decrease in osteogenesis and bone mass.Dkk1 and sclerostin/Sost play an important role in regulating bone mass as downstream effectors of BMPRIA signaling in bone taking balances between BMP signaling and Wnt signaling.

SOST/Sclerostin
Sost/sclerostin was originally reported as a member of the BMP antagonist DAN family (i.e. the Dan gene family of BMP antagonists) 49,50 .Although DAN family members modulate both BMP and Wnt signaling in Xenopus [51][52][53] , recent studies suggest a primary role of Sost/sclerostin in Wnt signaling in mouse and humans: Sost/sclerostin is not a BMP antagonist 54 but rather a Wnt inhibitor 55 that binds the Wnt co-receptor low density lipoprotein receptor-related protein 5 and 6 (LRP5 and LRP6) 32,56 .Conventional knockouts of Sost (i.e.Sost KO) are viable and exhibit increased bone mass 57 .In humans, loss-offunction and hypomorphic mutations in SOST cause sclerosteosis 58,59 and Van Buchem disease 60,61 , respectively, with a high bone mass (HBM) phenotype.These mutants share the HBM phenotype with other gain-of-function of LRP5 mutation, due to the defect in DKK1mediated regulation of LRP5 in humans [62][63][64] and overexpression of Lrp5 in mice 65 .In contrast, loss-of-function of LRP5 leads to OPPG with low bone mass 28 , which is similar to the bone phenotype of mice overexpressing Sost 50 .In addition, recent genome-wide SNPbased analyses identified a significant association between bone mineral density and the SOST gene locus [66][67][68] .

DKK1
Conventional knockouts of Dkk1 die in utero from defective head induction and limb formation 29 .Similar to Sost KO mice, mice heterozygous for Dkk1 (Dkk1 +/-mice), however, exhibit a high bone mass (HBM) phenotype 69 , while overexpression of Dkk1 in osteoblasts causes osteopenia 70 .In addition, increased DKK1 expression in bone marrow has also been associated with lytic bone lesions in patients with multiple myeloma 71 .Collectively, these results support the hypothesis that Dkk1 functions as a potent negative regulator of bone mass.

Sost/DKK1 expression in the Bmpr1a cKO mice
Conditional knockouts of Bmpr1a, which are deficient in the Dkk1 and Sost expression, show a HBM phenotype 40,42,72 .In particular, Sost expression levels were the most dramatically reduced in the cKO mice during embryonic stages 42 .Furthermore, both Sost and Dkk1 expression levels were increased by the addition of BMP2, a potent ligand for BMPRIA, using primary osteoblasts 40 .Similarly, both Sost and Dkk1 expression levels were significantly reduced in the Acvr1 cKO mice 73 .In addition, both Sost and Dkk1 expression levels were increased by the addition of BMP7, a potent ligand for ACVRI, using primary osteoblasts 73 .These facts support the new concept of molecular interactions between BMP signaling and Wnt signaling that Dkk1 and Sost/sclerostin act physiologically as inhibitors of canonical Wnt signaling as downstream targets of BMP receptors BMPRIA and ACVRI and that BMP signaling can negatively controls Wnt signaling in osteoblasts (Figure 3).

Effects of Wnt signaling on osteoclasts
There is accumulating evidence that Wnt signaling also plays a critical role in osteoclastogenesis regulated by osteoblasts through the RANKL (Receptor activator of nuclear factor kappa-B ligand)-OPG (Osteoprotegerin) pathway.Recently, two in vivo studies have suggested that the canonical Wnt signaling is important in the regulation of osteoclastogenesis by osteoblasts.One study provided evidence that the Wnt pathway positively regulates the expression of Opg in osteoblasts 74 .Overexpression of stabilized catenin in osteoblasts, which results in an increase of canonical Wnt signaling level, decreases osteoclast differentiation leading to increased bone volume in mice 74 .Another study showed that an osteoblast-specific deletion of -catenin leads to an impaired maturation and mineralization of bones in mice due to the elevated expression of Rankl and diminished Opg 75 .These facts suggest that the canonical Wnt pathway negatively regulates osteoblasts in their supporting function in osteoclastogenesis, and thus upregulation of Wnt signaling in osteoblasts can suppress osteoclast-mediated bone resorption 75 .Taken together, it is possible that the treatment of bones with BMPs can reduce Wnt activity in osteoblasts and in turn enhance osteoclast activity.

