A role for the BMP antagonist chordin in endochondral ossification

Function of BMPs and BMP antagonists in adult bone

The expression and function of BMPs and BMPs in bone tissues have been studied for a long time because of their remarkable activities. However, their biological functions in normal bone remodeling in adults were not fully understood until recently. Advanced technologies using gene manipulation were used to study their roles in adulthood. In addition, findings of new BMP antagonists and the effect of Wnt-canonical pathways on bone features also provided new insights in bone studies.

Bone morphogenetic proteins and their antagonists

Skeletal homeostasis is determined by systemic hormones and local factors. Bone morphogenetic proteins (BMPs) are unique because they induce the commitment of mesenchymal cells toward cells of the osteoblastic lineage and also enhance the differentiated function of the osteoblast. BMP activities in bone are mediated through binding to specific cell surface receptors and through interactions with other growth factors. BMPs are required for skeletal development and maintenance of adult bone homeostasis, and play a role in fracture healing. BMPs signal by activating the mothers against decapentaplegic (Smad) and mitogen activated protein kinase (MAPK) pathways, and their actions are tempered by intracellular and extracellular proteins. The BMP antagonists block BMP signal transduction at multiple levels including pseudoreceptor, inhibitory intracellular binding proteins, and factors that induce BMP ubiquitination. A large number of extracellular proteins that bind BMPs and prevent their binding to signaling receptors have emerged. The extracellular antagonists are differentially expressed in cartilage and bone tissue and exhibit BMP antagonistic as well as additional activities. Both intracellular and extracellular antagonists are regulated by BMPs, indicating the existence of local feedback mechanisms to modulate BMP cellular activities.

Optimal compressive force induces bone formation via increasing bone morphogenetic proteins production and decreasing their antagonists production by Saos-2 cells

Orthodontic tooth movement induced alveolar bone resorption and formation around the teeth applied mechanical force. Although mechanical force can promote bone formation, the molecular mechanism that underlies this phenomenon is not fully understood. The purposes of this study were to determine how mechanical stress affects the osteogenic response of human osteoblastic cells (Saos-2), and also to examine the optimal compressive force for osteogenesis in vitro. Saos-2 cells were cultured with or without continuously compressive force (0.5-3.0 g/cm2). The expression of bone morphogenetic proteins (BMPs), their antagonists, and transcription factors which involved in osteogenesis were measured using real-time PCR and/or Western blot analysis. Phosphorylation of Smad1 was determined by Western blot. Loading with 1.0 g/cm2 of compressive force significantly increased the expression of BMPs, Runx2 and osterix. In contrast, the expression of BMP antagonists and AJ18 was decreased with 1.0 g/cm2 of compressive force. Loading with 1.0 g/cm2 of compressive force also induced phosphorylation of Smad1. Noggin inhibited the compressive force-induced phosphorylation of Smad1 markedly, and also partially blocked compressive force-induced Runx2 mRNA expression. Moreover, the conditioned medium from 1.0 g/cm2 of compressive force applied cells apparently increased calcium content in mineralized nodules of Saos-2 culture. This study demonstrates that an optimal compressive force stimulates in vitro mineralization via increasing BMPs production and decreasing their antagonists production.

The role of inhibitory molecules in fracture healing

The balance between all the signalling molecules involved in bone formation with their inhibitors and most importantly between BMPs and their antagonists is critical determinant of osteogenesis, and therefore of skeletal development, fracture repair, and bone remodelling. The main identified inhibitory molecules of the osteogenic lineage, either from studies during embryonic development or from in vitro and in vivo studies are presented in the herein study. Potential treatments using these molecules either alone or in combination with BMPs to control the bone growth and overgrowth are already under investigation aiming in treatments that mimic as much as possible the natural process of bone generation in various situations including fracture healing, osteoporosis, and osteoarthritis and other metabolic disorders, in order to more closely resemble the original tissue.

