Efficient expression of a heterodimer of bone morphogenetic protein subunits using a baculovirus expression system

Highly conserved and extremely evolvable: BMP signalling in secondary axis patterning of Cnidaria and Bilateria

Bilateria encompass the vast majority of the animal phyla. As the name states, they are bilaterally symmetric, that is with a morphologically clear main body axis connecting their anterior and posterior ends, a second axis running between their dorsal and ventral surfaces, and with a left side being roughly a mirror image of their right side. Bone morphogenetic protein (BMP) signalling has widely conserved functions in the formation and patterning of the second, dorso-ventral (DV) body axis, albeit to different extents in different bilaterian species. Whilst initial findings in the fruit fly Drosophila and the frog Xenopus highlighted similarities amongst these evolutionarily very distant species, more recent analyses featuring other models revealed considerable diversity in the mechanisms underlying dorsoventral patterning. In fact, as phylogenetic sampling becomes broader, we find that this axis patterning system is so evolvable that even its core components can be deployed differently or lost in different model organisms. In this review, we will try to highlight the diversity of ways by which BMP signalling controls bilaterality in different animals, some of which do not belong to Bilateria. Future research combining functional analyses and modelling is bound to give us some understanding as to where the limits to the extent of the evolvability of BMP-dependent axial patterning may lie.

Heterodimerization-dependent secretion of bone morphogenetic proteins in Drosophila

Combinatorial signaling is key to instruct context-dependent cell behaviors. During embryonic development, adult homeostasis, and disease, bone morphogenetic proteins (BMPs) act as dimers to instruct specific cellular responses. BMP ligands can form both homodimers or heterodimers; however, obtaining direct evidence of the endogenous localization and function of each form has proven challenging. Here, we make use of precise genome editing and direct protein manipulation via protein binders to dissect the existence and functional relevance of BMP homodimers and heterodimers in the Drosophila wing imaginal disc. This approach identified in situ the existence of Dpp (BMP2/4)/Gbb (BMP5/6/7/8) heterodimers. We found that Gbb is secreted in a Dpp-dependent manner in the wing imaginal disc. Dpp and Gbb form a gradient of heterodimers, whereas neither Dpp nor Gbb homodimers are evident under endogenous physiological conditions. We find that the formation of heterodimers is critical for obtaining optimal signaling and long-range BMP distribution.

Intrinsically disordered BMP4 morphogen and the beak of the finch: Co-option of an ancient axial patterning system

Darwin's finches, with the primary diversity in the shape and size of their beaks, represent an excellent model system to study speciation and adaptive evolution. It is generally held that evolution depends on the natural selection of heritable phenotypic variations originating from the genetic mutations. However, it is now increasingly evident that epigenetic transgenerational inheritance of phenotypic variation can also guide evolutionary change. Several studies have shown that the bone morphogenetic protein BMP4 is a major driver of beak morphology. A recent study explored variability of the morphological, genetic, and epigenetic differences in the adjacent "urban" and "rural" populations of two species of ground Darwin's finches on the Galápagos Islands and revealed significant changes in methylation patterns in several genes including those involved in the BMP/TGFß pathway in the sperm DNA compared to erythrocyte DNA. These observations indicated that epigenetic changes caused by environmental fluctuations can be passed on to the offspring. Nonetheless, the mechanism by which dysregulated expression of BMP4 impacts beak morphology remains poorly understood. Here, we show that BMP4 is an intrinsically disordered protein and present a causal a link between epigenetic changes, BMP4 dysregulation and the evolution of the beak of the finch by natural selection.

The Body's Cellular and Molecular Response to Protein-Coated Medical Device Implants: A Review Focused on Fibronectin and BMP Proteins

Recent years have seen a marked rise in implantation into the body of a great variety of devices: hip, knee, and shoulder replacements, pacemakers, meshes, glucose sensors, and many others. Cochlear and retinal implants are being developed to restore hearing and sight. After surgery to implant a device, adjacent cells interact with the implant and release molecular signals that result in attraction, infiltration of the tissue, and attachment to the implant of various cell types including monocytes, macrophages, and platelets. These cells release additional signaling molecules (chemokines and cytokines) that recruit tissue repair cells to the device site. Some implants fail and require additional revision surgery that is traumatic for the patient and expensive for the payer. This review examines the literature for evidence to support the possibility that fibronectins and BMPs could be coated on the implants as part of the manufacturing process so that the proteins could be released into the tissue surrounding the implant and improve the rate of successful implantation.

The Role of Bone Morphogenetic Protein 7 (BMP-7) in Inflammation in Heart Diseases

Bone morphogenetic protein-7 is (BMP-7) is a potent anti-inflammatory growth factor belonging to the Transforming Growth Factor Beta (TGF-β) superfamily. It plays an important role in various biological processes, including embryogenesis, hematopoiesis, neurogenesis and skeletal morphogenesis. BMP-7 stimulates the target cells by binding to specific membrane-bound receptor BMPR 2 and transduces signals through mothers against decapentaplegic (Smads) and mitogen activated protein kinase (MAPK) pathways. To date, rhBMP-7 has been used clinically to induce the differentiation of mesenchymal stem cells bordering the bone fracture site into chondrocytes, osteoclasts, the formation of new bone via calcium deposition and to stimulate the repair of bone fracture. However, its use in cardiovascular diseases, such as atherosclerosis, myocardial infarction, and diabetic cardiomyopathy is currently being explored. More importantly, these cardiovascular diseases are associated with inflammation and infiltrated monocytes where BMP-7 has been demonstrated to be a key player in the differentiation of pro-inflammatory monocytes, or M1 macrophages, into anti-inflammatory M2 macrophages, which reduces developed cardiac dysfunction. Therefore, this review focuses on the molecular mechanisms of BMP-7 treatment in cardiovascular disease and its role as an anti-fibrotic, anti-apoptotic and anti-inflammatory growth factor, which emphasizes its potential therapeutic significance in heart diseases.

Specification of BMP Signaling

Bone Morphogenetic Proteins (BMPs) together with the Growth and Differentiation Factors (GDFs) form the largest subgroup of the Transforming Growth Factor (TGF)β family and represent secreted growth factors, which play an essential role in many aspects of cell communication in higher organisms. As morphogens they exert crucial functions during embryonal development, but are also involved in tissue homeostasis and regeneration in the adult organism. Their involvement in maintenance and repair processes of various tissues and organs made these growth factors highly interesting targets for novel pharmaceutical applications in regenerative medicine. A hallmark of the TGFβ protein family is that all of the more than 30 growth factors identified to date signal by binding and hetero-oligomerization of a very limited set of transmembrane serine-threonine kinase receptors, which can be classified into two subgroups termed type I and type II. Only seven type I and five type II receptors exist for all 30plus TGFβ members suggesting a pronounced ligand-receptor promiscuity. Indeed, many TGFβ ligands can bind the same type I or type II receptor and a particular receptor of either subtype can usually interact with and bind various TGFβ ligands. The possible consequence of this ligand-receptor promiscuity is further aggravated by the finding that canonical TGFβ signaling of all family members seemingly results in the activation of just two distinct signaling pathways, that is either SMAD2/3 or SMAD1/5/8 activation. While this would implicate that different ligands can assemble seemingly identical receptor complexes that activate just either one of two distinct pathways, in vitro and in vivo analyses show that the different TGFβ members exert quite distinct biological functions with high specificity. This discrepancy indicates that our current view of TGFβ signaling initiation just by hetero-oligomerization of two receptor subtypes and transduction via two main pathways in an on-off switch manner is too simplified. Hence, the signals generated by the various TGFβ members are either quantitatively interpreted using the subtle differences in their receptor-binding properties leading to ligand-specific modulation of the downstream signaling cascade or additional components participating in the signaling activation complex allow diversification of the encoded signal in a ligand-dependent manner at all cellular levels. In this review we focus on signal specification of TGFβ members, particularly of BMPs and GDFs addressing the role of binding affinities, specificities, and kinetics of individual ligand-receptor interactions for the assembly of specific receptor complexes with potentially distinct signaling properties.