BMP signaling and mouse genetics
Along with the huge advancement in technologies involving mouse genetics over the last decade, many of the BMP signaling related genes have been knocked out in mice.BMP2, BMP4, BMP6 and BMP7 and their receptor BMPRIA and ACVRI are abundantly expressed in bone.However, conventional knockout mice for these genes result in an early embryonic lethality and thus, it is not possible to investigate bone development and remodeling using these models [76][77][78][79][80][81][82] .To avoid the embryonic lethality, a strategy of conditional knockout mice using a Cre-loxP system has been employed.A bone-specific conditional deletion of Bmpr1a using an Og2-Cre mouse, in which a Cre recombination is restricted in differentiated osteoblasts under the osteocalcin promoter, was first reported in 2004 83 .Interestingly, this study demonstrated that the response of osteoblasts to BMP signaling is age-dependent; in the mutant mice, bone volume decreased in young mice but increased in aged mice.In addition, the activity of osteoclasts was reduced in the aged osteoblast-specific Bmpr1adeficient mice, which may have lead to the complex skeletal phenotype.These facts suggest that the BMP signaling in differentiated osteoblasts can control the balance between bone formation by osteoblasts and resorption by osteoclasts, thereby affecting the final outcome of the amount of bone mass in an age-dependent manner.The increased bone mass in the Bmpr1a-deficient mice appeared to be in opposition to the general concept of BMPs as osteogenic inducers; however, the concept is reasonable if the target cell for BMPs as osteogenic inducers is mesenchymal cells or chondrocytes,.It is expected that BMPs have multifaceted functions in vivo because different cell types exhibit differing responses to BMPs.In addition, the opposite outcome in the Bmpr1a-deficient mice was discussed from the point of molecular interaction in the sections 3.

BMP signaling in chondrocytes, mesenchymal cells, and osteoblasts
During skeletogenesis, bones are formed via two distinct processes: intramembranous and endochondral bone formation 84 .Intramembranous bone formation occurs primarily in flat bones (e.g., calvarial bones) where mesenchymal cells differentiate directly into osteoblasts 85 .Endochondral bone formation occurs primarily in long bones where condensed mesenchymal cells differentiate into chondrocytes to form cartilage templates, and then chondrocytes are replaced by osteoblasts 86 .Recently many studies have been designed to investigate the difference in the molecular mechanism by which BMP signaling regulates these cell types.Several Cre mouse lines have been used to target different cell types including osteoblast, chondrocyte, and mesenchymal cells (Table 2).BMP signaling in chondrocytes and mesenchymal cells both positively control bone size and mass while BMP signaling in osteoblasts can reduce them.

Chondrocytes
There are several lines of evidence that show that BMP signaling in chondrocytes is required for bone size and the amount of bone mass.BMP signaling through BMPRIA is essential for postnatal maintenance of articular cartilage, using a Gdf5-Cre mouse line specific for chondrocytes in joints 87 .0 .In parallel, studies focusing on BMP ligands and their antagonists provide further evidence that BMPs are critical for normal development of cartilage.A transgenic mouse line to overexpress Bmp4 in mesenchymal cells/chondrocytes using a type XI collagen promoter (Col11a2) was generated, and bone mass was increased in the mutant mice 89 .Another transgenic mouse line in which Noggin was overexpressed in the same cells (Col11a2-Noggin) demonstrated a decreased bone mass.As Noggin is an antagonist for BMPs (BMP2, BMP4, BMP5, BMP6, and BMP7) with various degrees of affinity 95 , these results suggest that BMP signaling positively controls proliferation and differentiation of chondrocytes.