BMP signaling in the cartilage growth plate

Transforming growth factor-beta (TGF-beta) superfamily members play diverse roles in all aspects of cartilage development and maintenance. It is well established that TGF-betas and bone morphogenetic proteins (BMPs) play distinct roles in the growth plate. This chapter discusses key experiments and experimental approaches that have revealed these roles, and progress toward the identification of previously unsuspected roles. Current understanding of the mechanisms by which different TGF-beta and BMP pathways exert their functions is discussed. Finally attempts to utilize this information to promote cartilage regeneration, and important issues for future research, are outlined.

Heparan sulfate proteoglycans including syndecan-3 modulate BMP activity during limb cartilage differentiation

Bone morphogenetic proteins (BMPs) are involved in multiple aspects of limb development including regulation of cartilage differentiation. Several BMPs bind strongly to heparin, and heparan sulfate proteoglycans (HSPGs) at the cell surface or in the extracellular matrix have recently been implicated as modulators of BMP signaling in some developing systems. Here we have explored the role of HSPGs in regulating BMP activity during limb chondrogenesis by evaluating the effects of exogenous heparan sulfate (HS), heparitinase treatment, and overexpression of the HSPG syndecan-3 on the ability of BMP2 to modulate the chondrogenic differentiation of limb mesenchymal cells in micromass culture. Exogenous HS dramatically enhances the ability of BMP2 to stimulate chondrogenesis and cartilage specific gene expression, and reduces the concentration of BMP2 needed to stimulate chondrogenesis. Furthermore, HS stimulates BMP2-mediated phosphorylation of Smad1, Smad5, and Smad8, transcriptional mediators of BMP2 signaling, indicating that HS enhances the interaction of BMP2 with its receptors. Pretreatment of micromass cultures with heparitinase to degrade endogenous HSPGs also enhances the chondrogenic activity of BMP2, and reduces the concentration of BMP2 needed to promote chondrogenesis. Taken together these results indicate that exogenous HS or heparitinase enhance the chondrogenic activity of BMP2 by interfering with its interaction with endogenous HSPGs that would normally restrict its interaction with its receptors. Consistent with the possibility that HSPGs are negative modulators of BMP signaling during chondrogenesis, we have found that overexpression of syndecan-3, which is one of the major HSPGs normally expressed during chondrogenesis, greatly impairs the ability of BMP2 to promote cartilage differentiation. Furthermore, retroviral overexpression of syndecan-3 inhibits BMP2-mediated Smad phosphorylation in the regions of the cultures in which chondrogenesis is inhibited and in which ectopic syndecan-3 protein is highly expressed. These results indicate that syndecan-3 interferes with the interaction of BMP2 with its receptors, and that this interference results in an inhibition of chondrogenesis. Taken together these results indicate that HSPGs including syndecan-3 normally modulate the strength of BMP signaling during limb cartilage differentiation by limiting the effective concentration of BMP available for signaling.

BMP signaling stimulates cellular differentiation at multiple steps during cartilage development

Bone morphogenetic proteins (BMPs) play important roles at multiple stages of endochondral bone formation. However, the roles of BMP signaling in chondrocytes in vivo are still contentious. In the present study, we overexpressed a constitutively active BMP receptor 1A (caBmpr1a) in chondrocytes by using two systems: caBmpr1a was directly driven by a rat type II collagen promoter in a conventional transgenic system and indirectly driven in a UAS-Gal4 binary system. CaBmpr1a expression caused shortening of the columnar layer of proliferating chondrocytes and up-regulation of maturation markers, suggesting acceleration of differentiation of proliferating chondrocytes toward hypertrophic chondrocytes. In addition to the acceleration of chondrocyte differentiation, conventional transgenic mice showed widening of cartilage elements and morphological alteration of perichondrial cells, possibly due to stimulation of differentiation of prechondrogenic cells. Moreover, bigenic expression of caBmpr1a rescued the differentiation defect of prechondrogenic cells in Bmpr1b-null phalanges. This finding indicates that BMP signaling is necessary for phalangeal prechondrogenic cells to differentiate into chondrocytes and that signaling of BMP receptor 1B in this context is replaceable by that of a constitutively active BMP receptor 1A. These results suggest that BMP signaling in prechondrogenic cells and in growth plate chondrocytes stimulates their chondrocytic differentiation and maturation toward hypertrophy, respectively.