Asymmetric paralog evolution between the "cryptic" gene Bmp16 and its well-studied sister genes Bmp2 and Bmp4

The vertebrate gene repertoire is characterized by “cryptic” genes whose identification has been hampered by their absence from the genomes of well-studied species. One example is the Bmp16 gene, a paralog of the developmental key genes Bmp2 and -4. We focus on the Bmp2/4/16 group of genes to study the evolutionary dynamics following gen(om)e duplications with special emphasis on the poorly studied Bmp16 gene. We reveal the presence of Bmp16 in chondrichthyans in addition to previously reported teleost fishes and reptiles. Using comprehensive, vertebrate-wide gene sampling, our phylogenetic analysis complemented with synteny analyses suggests that Bmp2, -4 and -16 are remnants of a gene quartet that originated during the two rounds of whole-genome duplication (2R-WGD) early in vertebrate evolution. We confirm that Bmp16 genes were lost independently in at least three lineages (mammals, archelosaurs and amphibians) and report that they have elevated rates of sequence evolution. This finding agrees with their more “flexible” deployment during development; while Bmp16 has limited embryonic expression domains in the cloudy catshark, it is broadly expressed in the green anole lizard. Our study illustrates the dynamics of gene family evolution by integrating insights from sequence diversification, gene repertoire changes, and shuffling of expression domains.

Bone physiology as inspiration for tissue regenerative therapies

The development, maintenance of healthy bone and regeneration of injured tissue in the human body comprise a set of intricate and finely coordinated processes. However, an analysis of current bone regeneration strategies shows that only a small fraction of well-reported bone biology aspects has been used as inspiration and transposed into the development of therapeutic products. Specific topics that include inter-scale bone structural organization, developmental aspects of bone morphogenesis, bone repair mechanisms, role of specific cells and heterotypic cell contact in the bone niche (including vascularization networks and immune system cells), cell-cell direct and soluble-mediated contact, extracellular matrix composition (with particular focus on the non-soluble fraction of proteins), as well as mechanical aspects of native bone will be the main reviewed topics. In this Review we suggest a systematic parallelization of (i) fundamental well-established biology of bone, (ii) updated and recent advances on the understanding of biological phenomena occurring in native and injured tissue, and (iii) critical discussion of how those individual aspects have been translated into tissue regeneration strategies using biomaterials and other tissue engineering approaches. We aim at presenting a perspective on unexplored aspects of bone physiology and how they could be translated into innovative regeneration-driven concepts.

Vg1-Nodal heterodimers are the endogenous inducers of mesendoderm

Nodal is considered the key inducer of mesendoderm in vertebrate embryos and embryonic stem cells. Other TGF-beta-related signals, such as Vg1/Dvr1/Gdf3, have also been implicated in this process but their roles have been unclear or controversial. Here we report that zebrafish embryos without maternally provided vg1 fail to form endoderm and head and trunk mesoderm, and closely resemble nodal loss-of-function mutants. Although Nodal is processed and secreted without Vg1, it requires Vg1 for its endogenous activity. Conversely, Vg1 is unprocessed and resides in the endoplasmic reticulum without Nodal, and is only secreted, processed and active in the presence of Nodal. Co-expression of Nodal and Vg1 results in heterodimer formation and mesendoderm induction. Thus, mesendoderm induction relies on the combination of two TGF-beta-related signals: maternal and ubiquitous Vg1, and zygotic and localized Nodal. Modeling reveals that the pool of maternal Vg1 enables rapid signaling at low concentrations of zygotic Nodal.

All animals begin life as just one cell – a fertilized egg. In order to make a recognizable adult, each embryo needs to make the three types of tissue that will eventually form all of the organs: endoderm, which will form the internal organs; mesoderm, which will form the muscle and bones; and ectoderm, which will generate the skin and nervous system.

All vertebrates – animals with backbones like fish and humans – use the so-called Nodal signaling pathway to make the endoderm and mesoderm. Nodal is a signaling molecule that binds to receptors on the surface of cells. If Nodal binds to a receptor on a cell, it instructs that cell to become endoderm or mesoderm. As such, Nodal is critical for vertebrate life. However, there has been a 30-year debate in the field of developmental biology about whether a protein called Vg1, which has a similar molecular structure as Nodal, plays a role in the early development of vertebrates.

Zebrafish are often used to study animal development, and Montague and Schier decided to test whether these fish need the gene for Vg1 (also known as Gdf3) by deleting it using a genome editing technique called CRISPR/Cas9. It turns out that female zebrafish can survive without this gene. Yet, when the offspring of these females do not inherit the instructions to make Vg1 from their mothers, they fail to form the endoderm and mesoderm. This means that the embryos do not have hearts, blood or other internal organs, and they die within three days. Two other groups of researchers have independently reported similar results.

The findings reveal that Vg1 is critical for the Nodal signaling pathway to work in zebrafish. Montague and Schier then showed that, in this pathway, Nodal does not activate its receptors on its own. Instead, Nodal must interact with Vg1, and it is this Nodal-Vg1 complex that activates receptors, and instructs cells to become endoderm and mesoderm.

Scientists currently use the Nodal signaling pathway to induce human embryonic stem cells growing in the laboratory to become mesoderm and endoderm. As such, these new findings could ultimately help researchers to grow tissues and organs for human patients.

Transfer of the bone morphogenetic protein 4 gene into rat periodontal ligament by in vivo electroporation

OBJECTIVE

Regulation of alveolar bone metabolism is required in clinical dentistry. The aim of the present study was to establish a method for gene transfer into the periodontal ligament (PDL) by in vivo electroporation with a plasmid vector and to investigate the effects of BMP-4 transfer into the PDL.

DESIGN

Plasmids containing mouse BMP-4 cDNA (pCAGGS-BMP4) were transfected into cultured rat PDL cells by in vitro electroporation, and BMP-4 production and secretion were detected by immunocytochemistry and western blotting. Next, pCAGGS-BMP4 was injected into the PDL of rats, and electroporation was performed in vivo, using original paired-needle electrodes. BMP-4 expression was examined by immunohistochemical staining 3, 7, 14, 21, and 28days after electroporation. Control groups were injected with pCAGGS by electroporation, injected with pCAGGS-BMP4 without electroporation, or subjected to neither injection nor electroporation.

RESULTS

In vitro-transfected rat PDL cells exhibited production and secretion of the mature-form BMP-4. After in vivo electroporation of pCAGGS-BMP4, site-specific BMP-4 expression peaked on day 3, gradually decreased until day 14, and was absent by day 21. We observed no unfavorable effects such as inflammation, degeneration, or necrosis.

CONCLUSIONS

Gene transfer by electroporation with plasmid DNA vectors has several advantages over other methods, including the non-viral vector, non-immunogenic effects, site-specific expression, simplicity, cost-effectiveness, and limited histological side effects. Our results indicate that the method is useful for gene therapy targeting the periodontal tissue, which regulates alveolar bone remodeling.