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New Aspect of Bone Morphogenetic Protein Signaling and Its Relationship with Wnt Signaling in Bone 291

Mesenchymal cells
Similar to chondrocytes, a few studies demonstrated a requirement of BMP signaling in mesenchymal cells for proper bone development and remodeling using a mesenchymal cellspecific Cre mouse line, Prx1-Cre, in which Cre is active in mesenchymal cells as early as embryonic day 9.5 96 .Using the Prx1-Cre mouse, the simultaneously conditional deletions of Bmp2 and Bmp4 in mesenchymal cells resulted in an impairment of osteogenesis during late embryogenesis 91,92 .In contrast, the conditional deletion of Bmp2 in mesenchymal cells does not show overt developmental abnormalities; however, the resulted mice lack an initiation of fracture healing 91,92 .Interestingly, Bmp7-deficiency in mesenchymal cells did not affect bone mass probably due to the compensation by Bmp4 97 .Taken together, it is possible that the defects in the BMP signaling in chondrocytes largely contribute to the phenotypes described above because chondrocytes are derived from mesenchymal cells and play an important role in the process of fracture repair.

Osteoblasts
As aforementioned, a differentiated osteoblast-specific deletion of Bmpr1a caused an increase in bone mass in aged mice 83 .Similar to this finding, an overexpression of a BMP antagonist, Noggin, in osteoblasts increases bone volume with a reduced osteoclast number and osteoclastogenesis both at embryonic day 17.5 (E17.5) and at 3 weeks 93 .In parallel, the overexpression of Bmp4 in osteoblasts reduced bone mass presumably due to the increase in the osteoclast number at E18.5 93 .Recently, Bmpr1a was conditionally disrupted in immature osteoblasts using a tamoxifen inducible Cre driven by a 3. bone remodeling stage at 22 weeks as well as the bone developmental stages at E18.5 and 3 weeks 42,72 (Figure 4A).This result is an interesting contrast to previous work that disruption of Bmpr1a in differentiated osteoblasts results in decrease of bone mass in young adult stages (3-4 weeks).The increased bone mass in the Bmpr1a-deficient mice resulted from severely suppressed bone resorption due to reduced osteoclastogenesis, despite a simultaneous small reduction in the rate of bone formation 72 .Levels of RANKL and OPG are changed in the Bmpr1a-deficient osteoblasts and fail to support osteoclastogenesis 42,72 .
In addition, the conditional disruption of Acvr1 in osteoblasts also demonstrated a dramatic increase in bone mass, similar to the bone phenotype of Bmpr1a-deficient mice (Figure 4B), although osteoclastic activity is still under investigation 73 .These findings suggest that BMP may have dual roles in osteoblasts; to stimulate both bone formation by osteoblasts and bone resorption supporting osteoclastogenesis.Disruption of BMP signaling in immature osteoblasts alters the balance of bone turn over to increase the bone mass, which is opposite to what people have expected for the past 4 decades.

Other cell type
Angiogenesis is another necessary step in new bone formation in skeletal development as well as in bone remodeling after fracture 98,99 .Both BMP2 and BMP7 are known to induce angiogenesis by associating with other growth factors such as VEGF (vascular endothelial growth factor), bFGF (basic fibroblast growth factor), and TGF-1 100 .A study using an adenovirus vector in muscle demonstrated that BMP9 induces ectopic bone formation similar to BMP2 101,102 .As BMP9 is abundantly expressed in endothelial cells that are primarily cell types for angiogenesis 103 , it is possible that BMP signaling in endothelial cells synergizes anabolic bone formation.The mechanism and origin of precursor cells for ectopic bone formation, which is physiologically observed in the patients with FOP (fibrodysplasia ossificans progressiva), is under investigation [104][105][106] but could be endothelial cells 107 .

