Interplay between bone morphogenetic proteins and cognate binding proteins in bone and cartilage development: noggin, chordin and DAN
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This commentary is a concise discussion of the interactions between bone morphogenetic proteins (BMPs) and their binding proteins in bone and cartilage morphogenesis. BMPs are a family of growth and differentiation factors, and they act on mesenchymal cells to induce cartilage and bone differentiation in concentration-dependent thresholds. The BMP–BMP receptor binding leads to a cascade of signaling and transcription of BMP response genes. BMP binding proteins, noggin, chordin and DAN, act as antagonists and determine the bioavailability of BMPs for binding to cognate receptors to elicit the biological response. Noggin null mice with unrestricted action of BMPs exhibit defects in joint morphogenesis. BMPs and their binding proteins may reciprocally regulate the dynamic topography of joints, muscle, tendons and ligaments during morphogenesis of the skeleton. In addition, BMP actions may be potentiated by twisted gastrulation. BMPs and their binding proteins may play a critical role in regeneration of cartilage in osteoarthritis.
KeywordsBMP BMP receptors extracellular matrix Smads
The sequelae of damage and degradation of articular cartilage is well known in osteoarthritis. Repair and regeneration of articular cartilage in experimental animals is initiated when the subchondral bone is penetrated, as in full thickness defects. On the other hand, there is no repair in partial thickness defects with the defect confined to cartilage only. Subchondral bone with associated matrix and hematopoietic stromal cells may thus play a role in cartilage repair. The difference in regenerative potential between bone and cartilage is immense. The fundamental differences between cartilage and bone in inherent repair potential may be due to differences in concentration of endogeneous growth and morphogenetic factors and their antagonists. The demineralized bone matrix is a repository of BMPs and this might explain, in part, the repair of articular cartilage in full thickness defects. The aim of this commentary is to discuss the interplay between the evolving family of BMPs and cognate BMP binding proteins in bone and cartilage development. In arthritis, with attendant cartilage damage, there is a derangement of the morphogenetic signals. It is thus possible to harness the recent progress in developmental biology of morphogens to design rational therapeutic approaches to cartilage regeneration. Regeneration is, after all, a recapitulation of embryonic development and morphogenesis, and includes redeployment of morphogens in regeneration.
Bone morphogenetic proteins
BMPs are a family of growth and differentiation factors [1,2]. BMPs are pleiotropic morphogens, and they induce new cartilage and bone formation in ectopic sites by a developmental sequence that mimics limb development and morphogenesis. BMPs have chemotactic, mitogenic and differentiation-inducing properties. The actions of BMPs are concentration dependent, and are based on thresholds. BMPs are dimeric proteins with a single interchain disulfide bond. The dimeric conformation is an absolute requirement for the biological action and interaction with receptors.
BMPs were initially identified as bioactive molecules in the demineralized bone matrix responsible for cartilage and bone differentiation [1,2]. BMPs regulate the lineage, pattern and differentiation of bone, and over 15 have been cloned and expressed in humans and mice. Genetic evidence points to actions of BMPs in tissues beyond bone. Knockout of BMP 2 and BMP 4 results in defects in mesoderm formation. Mutations in BMP 5 in mice are responsible for the short ear phenotype. BMP 7 null mice exhibit defects in eye, kidney and skeleton . BMP 4 stimulates chondrogenesis in limb bud mesenchymal cells  and maintains articular cartilage phenotype .
Cartilage-derived morphogenetic proteins
Luyten and coworkers identified cartilage-derived morphogenetic protein-1 (CDMP-1) in articular cartilage  and found it identical to growth/differentiation factor 5. The incisive work of Lee and coworkers cloned growth/differentiation factors 5, 6, and 7, with mutations in growth/differentiation factor 5 identified in mouse brachypodism . It is noteworthy that mutations in human CDMP-1 were identified in patients with Hunter-Thompson type chondrodysplasia, type C Brachydactyly and Grebe type chondrodysplasia with severe limb shortening and impaired morphogenesis [7,8]. BMPs and CDMPs are thus critical for bone and cartilage morphogenesis and beyond. BMPs should perhaps be called body morphogenetic proteins, which would take into account the wide-ranging actions of BMPs and obviate the need for tinkering with terminology.
BMP binding proteins as antagonists
During the course of a search for the elusive neural inducer, noggin was isolated from Xenopus based on rescue of dorsal development in ultraviolet-induced ventralized embryos . Injection of the putative cloned RNA into embryos resulted in large heads, hence the name noggin. Noggin is produced by the Spemann organizer and antagonizes the action of BMPs, induces neural tissue and dorsalizes ventral mesoderm. Noggin binds to BMP 2 and BMP 4 with high affinity and blocks interaction with BMP receptor . The bioavailability receptor is thus modulated by BMP antagonist noggin.