Computational analysis on conformational dynamics of bone morphogenetic protein-2 (BMP-2)

BMP-2 is widely used for bone regeneration because of its ability to induce osteoblast differentiation and proliferation. The pharmaceutical application of BMP-2 as bone implant makes the studies on stability and conformational dynamics very relevant as proteins are functional only in their native three-dimensional state. Knowing the factors affecting BMP-2 structure becomes essential for designing bone implants activated by BMP-2. In order to explore the influence of temperature and hydration on protein conformation, we have performed the molecular dynamics (MD) simulations at the time scale of 100 ns with two different force fields. We have examined the dynamic behaviour of BMP-2 monomer and dimer in aqueous medium as well as in vacuum at four different temperatures (300, 350, 400 and 450 K). MD simulation of BMP-2 monomer and dimer in water and vacuum environments shows the major contribution of water in structure stabilization. Temperature of the system affects the secondary structure differently in case of monomer and dimer simulation and the dynamics also depends on the environment viz. vacuum and aqueous. Vacuum simulations show very early loss of the major secondary structure content. On the other hand, BMP-2 monomer and dimer in aqueous environment show the unfolding of α-helix with increasing temperature. This unfolded α-helix is converted into β-sheet at 400 K in monomer of BMP-2. Contrary to this, we did not observe β-sheet formation in dimer BMP-2 even at 450 K indicating that monomers are more aggregation prone entity as compared to dimers of BMP-2.

A Fusion between Domains of the Human Bone Morphogenetic Protein-2 and Maize 27 kD γ-Zein Accumulates to High Levels in the Endoplasmic Reticulum without Forming Protein Bodies in Transgenic Tobacco

Human Bone Morphogenetic Protein-2 (hBMP2) is an osteoinductive agent physiologically involved in bone remodeling processes. A commercialized recombinant hBMP2 produced in mammalian cell lines is available in different clinical applications where bone regeneration is needed, but widespread use has been hindered due to an unfavorable cost/effective ratio. Protein bodies are very large insoluble protein polymers that originate within the endoplasmic reticulum by prolamine accumulation during the cereal seed development. The N-terminal domain of the maize prolamin 27 kD γ-zein is able to promote protein body biogenesis when fused to other proteins. To produce high yield of recombinant hBMP2 active domain (ad) in stably transformed tobacco plants we have fused it to the γ-zein domain. We show that this zein-hBMP2ad fusion is retained in the endoplasmic reticulum without forming insoluble protein bodies. The accumulation levels are above 1% of total soluble leaf proteins, indicating that it could be a rapid and suitable strategy to produce hBMP2ad at affordable costs.

Mechanisms of BMP-Receptor Interaction and Activation

Bone morphogenetic proteins (BMPs), together with the eponymous transforming growth factor (TGF) β and the Activins form the TGFβ superfamily of ligands. This protein family comprises more than 30 structurally highly related proteins, which determine formation, maintenance, and regeneration of tissues and organs. Their importance for the development of multicellular organisms is evident from their existence in all vertebrates as well as nonvertebrate animals. From their highly specific functions in vivo either a strict relation between a particular ligand and its cognate cellular receptor and/or a stringent regulation to define a distinct temperospatial expression pattern for the various ligands and receptor is expected. However, only a limited number of receptors are found to serve a large number of ligands thus implicating highly promiscuous ligand-receptor interactions instead. Since in tissues a multitude of ligands are often found, which signal via a highly overlapping set of receptors, this raises the question how such promiscuous interactions between different ligands and their receptors can generate concerted and highly specific cellular signals required during embryonic development and tissue homeostasis.

The bone morphogenetic protein-2/7 heterodimer is a stronger inducer of bone regeneration than the individual homodimers in a rat spinal fusion model

BACKGROUND CONTEXT

Bone morphogenetic proteins (BMPs) are a group of dimeric growth factors that belong to the transforming growth factor super family and are capable of eliciting new bone formation. Previous studies have suggested that the coexpression of two different BMP genes in a cell can result in the production of BMP heterodimers that are more potent than homodimers. However, because of the difficulty in optimizing the level of BMP gene expression, the coexpression of two different BMP genes also produces BMP homodimers as a by-product. These homodimers could, in theory, interact with the heterodimers.

PURPOSE

To elucidate the effects of a BMP-2/7 heterodimer, which were investigated in depth using purified BMP-2/7 heterodimers, BMP-2 homodimers, and BMP-7 homodimers in a rat spinal fusion model.

METHODS

Bilateral posterolateral fusion at L4-L5 was performed in four different groups: control group animals were implanted with collagen carriers alone; BMP-7 group animals with collagen carriers+1 μg of BMP-7 homodimer; BMP-2 group animals with collagen carriers+1 μg of BMP-2 homodimer; and BMP-2/7 group animals with collagen carriers+1 μg of the BMP-2/7 heterodimer. The following assessments were performed: bone microstructural analysis of the fusion mass and tissue volume (TV) with microcomputed tomography (micro-CT); fusion assessment with manual palpation testing and three-dimensional CT images; and bone histomorphometrical analysis of the fusion mass.

RESULTS

The fusion scores, as determined by radiography, and the TV of the newly formed bone, as determined by micro-CT, were significantly higher in the BMP-2/7 heterodimer group than the other groups (p<.0001). The microstructural indices of the newly formed bone did not differ between the groups. Moreover, histologic analysis of the fused spines revealed that the formation of the trabecular bone bridging the transverse process was the highest in this group.

CONCLUSIONS

This study demonstrated that BMP-2/7 heterodimer is a stronger inducer of bone regeneration than BMP-2 or -7 homodimers. The use of a purified BMP-2/7 heterodimer may represent an efficient alternative to the current clinical use of BMP-2 or -7 homodimers. Further studies as to the side effects of BMP-2/7 heterodimer are required.

The prodomain of BMP4 is necessary and sufficient to generate stable BMP4/7 heterodimers with enhanced bioactivity in vivo

Bone morphogenetic proteins 4 and 7 (BMP4 and BMP7) are morphogens that signal as either homodimers or heterodimers to regulate embryonic development and adult homeostasis. BMP4/7 heterodimers exhibit markedly higher signaling activity than either homodimer, but the mechanism underlying the enhanced activity is unknown. BMPs are synthesized as inactive precursors that dimerize and are then cleaved to generate both the bioactive ligand and prodomain fragments, which lack signaling activity. Our study reveals a previously unknown requirement for the BMP4 prodomain in promoting heterodimer activity. We show that BMP4 and BMP7 precursor proteins preferentially or exclusively form heterodimers when coexpressed in vivo. In addition, we show that the BMP4 prodomain is both necessary and sufficient for generation of stable heterodimeric ligands with enhanced activity and can enable homodimers to signal in a context in which they normally lack activity. Our results suggest that intrinsic properties of the BMP4 prodomain contribute to the relative bioactivities of homodimers versus heterodimers in vivo. These findings have clinical implications for the use of BMPs as regenerative agents for the treatment of bone injury and disease.

Fibrin Hydrogel Based Bone Substitute Tethered with BMP-2 and BMP-2/7 Heterodimers

Current clinically used delivery methods for bone morphogenetic proteins (BMPs) are collagen based and require large concentrations that can lead to dangerous side effects. Fibrin hydrogels can serve as osteoinductive bone substitute materials in non-load bearing bone defects in combination with BMPs. Two strategies to even further optimize such a fibrin based system include employing more potent BMP heterodimers and engineering growth factors that can be covalently tethered to and slowly released from a fibrin matrix. Here we present an engineered BMP-2/BMP-7 heterodimer where an N-terminal transglutaminase substrate domain in the BMP-2 portion provides covalent attachment to fibrin together with a central plasmin substrate domain, a cleavage site for local release of the attached BMP-2/BMP-7 heterodimer under the influence of cell-activated plasmin. In vitro and in vivo results revealed that the engineered BMP-2/BMP-7 heterodimer induces significantly more alkaline phosphatase activity in pluripotent cells and bone formation in a rat calvarial model than the engineered BMP-2 homodimer. Therefore, the engineered BMP-2/BMP-7 heterodimer could be used to reduce the amount of BMP needed for clinical effect.