Possible interpretation
Mesenchymal cells, chondrocytes, and endothelial cells respond to BMPs by inducing bone mass and size (Table 3).Recent histological findings suggest that the process of endochondral bone formation, which first forms cartilage template prior to the final bone following vessel formation (i.e.angiogenesis), plays a critical role in the process of ectopic bone formation 108 .The origin of precursor cells for the ectopic bone is under investigation 105, 106 ; however, it is possible that formation of ectopic bones by BMPs 1 is largely due to the stimulation of chondrocytes, mesenchymal cells, and/or endothelial cells in soft tissue, which results in an expansion of ectopic cartilage subsequently replaced by osteoblasts.There is another possibility that the BMP signaling directly affects osteoblasts to form ectopic bone.However, this possibility is less likely based on recent evidence that reduced

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BMP signaling in osteoblasts results in an increase in bone mass.As current methods of systemic and local treatment affect multiple cell types simultaneously in bone, it is important to evaluate the effects of BMPs on more than just osteoblasts.

Effect of BMP signaling on osteoclasts
Bone mass is determined by the balance between bone formation and bone resorption.Osteoclasts are multinuclear cells derived from hematopoietic stem cells to secrete enzymes for bone resorption 109 .Recent mouse genetic studies revealed the importance of BMP signaling for osteoclastic activity and bone resorption.

Regulation of osteoclast by osteoblast-dependent BMP signaling
It is expected that BMPs play roles in osteoclastogenesis and their functions, because receptors for BMPs are expressed in these cells 110 .Additionally, osteoblasts also play critical roles in bone resorption by regulating osteoclastogenesis because they produce RANK ligand (RANKL), essential to promote osteoclastogenesis, and its decoy receptor, osteoprotegerin (OPG) 111,112 .A balance between RANKL and OPG is important to determine the degree of osteoclastogenesis, i.e. more RANKL production by osteoblasts leads to more osteoclasts; thus more bone resorption is expected.As RANKL is an osteoblastic product and BMPs induce osteoblast maturation, BMPs indirectly stimulate osteoclastogenesis and thus, osteoclastogenesis is impaired when osteoblastogenesis is blocked with BMP antagonists in culture 113 .The physiological effects of BMP signaling in osteoblasts on osteoclastogenesis were determined later using an osteoblast-specific gain-of-function or loss-of-function mouse model.For the cases of the osteoblast-specific deletion of Bmpr1a and osteoblast-specific over expression of Noggin, osteoclastogenesis is highly compromised leading to an increase of bone mass 83,93 .In contrast, osteoblast-specific overexpression of Bmp4 increased osteoclastogenesis 93 .The regulation of RANKL by BMPs was suggested based on an in vitro study 114 .This concept was recently proven in mouse studies, as Bmpr1a-deficient osteoblasts were not able to support osteoclastogenesis due to an imbalance between RANKL and OPG 42,72 .It is therefore concluded that osteoblasts can respond to BMPs by inducing osteogenic (i.e.bone anabolic) action as well as osteoclastogenic (i.e.bone catabolic) action simultaneously presumably dependening on context and timing (Table 3).

Regulation of osteoclast by osteoclast-dependent BMP signaling
BMP receptors are expressed in osteoclasts 110 .When BMP signaling through BMPRIA was deficient in osteoclasts using a Catepsin K promoter (CtsK), bone mass was increased as expected 94 (Table 2).Interestingly, both bone formation rate and osteoblast number assessed by bone histomorphometry analysis were increased while osteoclast number was reduced in the mutant mice compared to their controls.It is possible that some coupling factors can control osteoblast function in an osteoclast-dependent manner in the mutant mice (i.e.osteoclast-derived coupling factors).Further studies are needed to determine whether such factors mediate BMPRIA-induced coupling from osteoclasts to osteoblasts.