Overexpression of twisted gastrulation inhibits bone morphogenetic protein action and prevents osteoblast cell differentiation in vitro

Twisted gastrulation (Tsg) is a secreted glycoprotein that binds bone morphogenetic protein-2 (BMP-2) and BMP-4 and can display both BMP agonist and antagonist functions. Tsg acts as a BMP agonist in chondrocytes, but its expression and actions on the differentiation of cells of the osteoblastic lineage are not known. We investigated the effects of Tsg overexpression by transducing murine ST-2 stromal and MC3T3 cells with a retroviral vector where Tsg is under control of the cytomegalovirus promoter and compared them to cells transduced with the parental vector alone. ST-2 cells were cultured in osteoblastic differentiating conditions in the presence or absence of BMP-2. Tsg overexpression precluded the appearance of mineralized nodules induced by BMP-2, led to a delay in the expression of osteoblastic gene markers, and decreased the responsiveness of ST-2 differentiating cells to PTH. BMP-2 induced the phosphorylation of signaling mothers against decapentaplegic-1/5/8, but not ERK, c-Jun N-terminal kinase, and p38. ST-2 cells overexpressing Tsg displayed an inhibition of BMP/signaling mother against decapentaplegic signaling. Tsg action was specific to BMP, because Tsg overexpression did not affect TGF-beta or Wnt/beta-catenin signaling pathways. Tsg also opposed MC3T3 cell differentiation and the expression of a mature osteoblast phenotype. In conclusion, Tsg overexpression inhibits BMP action in stromal and preosteoblastic cells and, accordingly, arrests their differentiation toward the osteoblastic pathway.

Sclerostin Inhibition of Wnt-3a-induced C3H10T1/2 Cell Differentiation Is Indirect and Mediated by Bone Morphogenetic Proteins

High bone mass diseases are caused both by activating mutations in the Wnt pathway and by loss of SOST, a bone morphogenetic protein (BMP) antagonist, leading to the activation of BMP signaling. Given the phenotypic similarity between mutations that activate these signaling pathways, it seems likely that BMPs and Wnts operate in parallel or represent components of the same pathway, modulating osteoblast differentiation. In this study, we show that in C3H10T1/2 cells, Wnt-3A and BMP-6 proteins were inducers of osteoblast differentiation, as measured by alkaline phosphatase (ALP) induction. Surprisingly, sclerostin, noggin, and human BMP receptor 1A (BMPR1A)-FC fusion proteins blocked Wnt-3A-induced ALP as well as BMP-6-induced ALP activity. Dkk-1, a Wnt inhibitor, blocked Wnt-induced ALP activity but not BMP-induced ALP activity. Early Wnt-3A signaling as measured by beta-catenin accumulation was not affected by the BMP antagonists but was blocked by Dkk-1. Wnt-3A induced the appearance of BMP-4 mRNA 12 h prior to that of ALP in C3H10T1/2 cells. We propose that sclerostin and other BMP antagonists do not block Wnt signaling directly. Sclerostin blocks Wnt-induced ALP activity by blocking the activity of BMP proteins produced by Wnt treatment. The expression of BMP proteins in this autocrine loop is essential for Wnt-3A-induced osteoblast differentiation.