Engineering TGF-β superfamily ligands for clinical applications

TGF-β superfamily ligands govern normal tissue development and homeostasis, and their dysfunction is a hallmark of many diseases. These ligands are also well defined both structurally and functionally. This review focuses on TGF-β superfamily ligand engineering for therapeutic purposes, in particular for regenerative medicine and musculoskeletal disorders. We describe the key discovery that structure-guided mutation of receptor-binding epitopes, especially swapping of these epitopes between ligands, results in new ligands with unique functional properties that can be harnessed clinically. Given the promising results with prototypical engineered TGF-β superfamily ligands, and the vast number of such molecules that remain to be produced and tested, this strategy is likely to hold great promise for the development of new biologics.

Synergistic effect of bone morphogenetic proteins 2 and 7 by ex vivo gene therapy in a rat spinal fusion model

BACKGROUND

Previous studies have suggested that the co-expression of two different bone morphogenetic protein (BMP) genes can result in the production of heterodimeric BMPs that may be more potent than homodimers. In this study, combined BMP-2 and BMP-7 gene transfer was performed ex vivo to compare the resulting new bone formation with that of single-BMP gene transfer in a rat spinal fusion model.

METHODS

Forty-four athymic rats underwent posterolateral fusion at L4-L5 and were implanted with a collagen sponge containing human adipose-derived stem cells. Group A received untreated cells, and the remaining groups received cells transfected with various genes in a lentivirus vector. The transferred genes were GFP (green fluorescent protein) in Group B, BMP-2 in Group C, BMP-7 in Group D, and both BMP-2 and BMP-7 in Group E. In vitro production of BMP-2 and BMP-7 was quantified by means of an enzyme-linked immunosorbent assay (ELISA) specific to BMP-2 or BMP-7. Osseous fusion was quantified with use of radiography and microcomputed tomography.

RESULTS

ELISA demonstrated that Group E, which was treated with both BMP-2 and BMP-7, produced less than one-fourth as much BMP as the groups treated with a single transfected BMP (Groups C and D). Radiographs showed that all of the spines in Groups C, D, and E appeared to be fused by eight weeks; the spines in Groups A and B showed minimal evidence of new bone formation. Measurements confirmed that the mean bone formation area was significantly greater in Groups C, D, and E compared with Groups A and B (p &lt; 0.001). In addition, the bone formation area was significantly greater in Group E compared with Groups C and D (p &lt; 0.001).

CONCLUSIONS

Combined BMP-2 and BMP-7 ex vivo gene transfer was found to be significantly more effective for inducing new bone formation compared with ex vivo gene transfer of an individual BMP in a rat spinal fusion model.

CLINICAL RELEVANCE

Combined BMP-2 and BMP-7 therapy may lead to efficient bone regeneration.

Prediction of heparin binding sites in bone morphogenetic proteins (BMPs)

Heparin is a glycosaminoglycan known to bind bone morphogenetic proteins (BMPs) and the growth and differentiation factors (GDFs) and has strong and variable effects on BMP osteogenic activity. In this paper we report our predictions of the likely heparin binding sites for BMP-2 and 14. The N-terminal sequences upstream of TGF-β-type cysteine-knot domains in BMP-2, 7 and 14 contain the basic residues arginine and lysine, which are key components of the heparin/HS-binding sites, with these residues being highly non-conserved. Importantly, evolutionary conserved surfaces on the beta sheets are required for interactions with receptors and antagonists. Furthermore, BMP-2 has electropositive surfaces on two sides compared to BMP-7 and BMP-14. Molecular docking simulations suggest the presence of high and low affinity binding sites in dimeric BMP-2. Histidines were found to play a role in the interactions of BMP-2 with heparin; however, a pK(a) analysis suggests that histidines are likely not protonated. This is indicative that interactions of BMP-2 with heparin do not require acidic pH. Taken together, non-conserved amino acid residues in the N-terminus and residues protruding from the beta sheet (not overlapping with the receptor binding sites and the dimeric interface) and not C-terminal are found to be important for heparin-BMP interactions.

The signaling and functions of heterodimeric bone morphogenetic proteins

Heterodimeric bone morphogenetic proteins (BMPs) consist of disulfide-linked dimeric monomers derived from different BMP members. Owing to this specific constitution pattern, they bear high affinity to both type I and type II BMP receptors simultaneously. Meanwhile, the antagonism efficiency of extracellular antagonists to heterodimeric BMPs is also significantly lower than that to homodimeric ones. All these specific properties confer heterodimeric BMPs with distinct signaling and bio-functions that are characterized by more speediness, lower concentration/dose threshold and higher efficiency than homodimeric BMPs. Consequently, heterodimeric BMPs bear promising application potential in inducing osteogenesis. In addition, they may play indispensible roles in organogenesis. In this review, we summarize the current knowledge of heterodimeric BMPs in their signaling pathways and bio-functions.

Promiscuity and specificity in BMP receptor activation

Bone Morphogenetic Proteins (BMPs), together with Transforming Growth Factor (TGF)-β and Activins/Inhibins constitute the TGF-β superfamily of ligands. This superfamily is formed by more than 30 structurally related secreted proteins. Since TGF-β members act as morphogens, either a strict relation between a particular ligand to a distinct cellular receptor and/or temporospatial expression patterns of ligands and receptors is expected. Instead, only a limited number of receptors exist implicating promiscuous interactions of ligands and receptors. Furthermore, in complex tissues a multitude of different ligands can be found, which signal via overlapping subsets of receptors. This raises the intriguing question how concerted interactions of different ligands and receptors generate highly specific cellular signals, which are required during development and tissue homeostasis.

The BMP2/7 heterodimer inhibits the human breast cancer stem cell subpopulation and bone metastases formation

Accumulating evidence suggests that a subpopulation of breast cancer cells, referred to as cancer stem cells (CSCs), have the ability to propagate a tumor and potentially seed new metastases. Furthermore, stimulation of an epithelial-to-mesenchymal transition by factors like transforming growth factor-β (TGFβ) is accompanied with the generation of breast CSCs. Previous observations indicated that bone morphogenetic protein-7 (BMP7) antagonizes the protumorigenic and prometastatic actions of TGFβ, but whether BMP7 action is mechanistically linked to breast CSCs has remained elusive. Here, we have studied the effects of BMP7, BMP2 and a BMP2/7 heterodimer on the formation of human breast CSCs (ALDH(hi)/CD44(hi)/CD24(-/low)) and bone metastases formation in a preclinical model of intra-cardiac injection of MDA-MB-231 cells in athymic nude (Balb/c nu/nu) mice. The BMP2/7 heterodimer was the most efficient stimulator of BMP signaling and very effectively reduced TGFβ-driven Smad signaling and cancer cell invasiveness. The tested BMPs-particularly the heterodimeric BMP2/7-strongly reduced the size of the ALDH(hi)/CD44(hi)/CD24(-/low) CSC subpopulation. In keeping with these in vitro observations, pretreatment of cancer cells with BMPs for 72 h prior to systemic inoculation of the cancer cells inhibited the formation of bone metastases. Collectively, our data support the notion that breast CSCs are involved in bone metastasis formation and describe heterodimeric BMP2/7 as a powerful TGFβ antagonist with anti-metastatic potency.