Future direction of BMPs and Wnt
As is discussed in the former part of this review, it is important to understand that BMPs have variable and context-sensitive effects on diverse cell types in bone including chondrocytes, osteoblasts, and osteoclasts.Studies focusing on BMP receptors in chondrocytes including mesenchymal cells suggest that these cells can respond to BMP signaling by increasing bone mass during the endochondral formation process.As discussed in the latter part, BMP signals can consistently inhibit Wnt signaling and bone mass while exerting concordant effects on Dkk1 and Sost.This revision of traditional understanding of the BMP signaling pathway in clinical therapeutics might suggest that in some circumstances, BMP inhibition would be desirable for promoting bone mass.More importantly, if BMP signaling reduces bone mass by inhibiting Wnt signaling through SOST/DKK1 in osteoblasts, small molecule antagonists for BMPs or BMP receptors can conversely increase bone mass and size.Therefore, development of these molecules would be a next step towards disease conditions in which bone mass is reduced such as osteoporosis and bone fracture.Although antibodies for SOST and DKk1 have been developed in order to increase bone mass, the small molecule antagonists which can be an upstream of SOST and DKK1 would be used as more potent therapeutic agents for osteoporosis.Last, the function of the BMP signaling in osteoclasts remains largely unknown in terms of coupling factors and merits future study, although the BMP signaling regulates osteoblast-dependent osteoclastogenesis via the RANKL-OPG pathway.

Conclusion
Understanding the complex roles of the BMP signaling pathway and its molecular interaction with other signaling pathway (i.e.Wnt) in a variety of cell-types in bone including chondrocytes, osteoblasts and osteoclasts, which contribute to normal physiological conditions (i.e.bone development, homeostasis, and remodeling) will not only help to improve current knowledge of the pathological conditions (i.e.bone fracture, osteoporosis, and other congenital and aging-related bone diseases) but may provide novel therapeutically useful strategies.

Fig. 3 .
Fig. 3. Possible regulation between BMP and Wnt in Osteoblasts.A proposed model of the relationship between the BMP signaling via BMPRIA and the canonical Wnt signaling in osteoblasts.Both Dkk1 and sclerostin/Sost are downstream targets of the BMP signaling.The BMP signaling upregulates the Sost expression primarily through the Smad-dependent signaling while it upregulates the Dkk1 expression through both the Smad and non-Smad signaling (p38 MAPK).As Dkk1 and sclerostin/Sost act as Wnt signaling inhibitors, BMP signaling in osteoblasts, in turn, leads to a decrease in osteogenesis and bone mass.Dkk1 and sclerostin/Sost play an important role in regulating bone mass as downstream effectors of BMPRIA signaling in bone taking balances between BMP signaling and Wnt signaling.

Fig. 4 .
Fig. 4. Increased bone mass in the osteoblast-specific conditional knockout (cKO) mice for BMP receptors BMPRIA or ACVRI at the adult stage.Bmpr1a or Acvr1 cKO mice were generated by crossing a floxed mouse line for Bmpr1a(Bmpr1afx/fx) or Acvr1(Acvr1fx/fx) with a transgenic mouse line harboring a tamoxifen-inducible Cre driven by a 3.2 kb mouse procollagen 1(I) promoter.The Cre recombination was induced specifically in the osteoblasts by 10 weeks of tamoxifen administration from 10 weeks after birth, and bones were removed at 22 weeks.Radiodensity of rib bones was assessed by X-ray.(A) The radiodensity was dramatically increased in the Bmpr1a cKO mice (Cre+, Bmpr1afx/fx) compared with controls (Cre-, Bmpr1afx/fx).(B) The radiodensity was dramatically increased in the Acvr1 cKO mice (Cre+, Acvr1fx/fx) compared with controls (Cre-, Bmpr1afx/fx).
2. A potential relationship between the two major signaling BMP and Wnt in osteoblasts based on in vitro studies.1) Both signaling pathways function in a positive loop.2) Both signaling pathways function in a negative loop.It is expected that these two signaling pathways may regulate each other in an age-dependent and context-dependent manner.

Table 2 .
. Moreover, in chondrocytes a simultaneous deficiency in Smad 1 and Smad 5, Bone mass observed in genetically engineered mutant mice of BMP signaling which are BMPs' downstream target molecules, reduces bone mass

Table 3 .
A variety of cell types in bone that mediate bone mass in response to BMPs www.intechopen.comNew Aspect of Bone Morphogenetic Protein Signaling and Its Relationship with Wnt Signaling in Bone