Parathyroid hormone-related peptide (PTHrP) inhibits Runx2 expression through the PKA signaling pathway

The bone-related transcription factor Runx2 (Cbfa1) has been extensively shown to regulate osteoblast differentiation and function. Recent studies demonstrate that Runx2 is also a positive regulator of chondrocyte maturation and vascular invasion in cartilage. Runx2 activity can be modulated in several ways, including direct stimulation of gene expression, post-translational modification, and protein-protein interactions. We have previously reported cooperative effects between BMP and RA downstream signaling involving Smad proteins and Runx2. Furthermore, our previous studies showed that PTHrP inhibits chondrocyte maturation primarily through CREB and AP-1 signaling pathways. In the present study, we investigated the effect of PTHrP on Runx2 expression in chick upper sternal chondrocytes (USCs). We further determined the signaling pathways through which PTHrP regulates Runx2 transcription. Our results show that PTHrP inhibits Runx2 expression at both the mRNA and protein levels concomitant with a PTHrP-mediated suppression of the phenotypic marker of hypertrophy, type X collagen. We further determined potential signaling pathways through which PTHrP inhibits Runx2 expression using protein kinase inhibitors, H89 (PKA inhibitor): Go-6976 (PKC inhibitor): SB203850 (p38 MAPK inhibitor), and U0126 (MEK inhibitor). We show that pretreatment with PKA and, to a lesser extent, PKC inhibitors significantly blocked PTHrP suppression of Runx2, while p38 MAPK and MEK inhibitors had no significant effect. Furthermore, PTHrP suppression of Runx2 mRNA was partially blocked in USCs infected with RCAS-A-CREB, a dominant negative reagent that abrogates CREB activity. Overall, our results demonstrate that PTHrP downregulates Runx2 expression primarily through the PKA signaling pathway.

Noggin inhibits chondrogenic but not osteogenic differentiation in mesodermal stem cell line C1 and skeletal cells

Osteoblasts and chondroblasts are derived from common mesenchymal progenitors. Although bone morphogenetic protein induces mesenchymal differentiation into both osteogenic and chodrogenic lineage cells in vitro, its inhibitor, Noggin, is expressed exclusively during chondrogenic but not osteogenic differentiation in an embryonal carcinoma-derived mesodermal cell line, C1. We hypothesized that Noggin may regulate cell differentiation in a lineage-specific manner. To test this hypothesis, Noggin was overexpressed using recombinant adenovirus (Ad/Noggin) in mesodermal C1 cells to examine whether Noggin specifically inhibits chondrogenic differentiation. Noggin overexpression by recombinant adenovirus infection reduced Sox9, patched, Ihh, and type II, X, and XI collagen mRNA expression levels in C1 cell aggregates that were induced to differentiate into chondrocyte lineage by culturing in differentiation medium. In contrast, Noggin overexpression did not affect osteogenic differentiation in C1 cells because osteoblast phenotypic markers such as osteocalcin and alkaline phosphatase mRNA levels were not altered. We further examined whether Noggin also differentially affects chondrogenesis and osteogenesis in limb development by using organ cultures of long bone. Ad/Noggin infection into 15.5 d post conception limb skeletal rudiments that were cultured on filter membrane in vitro or on the chorioallantoic membranes in ovo inhibited the levels of chondrogenesis, which were evaluated based on alcian blue staining. These results suggest that Noggin specifically blocks chondrogenic differentiation, rather than osteogenic differentiation, in mesodermal stem cell line C1 and skeletal cells.

Transforming growth factor-beta1 induced alteration of skeletal morphogenesis in vivo

Transforming growth factor beta (TGF-beta) is expressed in the growth plate and is an important regulator of chondrocyte maturation. Loss of function results in premature chondrocyte maturation both in vitro and in vivo. While TGF-beta inhibits chondrocyte maturation in cell cultures, the effect of increased TGF-beta has not been well characterized in an in vivo development model. Addition of Affi-gel agarose beads loaded with TGF-beta1 (10 ng/microl) to developing stage 24-25 chick limb buds resulted in limb shortening and altered morphology. In situ hybridization studies showed down regulation of Indian hedgehog (ihh), bone morphogenetic protein 6 (bmp6), and collagen type X (colX) expression, markers of chondrocyte maturation, in TGF-beta1 treated limbs. TGF-beta1 also decreased chondrocyte proliferation in the developing anlage. The findings confirm a critical role for TGF-beta during skeletal development. A more complete understanding of the role of TGF-beta and its down-stream signals will lead to improved understanding and treatment of cartilage diseases.