Maternal and zygotic control of zebrafish dorsoventral axial patterning

Vertebrate development begins with precise molecular, cellular, and morphogenetic controls to establish the basic body plan of the embryo. In zebrafish, these tightly regulated processes begin during oogenesis and proceed through gastrulation to establish and pattern the axes of the embryo. During oogenesis a maternal factor is localized to the vegetal pole of the oocyte that is a determinant of dorsal tissues. Following fertilization this vegetally localized dorsal determinant is asymmetrically translocated in the egg and initiates formation of the dorsoventral axis. Dorsoventral axis formation and patterning is then mediated by maternal and zygotic factors acting through Wnt, BMP (bone morphogenetic protein), Nodal, and FGF (fibroblast growth factor) signaling pathways, each of which is required to establish and/or pattern the dorsoventral axis. This review addresses recent advances in our understanding of the molecular factors and mechanisms that establish and pattern the dorsoventral axis of the zebrafish embryo, including establishment of the animal-vegetal axis as it relates to formation of the dorsoventral axis.

Osteoinduction by repeat plasmid injection of human bone morphogenetic protein-2

BACKGROUND

Bone morphogenetic protein-2 (BMP-2) is an osteoinductive protein and is considered useful for the treatment of skeletal disorders. Previous studies using BMP-2 in clinical applications have encountered difficulties, including the lack of an efficient, safe, inexpensive and simple delivery system. The gene transfer approach is a promising option for utilizing BMP-2. Although viral vector-mediated gene transfer is efficient, safety concerns prevent its clinical application for common diseases. On the other hand, plasmid-based gene transfer is a safe method and can be harnessed for practical applications.

METHODS

A plasmid encoding human BMP-2 (pCAGGS-BMP-2) was used and injected repeatedly (one to eight times) into the skeletal muscle of mice at a divided dose. We compared the capability of osteoinduction in the skeletal muscle of mice after gene transfer by repeat injection. BMP-2 production was assessed via immunohistochemistry, and osteoinduction was evaluated using radiography, histology and biochemical assays.

RESULTS

The BMP-2 gene was transferred into the skeletal muscle of mice by repeat injection using pCAGGS-BMP-2. Mature bone was frequently observed in mice injected repeatedly with pCAGGS-BMP-2 at a divided dose. This confirms that, if the total dose is fixed, repeat injection with pCAGGS-BMP-2 at a divided dose causes osteoinduction more frequently in the skeletal muscle of mice.

CONCLUSIONS

These results suggest the possibility of the effective clinical use of human BMP-2 gene therapy by direct DNA injection, and facilitate the clinical application of BMP-2 gene therapy.

Osteoblastic differentiation of human and equine adult bone marrow-derived mesenchymal stem cells when BMP-2 or BMP-7 homodimer genetic modification is compared to BMP-2/7 heterodimer genetic modification in the presence and absence of dexamethasone

Bone marrow-derived mesenchymal stem cells (BMDMSCs) have been targeted for use in enhancement of bone healing; and their osteogenic potential may be further augmented by genes encoding bone morphogenetic proteins (BMP's). The purpose of this study was to compare the effect of genetic modification of human and equine BMDMSCs with BMP-2 or -7 or BMP-2 and -7 on their osteoblastogenic differentiation in the presence or absence of dexamethasone. The BMDMSCs were harvested from the iliac crest of three human donors and tuber coxae of three equine donors. Monolayer cells were genetically modified using adenovirus vectors encoding BMP-2, -7 or both and cultured in the presence or absence of dexamethasone. Expression of BMPs was confirmed by enzyme linked immunosorbent assay (ELISA). To evaluate osteoblastic differentiation, cellular morphology was assessed every other day and expression and secretion of alkaline phosphatase (ALP), as well as expression levels of osteonectin (OSTN), osteocalcin (OCN), and runt-related transcription factor-2 (Runx2) were measured for up to 14 days. Human and equine BMDMSCs showed a capacity for osteogenic differentiation regardless of genetic modification or dexamethasone supplementation. Dexamethasone supplementation was more important for osteoblastogenic differentiation of equine BMDMSCs than human BMDMSCs. Genetic modification of BMDMSCs increased ALP secretion with AdBMP-2 homodimer having the greatest effect in both human and equine cells compared to AdBMP 7 or AdBMP 2/7. BMP protein elution rates reached their maximal concentration between day 4 and 8 and remained relatively stable thereafter, suggesting that genetically modified BMDMSCs could be useful for cell-based delivery of BMPs to a site of bone formation.

BMP signaling in vascular development and disease

Genetic and functional studies indicate that common components of the bone morphogenetic protein (BMP) signaling pathway play critical roles in regulating vascular development in the embryo and in promoting vascular homeostasis and disease in the adult. However, discrepancies between in vitro and in vivo findings and distinct functional properties of the BMP signaling pathway in different vascular beds, have led to controversies in the field that have been difficult to reconcile. This review attempts to clarify some of these issues by providing an up to date overview of the biology and genetics of BMP signaling relevant to the intact vasculature.

Bone morphogenetic protein-2 and -6 heterodimer illustrates the nature of ligand-receptor assembly

TGF-beta superfamily ligands are homo- or heterodimeric and recruit two type I and two type II Ser/Thr kinase receptors to initiate a transmembrane signaling cascade. Even with the known structure of the homodimer ligands in complex with extracellular domains of both receptor types, the sequential assembly of the signaling complex with its cognate receptors in the cell membrane remains elusive. We generated a bone morphogenetic protein-2/-6 heterodimer carrying two asymmetric interfaces for each receptor type. We demonstrate that the heterodimer possesses high affinity to both receptor types and increased Smad1-dependent signaling activity by both cell-based and chondrogenesis assays. Furthermore, we find that the minimal signaling complex consists of two type II receptors and one type I receptor per dimer. Our study reveals how the engineered heterodimers may use their independent binding interfaces to differentially recruit the different receptors for each receptor type to create new biological properties.

Identifying concerted evolution and gene conversion in mammalian gene pairs lasting over 100 million years

BACKGROUND

Concerted evolution occurs in multigene families and is characterized by stretches of homogeneity and higher sequence similarity between paralogues than between orthologues. Here we identify human gene pairs that have undergone concerted evolution, caused by ongoing gene conversion, since at least the human-mouse divergence. Our strategy involved the identification of duplicated genes with greater similarity within a species than between species. These genes were required to be present in multiple mammalian genomes, suggesting duplication early in mammalian divergence. To eliminate genes that have been conserved due to strong purifying selection, our analysis also required at least one intron to have retained high sequence similarity between paralogues.

RESULTS

We identified three human gene pairs undergoing concerted evolution (BMP8A/B, DDX19A/B, and TUBG1/2). Phylogenetic investigations reveal that in each case the duplication appears to have occurred prior to eutherian mammalian radiation, with exactly two paralogues present in all examined species. This indicates that all three gene duplication events were established over 100 million years ago.

CONCLUSION

The extended duration of concerted evolution in multiple distant lineages suggests that there has been prolonged homogenization of specific segments within these gene pairs. Although we speculate that selection for homogenization could have been utilized in order to maintain crucial homo- or hetero- binding domains, it remains unclear why gene conversion has persisted for such extended periods of time. Through these analyses, our results demonstrate additional examples of a process that plays a definite, although unspecified, role in molecular evolution.

Unveiling the bmp13 enigma: redundant morphogen or crucial regulator?

Bone morphogenetic proteins are a diverse group of morphogens with influences not only on bone tissue, as the nomenclature suggests, but on multiple tissues in the body and often at crucial and influential periods in development.