What has noggin to do with arthritis and articular cartilage? The precise boundary between muscle and cartilage with the interspersed perichondrium, the tendon/ligament interface, requires precisely regulated boundary conditions during joint morphogenesis. One can envisage, given the role of dominant morphogens such as BMPs and CDMPs, BMP/CDMP binding proteins (antagonists) to play a role in defining boundaries. Experimental evidence has in fact been adduced to precisely demonstrate this, using noggin null mice generated by homologous recombination . Regulated cell death and joint cavitation follow mesenchymal cell condensation in regions of presumptive joint morphogenesis, and CDMP-1 has been implicated in this process . It is possible in mice lacking noggin that the unfettered actions of BMPs/CDMPs may lead to impaired joint formation. Defects in vertebrae, ribs and limbs were observed in homozygous mutants; heterozygous mice appeared normal. The limbs are shorter in the mutant, the joints were not demarcated and fusion of the joints was commonplace. Optimal expression of BMPs/CDMP is thus needed for the normal joint morphogenesis. Future investigations with conditional knockouts of noggin in mice will shed more light on the role of noggin in joint morphogenesis. BMP 2 induces the expression of noggin in osteoblasts  and bone marrow cells , implying regulated expression of the BMP antagonist, perhaps to downregulate the agonist response to BMP 2. This is akin to a molecular thermostat to maintain skeletal homeostasis.
During the course of investigation on Xenopus pattern formation, chordin was identified as BMP 4 binding protein . Chordin has a homolog in Drosophila, short gastrulation (sog), known to bind to decapentaplegic, a BMP 2/4 homolog. These BMP antagonists have thus been conserved for several million years. Chordin binds to BMP 2 and BMP 4, and is further regulated by a metalloprotease BMP 1 and tolloid and Xenopus xolloid [17,18,19]. Antagonists such as chordin thus govern the intricate pattern formation by morphogens such as BMP, and in turn are proteolytically inactivated by a metalloprotease BMP 1, related to the astacin family. BMP 1 was originally identified in osteogenic extracts derived from bovine bone as it copurified with bonafide bone morphogenetic proteins 2–7 . BMP 1 also functions as a procollagen c-protease. In retrospect, the term BMP-1 is a misnomer, as this BMP does not induce bone morphogenesis.
The DAN gene was isolated as a candidate tumor suppressor gene in a differential hybridization screen . Secreted DAN suppressed DNA synthesis in transformed cells. The head inducer gene Cereberus codes for a secreted protein and can induce heads in Xenopus embryos, and it is related to DAN in the cysteine rich domain . Gremlin is a Xenopus homolog related to DAN that inhibits BMP 2 action and was identified by screening an ovarian cDNA library for activities inducing the secondary axis . DAN family members are thus newly identified BMP antagonists [23,24,25]. It is not yet clear if they play a role in articular cartilage and in arthritis.
BMP potentiating agonist: twisted gastrulation
One of the emerging themes in developmental biology is the regulation of pattern formation and morphogenesis by sets of genes. The anterior–posterior axis, for example, is governed by hox genes. The dorsal–ventral axis is the result of the activity and expression pattern of bone morphogenetic proteins, BMP antagonist chordin and metalloprotease xolloid. Recent work in Xenopus has identified a gene 'twisted gastrulation' (xTsg), as an agonist of BMP actions . In vertebrates, the dorsal–ventral pattern is regulated by a gradient of BMP activity. Although BMPs are expressed uniformly, the expression of BMP antagonists such as chordin generates the dorsal–ventral gradients. Further control is exerted by xolloid, a zinc-dependent metalloprotease. The xTsg binds to BMPs with a dissociation constant in the low nanomolar range. It is noteworthy that microinjection of xTsg results in potentiation of BMP signaling, leading to ventralization of Xenopus embryo. Twisted gastrulation is thus an agonist of BMP signaling. xTsg may thus activate the inactive BMP–chordin cysteine rich domain complex by releasing the bioactive BMPs. It is important to bear in mind, given BMPs bind to extracellular matrix [10,27] components such as collagens, heparan sulfate and heparin, additional regulation is inevitable, and is a question of time and space!
The recent advances in the developmental biology of BMP binding proteins will allow a rational approach to the challenges of articular cartilage repair by morphogens. It is remarkable how Nature zealously regulates the intricate interplay between BMPs, BMP receptors, extracellular matrix, BMP antagonists such as noggin, chordin and the DAN family, and BMP potentiating twisted gastrulation.
The author wishes to thank Mrs Rita Rowlands for outstanding assistance in the preparation of this manuscript. This research is supported by grants from Shriners Hospitals and the Lawrence Ellison Chair in Musculoskeletal Molecular Biology. The author also thanks the anonymous reviewers for their invaluable critique that improved our manuscript immensely.
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