Primary murine limb bud mesenchymal cells in long-term culture complete chondrocyte differentiation: TGF-beta delays hypertrophy and PGE2 inhibits terminal differentiation

In vitro models of endochondral bone formation using both primary and immortalized cells have provided insight regarding factors and signaling pathways involved in chondrocyte maturation and endochondral bone formation. However, primary murine cell culture models of chondrocyte differentiation have not been established but have enormous potential due to the possible use of cells from transgenic and knockout animals. Here, we show that stage E11.5 embryonic murine limb bud mesenchymal stem cells in micromass cell culture progress through the stages of chondrogenesis, chondrocyte hypertrophy, terminal differentiation, and matrix calcification. This cell culture system recapitulated the sequential expression of genes that characterize chondrocyte differentiation, including Sox9, col2, colX, MMP13, VEGF, and osteocalcin. TGF-beta treatment for up to 21 days markedly delayed the rate of chondrocyte maturation and inhibited matrix calcification and its related gene expression. In TGF-beta-treated cultures, the hypertrophic and terminal differentiation markers colX, VEGF, MMP13, and osteocalcin were reduced or absent. PGE2 had minimal effects on chondrocyte hypertrophy but delayed terminal differentiation and matrix calcification. Thus, primary murine mesenchymal cells sequentially differentiate through the various stages of chondrocyte maturation and establish a model whereby the role of specific signaling molecules can be examined in cells derived from transgenic or knockout mice.

CREB Cooperates with BMP-stimulated Smad signaling to enhance transcription of the Smad6 promoter

Growth plate chondrocytes integrate a multitude of growth factor signals during maturation. PTHrP inhibits maturation through stimulation of PKA/CREB signaling while the bone morphogenetic proteins (BMPs) stimulate maturation through Smad mediated signaling. In this manuscript, we show that interactions between CREB and the BMP associated Smads are promoter specific, and demonstrate for the first time the requirement of CREB signaling for Smad mediated activation of a BMP responsive region of the Smad6 promoter. The 28 base pairs (bp) BMP responsive element of the Smad6 promoter contains an 11 bp Smad binding region and an adjacent 17 bp region in which we characterize a putative CRE site. PKA/CREB gain of function enhanced BMP stimulation of this reporter, while loss of CREB function diminished transcriptional activity. In contrast, ATF-2 and AP-1 transcription factors had minimal effects. Electrophoretic mobility shift assay (EMSA) confirmed CREB binding to the Smad6 promoter element. Mutations eliminating binding resulted in loss of transcriptional activity, while mutations that maintained CREB binding had continued reporter activation by CREB and BMP-2. The Smad6 gene was similarly regulated by CREB. Dominant negative CREB reduced BMP-2 stimulated Smad6 gene transcription by 50%, but markedly increased BMP-2 mediated stimulation of colX and Ihh expression. In contrast, PTHrP which activates CREB signaling, blocked the stimulatory effect of BMP-2 on colX and Ihh, but minimally inhibited the stimulatory effect of BMP on Smad6. These findings are the first to demonstrate a cooperative association between CREB and BMP regulated Smads in cells from vertebrates and demonstrate that promoter-specific rather than generalized interactions between PKA/CREB and BMP signaling regulate gene expression in chondrocytes.