The purpose of this review is to identify and discuss current knowledge of one vertebrate BMP, Bone Morphogenetic Protein 13 (BMP13), from a variety of research fields, in order to clarify BMP13's functional contribution to developing and maintaining healthy tissues, and to identify potential future research directions for this intriguing morphogen. BMP13 is highly evolutionarily conserved (active domain >95%) across diverse species from Zebrafish to humans, suggesting a crucial function. In addition, mutations in BMP13 have recently been associated with Klippel-Feil Syndrome, causative of numerous skeletal and developmental defects including spinal disc fusion. The specific nature of BMP13's crucial function is, however, not yet known.

The literature for BMP13 is focused largely on its activity in the healing of tendon-like tissues, or in comparisons with other BMP family molecules for whom a clear function in embryo development or osteogenic differentiation has been identified. There is a paucity of detailed information regarding BMP13 protein activity, structure or protein processing. Whilst some activity in the stimulation of osteogenic or cartilaginous gene expression has been reported, and BMP13 expression is found in post natal cartilage and tendon tissues, there appears to be a redundancy of function in the BMP family, with several members capable of stimulating similar tissue responses. This review aims to summarise the known or potential role(s) for BMP13 in a variety of biological systems.

BMP2 and BMP7 play antagonistic roles in feather induction

Feathers, like hairs, first appear as primordia consisting of an epidermal placode associated with a dermal condensation that is necessary for the continuation of their differentiation. Previously, the BMPs have been proposed to inhibit skin appendage formation. We show that the function of specific BMPs during feather development is more complex. BMP2 and BMP7, which are expressed in both the epidermis and the dermis, are involved in an antagonistic fashion in regulating the formation of dermal condensations, and thus are both necessary for subsequent feather morphogenesis. BMP7 is expressed earlier and functions as a chemoattractant that recruits cells into the condensation, whereas BMP2 is expressed later, and leads to an arrest of cell migration, likely via its modulation of the EIIIA fibronectin domain and alpha4 integrin expression. Based on the observed cell proliferation, chemotaxis and the timing of BMP2 and BMP7 expression, we propose a mathematical model, a reaction-diffusion system, which not only simulates feather patterning, but which also can account for the negative effects of excess BMP2 or BMP7 on feather formation.

Uptake of nickel from 316L stainless steel into contacting osteoblastic cells and metal ion interference with BMP-2-induced alkaline phosphatase

Bone cells contacting nickel (Ni)-containing implant materials may be affected by Ni species via disturbed signaling pathways involved in bone cell development. Here we analyze effects of the Ni-containing steel 316L and major metal constituents thereof on bone morphogenetic protein-2 (BMP-2)-induced alkaline phosphatase (ALP) of MC3T3-E1 cells. While cells grew normally on 316L, cellular Ni content increased 10-fold vs. control within 4 days. With respect to the major components of 316L, Ni2+ (3-50 microM) was most inhibitory to BMP-2-induced ALP, whereas even 50 microM Fe3+, Cr3+, Mo5+, or Mn2+ had no such effect. In line with this, BMP-2-induced ALP was significantly reduced in cells on 316L. This effect was not prevented by the metal ion chelator diethylenetriaminepentaacetic acid (DTPA). Instead, DTPA abolished the stimulatory effect of BMP-2 on ALP, pointing to chelatable metal ions involved. Zn2+, as one possible candidate, antagonized the Ni2+ inhibition of BMP-2-induced ALP in both MC3T3-E1 and human bone marrow stromal cells. Results show that cells contacting 316L steel are exposed to increased concentrations of Ni which suffice to impair BMP-2-induced ALP activity. Zn2+, as a competitor of this inhibition, may help to restore normal osteoblastic function and bone development under these conditions.

Analysis of osteochondro-induction using growth and differentiation factor-5 in rat muscle

Growth and differentiation factor-5 (GDF-5) belongs to the TGF-beta super family, and reportedly plays an important role in cartilage development and differentiation. In this study, we implanted GDF-5 in rat leg muscle, and evaluated its in vivo osteochondro-inducing activity by histological and X-ray examinations. GDF-5 (0, 100, 300, and 500 microg) and the carrier type I collagen were mixed, and the mixture was implanted into rat leg muscle. Three weeks later, the site of implantation was examined by soft X-ray, and examined histologically. The GDF-5 0 and 100 microg groups showed no osteochondro-induction. The GDF-5 300 microg group showed aggregates of cartilage and some bone tissue in the carrier. The GDF-5 500 microg group revealed bone and no cartilage. This is the first report of the dose-dependent effect of GDF-5 inducing osteochondrogenesis or osteogenesis.

Heat treatment of BMP-2 depots on implant materials generates an immobilized layer of BMP-2 with pronounced bioactivity

Bone cells seeded directly on depots of bone morphogenetic protein-2 (BMP-2) increase alkaline phosphatase (ALP) expression. Heating of such BMP-2 depots to 100 degrees C augmented the intensity of this local ALP induction. To understand this unexpected finding, we investigated the effect of heat treatment on BMP-2 depots more closely. Using a novel bioassay based on ALP-induction of remote cells, we found that the amount of released bioactive BMP-2 from heat-treated depots decays within days and could be described by an exponential function. From this function, we expected that pre-incubation of BMP-2 depots in culture medium for 4 weeks renders them insufficient to induce ALP. However, preincubated, heat-treated depots still induced maximal ALP, unless treated with the selective BMP-2 inhibitor noggin. Furthermore, heat treatment of BMP-2 depots generated a layer of immunoreactive BMP-2 at the surface of the carrier. In contrast, BMP-2 was washed off completely if heat treatment of adsorbed protein was omitted. Results show that heat treatment generates both a soluble pool of BMP-2 and a material-bound layer of BMP-2 in which the protein is protected against degradation. Therefore, heat treatment appears useful to locally immobilize BMP-2 on various implant surfaces.

Simultaneous gene transfer of bone morphogenetic protein (BMP) -2 and BMP-7 by in vivo electroporation induces rapid bone formation and BMP-4 expression

BACKGROUND

Transcutaneous in vivo electroporation is expected to be an effective gene-transfer method for promoting bone regeneration using the BMP-2 plasmid vector. To promote enhanced osteoinduction using this method, we simultaneously transferred cDNAs for BMP-2 and BMP-7, as inserts in the non-viral vector pCAGGS.

METHODS

First, an in vitro study was carried out to confirm the expression of BMP-2 and BMP-7 following the double-gene transfer. Next, the individual BMP-2 and BMP-7 plasmids or both together were injected into rat calf muscles, and transcutaneous electroporation was applied 8 times at 100 V, 50 msec.

RESULTS

In the culture system, the simultaneous transfer of the BMP-2 and BMP-7 genes led to a much higher ALP activity in C2C12 cells than did the transfer of either gene alone. In vivo, ten days after the treatment, soft X-ray analysis showed that muscles that received both pCAGGS-BMP-2 and pCAGGS-BMP-7 had better-defined opacities than those receiving a single gene. Histological examination showed advanced ossification in calf muscles that received the double-gene transfer. BMP-4 mRNA was also expressed, and RT-PCR showed that its level increased for 3 days in a time-dependent manner in the double-gene transfer group. Immunohistochemistry confirmed that BMP-4-expressing cells resided in the matrix between muscle fibers.

CONCLUSION

The simultaneous transfer of BMP-2 and BMP-7 genes using in vivo electroporation induces more rapid bone formation than the transfer of either gene alone, and the increased expression of endogenous BMP-4 suggests that the rapid ossification is related to the induction of BMP-4.