Osteocyte control of bone formation via sclerostin, a novel BMP antagonist

There is an unmet medical need for anabolic treatments to restore lost bone. Human genetic bone disorders provide insight into bone regulatory processes. Sclerosteosis is a disease typified by high bone mass due to the loss of SOST expression. Sclerostin, the SOST gene protein product, competed with the type I and type II bone morphogenetic protein (BMP) receptors for binding to BMPs, decreased BMP signaling and suppressed mineralization of osteoblastic cells. SOST expression was detected in cultured osteoblasts and in mineralizing areas of the skeleton, but not in osteoclasts. Strong expression in osteocytes suggested that sclerostin expressed by these central regulatory cells mediates bone homeostasis. Transgenic mice overexpressing SOST exhibited low bone mass and decreased bone strength as the result of a significant reduction in osteoblast activity and subsequently, bone formation. Modulation of this osteocyte-derived negative signal is therapeutically relevant for disorders associated with bone loss.

Differential gene expression and regulation of the bone morphogenetic protein antagonists follistatin and gremlin in normal and osteoarthritic human chondrocytes and synovial fibroblasts

OBJECTIVE

To compare gene expression in normal and osteoarthritic (OA) human chondrocytes using microarray technology. Of the novel genes identified, we selected follistatin, a bone morphogenetic protein (BMP) antagonist, and investigated its expression/regulation as well as that of 3 other antagonists, gremlin, chordin, and noggin, in normal and OA chondrocytes and synovial fibroblasts.

METHODS

Basal and induced gene expression were determined using real-time polymerase chain reaction. Gene regulation was monitored following treatment with inflammatory, antiinflammatory, growth, and developmental factors. Follistatin protein production was measured using a specific enzyme-linked immunosorbent assay, and localization of follistatin and gremlin in cartilage was determined by immunohistochemical analysis.

RESULTS

All BMP antagonists except noggin were expressed in chondrocytes and synovial fibroblasts. Follistatin and gremlin were significantly up-regulated in OA chondrocytes but not in OA synovial fibroblasts. Chordin was weakly expressed in normal and OA cells. Production of follistatin protein paralleled the gene expression pattern. Follistatin and gremlin were expressed preferentially by the chondrocytes at the superficial layers of cartilage. Tumor necrosis factor alpha and interferon-gamma significantly stimulated follistatin expression but down-regulated expression of gremlin. Interleukin-1beta (IL-1beta) had no effect on follistatin but reduced gremlin expression. Conversely, BMP-2 and BMP-4 significantly stimulated expression of gremlin but down-regulated that of follistatin. IL-13, dexamethasone, transforming growth factor beta1, basic fibroblast growth factor, platelet-derived growth factor type BB, and endothelial cell growth factor down-regulated the expression of both antagonists.

CONCLUSION

This study is the first to show the possible involvement of follistatin and gremlin in OA pathophysiology. The increased activin/BMP-binding activities of these antagonists could affect tissue remodeling. The data suggest that follistatin and gremlin might appear at different stages during the OA process, making them interesting targets for the treatment of this disease.

Digit regeneration is regulated by Msx1 and BMP4 in fetal mice

The regeneration of digit tips in mammals, including humans and rodents, represents a model for organ regeneration in higher vertebrates. We had previously characterized digit tip regeneration during fetal and neonatal stages of digit formation in the mouse and found that regenerative capability correlated with the expression domain of the Msx1 gene. Using the stage 11 (E14.5) digit, we now show that digit tip regeneration occurs in organ culture and that Msx1, but not Msx2, mutant mice display a regeneration defect. Associated with this phenotype, we find that Bmp4 expression is downregulated in the Msx1 mutant digit and that mutant digit regeneration can be rescued in a dose-dependent manner by treatment with exogenous BMP4. Studies with the BMP-binding protein noggin show that wild-type digit regeneration is inhibited without inhibiting the expression of Msx1, Msx2 or Bmp4. These data identify a signaling pathway essential for digit regeneration, in which Msx1 functions to regulate BMP4 production. We also provide evidence that endogenous Bmp4 expression is regulated by the combined activity of Msx1 and Msx2 in the forming digit tip; however, we discovered a compensatory Msx2 response that involves an expansion into the wild-type Msx1 domain. Thus, although both Msx1 and Msx2 function to regulate Bmp4 expression in the digit tip, the data are not consistent with a model in which Msx1 and Msx2 serve completely redundant functions in the regeneration response. These studies provide the first functional analysis of mammalian fetal digit regeneration and identify a new function for Msx1 and BMP4 as regulators of the regenerative response.

ALK2 functions as a BMP type I receptor and induces Indian hedgehog in chondrocytes during skeletal development

Growth plate chondrocytes integrate multiple signals during normal development. The type I BMP receptor ALK2 is expressed in cartilage and expression of constitutively active (CA) ALK2 and other activated type I BMP receptors results in maturation-independent expression of Ihh in chondrocytes in vitro and in vivo. The findings suggest that BMP signaling modulates the Ihh/PTHrP signaling pathway that regulates the rate of chondrocyte differentiation.

INTRODUCTION

Bone morphogenetic proteins (BMPs) have an important role in vertebrate limb development. The expression of the BMP type I receptors BMPR-IA (ALK3) and BMPR-IB (ALK6) have been more completely characterized in skeletal development than ALK2.

METHODS

ALK2 expression was examined in vitro in isolated chick chondrocytes and osteoblasts and in vivo in the developing chick limb bud. The effect of overexpression of CA ALK2 and the other type I BMP receptors on the expression of genes involved in chondrocyte maturation was determined.

RESULTS

ALK2 was expressed in isolated chick osteoblasts and chondrocytes and specifically mediated BMP signaling. In the developing chick limb bud, ALK2 was highly expressed in mesenchymal soft tissues. In skeletal elements, expression was higher in less mature chondrocytes than in chondrocytes undergoing terminal differentiation. CA ALK2 misexpression in vitro enhanced chondrocyte maturation and induced Ihh. Surprisingly, although parathyroid hormone-related peptide (PTHrP) strongly inhibited CA ALK2 mediated chondrocyte differentiation, Ihh expression was minimally decreased. CA ALK2 viral infection in stage 19-23 limbs resulted in cartilage expansion with joint fusion. Enhanced periarticular expression of PTHrP and delayed maturation of the cartilage elements were observed. In the cartilage element, CA ALK2 misexpression precisely colocalized with the expression with Ihh. These findings were most evident in partially infected limbs where normal morphology was maintained. In contrast, BMP-6 had a normal pattern of differentiation-related expression. CA BMPR-IA and CA BMPR-IB overexpression similarly induced Ihh and PTHrP.

CONCLUSIONS

The findings show that BMP signaling induces Ihh. Although the colocalization of the activated type I receptors and Ihh suggests a direct BMP-mediated signaling event, other indirect mechanisms may also be involved. Thus, while BMPs act directly on chondrocytes to induce maturation, this effect is counterbalanced in vivo by induction of the Ihh/PTHrP signaling loop. The findings suggest that BMPs are integrated into the Ihh/PTHrP signaling loop and that a fine balance of BMP signaling is essential for normal chondrocyte maturation and skeletal development.

Gremlin is the BMP antagonist required for maintenance of Shh and Fgf signals during limb patterning

During limb outgrowth, signaling by bone morphogenetic proteins (BMPs) must be moderated to maintain the signaling loop between the zone of polarizing activity (ZPA) and the apical ectodermal ridge (AER). Gremlin, an extracellular Bmp antagonist, has been proposed to fulfill this function and therefore be important in limb patterning. We tested this model directly by mutating the mouse gene encoding gremlin (Cktsf1b1, herein called gremlin). In the mutant limb, the feedback loop between the ZPA and the AER is interrupted, resulting in abnormal skeletal pattern. We also show that the gremlin mutation is allelic to the limb deformity mutation (ld). Although Bmps and their antagonists have multiple roles in limb development, these experiments show that gremlin is the principal BMP antagonist required for early limb outgrowth and patterning.