Noggin regulation of bone morphogenetic protein (BMP) 2/7 heterodimer activity in vitro

Bone morphogenic proteins (BMPs) are growth factors important for skeletal development and bone growth. Noggin, one of the soluble BMP antagonists, regulates the action of BMPs on mesenchymal precursor cells, partially through a feedback type of inhibition. In this study, we constructed a novel BMP2/7 'fusion gene' that encodes both BMP2 and BMP7 genes in tandem by a linker. Polymerase chain reaction (PCR) and Western blotting showed that the BMP2/7 fusion gene construct led to the production of BMP2/7 heterodimers in A549 'producer' cells. When applied to C2C12 myoblastic cells, BMP2/7 heterodimers increased alkaline phosphatase (ALP) activity and osteocalcin (OCN) expression (markers of osteoblastic differentiation) more effectively than either BMP2 or BMP7 homodimers. Moreover, this heterodimer induced significantly lower levels of Noggin expression in C2C12 cells than respective homodimers at similar doses. The addition of Noggin did not affect the heterodimer's activities in increasing osteoblastic differentiation in C2C12 cells. In contrast, BMP2 and BMP7 homodimers were largely inhibited by Noggin. Our finding suggests that the 'fusion gene' construct led to the production of bioactive BMP2/7 heterodimers, which were not antagonized by Noggin as effectively as it to BMP homodimers. The weaker Noggin antagonism on BMP heterodimers compared to homodimers may contribute to increased osteogenic potency of heterodimers in vitro and in vivo.

Human BMP-2 gene transfer using transcutaneous in vivo electroporation induced both intramembranous and endochondral ossification

It has been generally accepted that bone morphogenetic protein-2 (BMP-2) can induce osteogenesis in skeletal muscles via endochondral ossification. However, it is not clear how the ossification process occurs after the BMP-2 gene transfer to skeletal muscles in rats using in vivo electroporation. In this study, we evaluated the ossification process by BMP-2 gene transfer using in vivo electroporation. The gastrocnemius muscles of Wistar rats were injected with human BMP-2 gene expression vector (pCAGGS-BMP-2), followed by electroporation under the condition of 100 V, 50 msec per 1 sec, x8. Light and electron microscopic and radiographic analyses were performed at 1, 3, 5, 7, and 10 days after treatment. At 7 days, no sign of cartilage and/or bone formation was detected. However, at 10 days after in vivo electroporation, soft X-ray analysis revealed small lucent areas around the plasmid-injected region. Clusters of both cartilage tissues, leading to endochondral ossification and intramembranous bones of various sizes, were observed between muscle fibers. RT-PCR detected osteocalcin mRNA, showing bone formation at 10 days. Our findings strongly suggest that BMP-2 gene transfer using in vivo electroporation induces not only endochondral ossification but also intramembranous ossification.

Modern biologics used in orthopaedic surgery

PURPOSE OF REVIEW

In this review we summarize some of the most recent research in the area of local bone regeneration. These innovations may be relevant in the orthopaedic treatment of patients with rheumatoid arthritis, or other inflammatory arthridities, as such patients often present with inadequate bone stock.

RECENT FINDINGS

Bone grafting remains the standard treatment for bone deficiency. Several new approaches, such as the use of concentrated blood products or osteoprogenitor cells in conjunction with grafts, have been developed but remain to be tested clinically. Experimental studies have elucidated important aspects of the biology of bone graft remodeling and osteoprogenitor cell differentiation. Materials that can serve as graft alternatives continue to be developed. Positive experimental findings have resulted from combinations of such materials with osteoprogenitor cells or osteoinductive factors such as bone morphogenetic proteins.

SUMMARY

While few studies to date have examined the specific use of these new strategies in the setting of rheumatoid arthritis, many hold promise for patients with rheumatoid arthritis and other inflammatory and metabolic conditions that affect bone quality and quantity.

Osteoinductivity of partially purified native ostrich (Struthio camelus) bone morphogenetic protein: Comparison with mammalian species

Bone morphogenetic proteins (BMPs) are members of the TGF-beta superfamily. They are capable of inducing ectopic bone formation. Until now, the main interest has been focused on mammalian osteoinductive BMPs, and there are no reports of native BMP extracts of birds. In this study, we isolated and characterized native BMPs of ostrich (Struthio camelus) and compared them with identically isolated native bovine (cow) and reindeer BMPs with regard to BMP pattern and osteoinductive capacity. The ostrich BMP pattern differed markedly from that of cow and reindeer BMP in non-reduced SDS-PAGE, reduced SDS-PAGE and Western blot. The differences in isoelectric focusing analysis were smaller. However, the ostrich BMP extract had a peak at pH 5.1, clearly differing from the BMPs of cow and reindeer. The osteoinductive capacity and density of ectopic bone, induced by BMP extracts in a mouse thigh muscle pouch, were determined radiographically. The ostrich BMP extract displayed significantly lower osteoinductive capacity and density of induced bone than the bovine and reindeer BMP extracts. In conclusion, our results indicate that the BMP pattern of birds differs considerably from that of mammals, and that the osteoinductive capacity of BMPs and the density of induced bone are lower in birds than in mammals. They also suggest that the bone metabolism of birds is adapted to make light bones suitable for flying.

Combinatorial gene therapy for bone regeneration: Cooperative interactions between adenovirus vectors expressing bone morphogenetic proteins 2, 4, and 7

Bone morphogenetic proteins (BMPs) have demonstrated effectiveness as bone regeneration agents whether delivered as recombinant proteins or via gene therapy. Current gene therapy approaches use vectors expressing single BMPs. In contrast, multiple BMPs are coordinately expressed during bone development and fracture healing. Furthermore, BMPs likely exist in vivo as heterodimeric molecules having enhanced biological activity. In the present study, we test the hypothesis that gene therapy-based bone regeneration can be enhanced by expressing combinations of BMPs. For in vitro studies, mesenchymal cell lines were transduced with individual adenoviruses containing BMP2, 4, or 7 cDNA under control of a CMV promoter (AdBMP2, 4, 7) or virus combinations. Significantly, combined transduction with AdBMP2 plus AdBMP7 or AdBMP4 plus AdBMP7 resulted in a synergistic stimulation of osteoblast differentiation. This synergy is best explained by formation of BMP2/7 and 4/7 heterodimers. To test in vivo biological activity, fibroblasts were transduced with specific virus combinations and implanted into C57BL6 mice. Consistent with in vitro results, strong synergy was observed using combined AdBMP2/BMP7 treatment, which induced twofold to threefold more bone than would be predicted based on the activity of individual AdBMPs. These studies show that dramatic enhancement of osteogenesis can be achieved using gene therapy to express specific combinations of interacting regenerative molecules.

Combined bone morphogenetic protein-2 and -7 gene transfer enhances osteoblastic differentiation and spine fusion in a rodent model

To enhance the osteogenic activity of BMP, combination BMP2 and BMP7 gene transfer was performed. This approach led to a significant increase in osteoblastic differentiation of mesenchymal precursors compared with single BMP gene transfer in vitro. When tested in 78 rats, combination gene transfer enhanced mechanically stable spine fusion and bone formation rate versus single BMP gene transfer.

INTRODUCTION

Although clinical bone morphogenetic protein (BMP) therapy is effective, required doses are very high. Previous studies have suggested that the co-expression of two different BMP genes can result in the production of heterodimeric BMPs that may be more potent than homodimers. In this study, combined BMP2 and BMP7 gene transfer was performed to test whether this approach improves osteoblastic differentiation and bone formation compared with single BMP gene transfer.