Physiology and pathophysiology of the growth plate

Longitudinal growth of the skeleton is a result of endochondral ossification that occurs at the growth plate. Through a sequential process of cell proliferation, extracellular matrix synthesis, cellular hypertrophy, matrix mineralization, vascular invasion, and eventually apoptosis, the cartilage model is continually replaced by bone as length increases. The regulation of longitudinal growth at the growth plate occurs generally through the intimate interaction of circulating systemic hormones and locally produced peptide growth factors, the net result of which is to trigger changes in gene expression by growth plate chondrocytes. This review highlights recent advances in genetics and cell biology that are illuminating the important regulatory mechanisms governing the structure and biology of the growth plate, and provides selected examples of how studies of human mutations have yielded a wealth of new knowledge regarding the normal biology and pathophysiology of growth plate cartilage.

Bone morphogenetic proteins, their antagonists, and the skeleton

Skeletal homeostasis is determined by systemic hormones and local factors. Bone morphogenetic proteins (BMP) are unique because they induce the differentiation of mesenchymal cells toward cells of the osteoblastic lineage and also enhance the differentiated function of the osteoblast. However, the activity of BMPs needs to be tempered by intracellular and extracellular antagonists. BMPs bind to specific receptors and signal by phosphorylating the cytoplasmic proteins mothers against decapentaplegic (Smad) 1 and 5, which form heterodimers with Smad 4, and after nuclear translocation regulate transcription. BMP antagonists can be categorized as pseudoreceptors that compete with signaling receptors, inhibitory Smads that block signaling, intracellular binding proteins that bind Smad 1 and 5, and factors that induce ubiquitination and proteolysis of signaling Smads. In addition, a large number of extracellular proteins that bind BMPs and prevent their binding to signaling receptors have emerged. They are the components of the Spemann organizer, noggin, chordin, and follistatin, members of the Dan/Cerberus family, and twisted gastrulation. The antagonists tend to be specific for BMPs and are regulated by BMPs, indicating the existence and need of local feedback mechanisms to temper BMP cellular activities.

Pleiotrophin/Osteoblast-stimulating factor 1: dissecting its diverse functions in bone formation

OSF-1, more commonly known as pleiotrophin (PTN) or heparin-binding growth-associated molecule (HB-GAM), belongs to a new family of secreted HB proteins, which are structurally unrelated to any other growth factor family. The aims of this study were to dissect the diverse functions of PTN in bone formation. The study showed that PTN was synthesized by osteoblasts at an early stage of osteogenic differentiation and was present at sites of new bone formation, where PTN was stored in the new bone matrix. Low concentrations (10 pg/ml) of PTN stimulated osteogenic differentiation of mouse bone marrow cells and had a modest effect on their proliferation, whereas higher concentrations (ng/ml) had no effect. However, PTN did not have the osteoinductive potential of bone morphogenetic proteins (BMPs) because it failed to convert C2C12 cells, a premyoblastic cell line, to the osteogenic phenotype, whereas recombinant human BMP-2 (rhBMP-2) was able to do so. When PTN was present together with rhBMP-2 during the osteoinductive phase, PTN inhibited the BMP-mediated osteoinduction in C2C12 cells at concentrations between 0.05 pg/ml and 100 ng/ml. However, when added after osteoinduction had been achieved, PTN enhanced further osteogenic differentiation. An unusual effect of PTN (50 ng/ml) was the induction of type I collagen synthesis by chondrocytes in organ cultures of chick nasal cartilage and rat growth plates. Thus, PTN had multiple effects on bone formation and the effects were dependent on the concentration of PTN and the timing of its presence. To explain these multiple effects, we propose that PTN is an accessory signaling molecule, which is involved in a variety of processes in bone formation. PTN enhances or inhibits primary responses depending on the prevailing concentrations, the primary stimulus, and the availability of appropriate receptors.