MATERIALS AND METHODS

A producer cell (A549) was co-transfected with adenovirus vectors encoding BMP2 (AdBMP2) and BMP7 (AdBMP7) or, as controls, each vector alone, AdNull (with no transgene) or no virus. Supernatants were compared for their ability to stimulate osteoblastic differentiation of C2C12 myoblasts and MC3T3-E1 pre-osteoblasts. In a rat posterolateral spine fusion model, co-administration of AdBMP2 and AdBMP7 was compared with treatment with each vector alone, AdNull or no virus in 78 rats. The spines were assessed 8 weeks after surgery for radiographic and mechanical fusion, bone formation, and mineralization.

RESULTS

BMP2 and BMP7 were co-precipitated from supernatants of cells co-transfected with AdBMP2 and AdBMP7, indicating the presence of BMP2/7 heterodimer. Supernatants of co-transfected cells containing relatively low doses (7-140 ng/ml) of BMPs induced osteocalcin expression and alkaline phosphatase activity in both C2C12 and MC3T3-E1 cells, that were up to 6- and 40-fold higher, respectively, than levels induced by maximal doses (200-1000 ng/ml) of either BMP2 or BMP7 alone. In the spine fusion model, co-administration of AdBMP2 and AdBMP7 resulted in a significantly greater number of mechanically stable fusions and also 2-fold higher mineralization rate and bone volume in the fusion mass versus single BMP gene transfer (p < 0.02, all comparisons).

CONCLUSION

Combined BMP2 and BMP7 gene transfer is significantly more effective in inducing osteoblastic differentiation and spine fusion than individual BMP gene transfer.

Ectopic bone formation by human bone morphogenetic protein-2 gene transfer to skeletal muscle using transcutaneous electroporation

Therapy using recombinant human bone morphogenetic protein-2 (rhBMP-2) is expected to promote bone healing and regeneration. Previous studies using protein or virus vectors for direct clinical application had problems, including a lack of efficiency, safety, and simplicity of the delivery system, and required an expensive protein, carrier matrix, or antigenic viral vector. In vivo gene transfer by electroporation is a simple and inexpensive method that only requires a plasmid and an electroporation device. Here, we created a plasmid-based human BMP-2 construct (pCAGGS-BMP-2) and examined the induction of bone in the skeletal muscle of rats after transferring different doses of this plasmid (25 microg, 100 microg, and 400 microg) by transcutaneous electroporation (8 electrical pulses of 100 V and 50 msec, in 1 to 5 sessions). First, we verified the gene transfer by transcutaneous electroporation using pCAGGS-lacZ. Next, the BMP-2 gene transfer and the production and localization of BMP-2 were identified by reverse transcription-polymerase chain reaction (RT-PCR), Western blots, and immunohistochemistry. Ectopic bone formation was verified by radiography, histologic and immunohistochemical analyses, and quantitative examination. Ectopic bone formation, consisting of active osteoblasts and osteoclasts, was observed in all rats treated with electroporation. Thus, transcutaneous electroporation with pCAGGS-BMP-2 induced ectopic bone formation in the skeletal muscle of rats. This supports the possibility of applying human BMP-2 gene transfer using transcutaneous electroporation clinically.

Rat strain differences in the ectopic osteogenic potential of recombinant human BMP adenoviruses

Different animal strains have different genetic backgrounds that influence their physiological function and pathological process. The differences in genetic background may affect the efficiency of adenoviral infection and target gene expression and further cause different gene therapy results when target genes are delivered with adenoviral vectors. In this study, ectopic bone was not seen in ADCMVBMP4 injection sites, but was formed in ADCMVBMP9 injection sites in all rat strains. The mean volumes of bone induced with ADCMVBMP9 were 0.87 +/- 0.2 cm3 in Wistar, 0.26 +/- 0.1 cm3 in Long-Evans, 0.34 +/- 0.2 cm3 in Sprague-Dawley, 0.44 +/- 0.1 cm3 in ACI, 0.66 +/- 0.2 cm3 in PVG, and 0.58 +/- 0.1 cm3 in Fischer 344 rats. This indicates that ADCMVBMP9 has different bone formation potentials in different immunocompetent rat strains (P = 0.02). The basic levels of CD4+ and CD8+ T cells in blood before viral infection and titers of adenoviral neutralizing antibodies 30 days post-viral infection were significantly different among rat strains (P < 0.01). The efficiencies of target gene expression delivered with adenovirus were also significantly different in primary muscle cell cultures from different rat strains (P < 0.01). The different osteogenic potentials of ADCMVBMP9 among rat strains may be, in part, due to the differences in immune factors and target gene expression efficiency in muscle tissue.

Sulfated polysaccharides enhance the biological activities of bone morphogenetic proteins

Bone morphogenetic proteins (BMPs), which have been shown to be heparin-binding proteins, induce osteoblast differentiation in mesenchymal cells. In the present study, we examined the effects of heparin on the BMP activities in C2C12 myoblasts. Heparin dose dependently enhanced the osteoblast differentiation induced by not only homodimers of BMP-2 or BMP-4 but also heterodimers of BMP-2/6 or BMP-2/7. However, the osteoblast differentiation induced by the constitutively active BMPR-IA, a functional BMP type I receptor, was not affected by heparin. Heparan sulfate and dextran sulfate also enhanced the BMP-2 activity, although the chemically desulfated heparin-derivatives have lost this stimulatory capacity. Heparin dose-dependently suppressed the accumulation of BMP-2 from the culture media into the cell layer or BMPR-IA, and retained a large amount of BMP-2 in the culture media. The biological activity of BMP-2, which was evaluated using a BMP-responsive reporter gene expression, was prolonged in the presence of heparin. Taken together, these results suggest that sulfated polysaccharides enhance the biological activity of both homodimers and heterodimers of BMPs by continuously serving the ligands to their signaling receptors expressed on cell membranes.

Osteogenic potential of five different recombinant human bone morphogenetic protein adenoviral vectors in the rat

Bone morphogenetic protein (BMP) adenoviral vectors for the induction of osteogenesis are being developed for the treatment of bone pathology. However, it is still unknown which BMP adenoviral vector has the highest potential to stimulate bone formation in vivo. In this study, the osteogenic activities of recombinant human BMP-2, BMP-4, BMP-6, BMP-7, and BMP-9 adenoviruses were compared in vitro, in athymic nude rats, and in Sprague-Dawley rats. In vitro osteogenic activity was assessed by measuring the alkaline phosphatase activity in C2C12 cells transduced by the various BMP vectors. The alkaline phosphatase activity induced by 2 x 10(5) PFU/well of BMP viral vector was 4890 x 10(-12) U/well for ADCMVBMP-9, 302 x 10(-12) U/well for ADCMVBMP-4, 220 x 10(-12) U/well for ADCMVBMP-6, 45 x 10(-12) U/well for ADCMVBMP-2, and 0.43 x 10(-12) U/well for ADCMVBMP-7. The average volume of new bone induced by 10(7) PFU of BMP vector in athymic nude rats was 0.37+/-0.03 cm(3) for ADCMVBMP-2, 0.89+/-0.07 cm(3) for ADCMVBMP-4, 1.02+/-0.07 cm(3) for ADCMVBMP-6, 0.24+/-0.05 cm(3) for ADCMVBMP-7, and 0.63+/-0.07 cm(3) for ADCMVBMP-9. In immunocompetent Sprague-Dawley rats, no bone formation was demonstrated in the ADCMVBMP-2, ADCMVBMP-4, and ADCMVBMP-7 groups. ADCMVBMP-6 at a viral dose of 10(8) PFU induced 0.10+/-0.03 cm(3) of new bone, whereas ADCMVBMP-9 at a lower viral dose of 10(7) PFU induced more bone, with an average volume of 0.29+/-0.01 cm(3).