Lovorka Grgurevic

BMP6 is a key endogenous regulator of hepcidin expression and iron metabolism

Juvenile hemochromatosis is an iron-overload disorder caused by mutations in the genes encoding the major iron regulatory hormone hepcidin (HAMP) and hemojuvelin (HFE2). We have previously shown that hemojuvelin is a co-receptor for bone morphogenetic proteins (BMPs) and that BMP signals regulate hepcidin expression and iron metabolism. However, the endogenous BMP regulator(s) of hepcidin in vivo is unknown. Here we show that compared with soluble hemojuvelin (HJV.Fc), the homologous DRAGON.Fc is a more potent inhibitor of BMP2 or BMP4 but a less potent inhibitor of BMP6 in vitro. In vivo, HJV.Fc or a neutralizing antibody to BMP6 inhibits hepcidin expression and increases serum iron, whereas DRAGON.Fc has no effect. Notably, Bmp6-null mice have a phenotype resembling hereditary hemochromatosis, with reduced hepcidin expression and tissue iron overload. Finally, we demonstrate a physical interaction between HJV.Fc and BMP6, and we show that BMP6 increases hepcidin expression and reduces serum iron in mice. These data support a key role for BMP6 as a ligand for hemojuvelin and an endogenous regulator of hepcidin expression and iron metabolism in vivo.

Systemically Administered Bone Morphogenetic Protein-6 Restores Bone in Aged Ovariectomized Rats by Increasing Bone Formation and Suppressing Bone Resorption

Although recombinant human bone morphogenetic proteins (BMPs) are used locally for treating bone defects in humans, their systemic effect on bone augmentation has not been explored. We have previously demonstrated that demineralized bone (DB) from ovariectomized (OVX) rats cannot induce bone formation when implanted ectopically at the subcutaneous site. Here we showed in vitro that 17β-estradiol (E2) specifically induced expression of Bmp6 mRNA in MC3T3-E1 preosteoblastic cells and that bone extracts from OVX rats lack BMPs. Next we demonstrated that 125I-BMP-6 administered systemically accumulated in the skeleton and also restored the osteoinductive capacity of ectopically implanted DB from OVX rats. BMP-6 applied systemically to aged OVX rats significantly increased bone volume and mechanical characteristics of both the trabecular and cortical bone, the osteoblast surface, serum osteocalcin and osteoprotegerin levels, and decreased the osteoclast surface, serum C-telopeptide, and interleukin-6. E2 was significantly less effective, and was not synergistic with BMP-6. Animals that discontinued BMP-6 therapy maintained bone mineral density gains for another 12 weeks. BMP-6 increased in vivo the bone expression of Acvr-1, Bmpr1b, Smad5, alkaline phosphatase, and collagen type I and decreased expression of Bmp3 and BMP antagonists, chordin and cerberus. These results show, for the first time, that systemically administered BMP-6 restores the bone inductive capacity, microarchitecture, and quality of the skeleton in osteoporotic rats.

Regulation of TMPRSS6 by BMP6 and iron in human cells and mice

Mutations in transmembrane protease, serine 6 (TMPRSS6), encoding matriptase-2, are responsible for the familial anemia disorder iron-refractory iron deficiency anemia (IRIDA). Patients with IRIDA have inappropriately elevated levels of the iron regulatory hormone hepcidin, suggesting that TMPRSS6 is involved in negatively regulating hepcidin expression. Hepcidin is positively regulated by iron via the bone morphogenetic protein (BMP)-SMAD signaling pathway. In this study, we investigated whether BMP6 and iron also regulate TMPRSS6 expression. Here we demonstrate that, in vitro, treatment with BMP6 stimulates TMPRSS6 expression at the mRNA and protein levels and leads to an increase in matriptase-2 activity. Moreover, we identify that inhibitor of DNA binding 1 is the key element of the BMP-SMAD pathway to regulate TMPRSS6 expression in response to BMP6 treatment. Finally, we show that, in mice, Tmprss6 mRNA expression is stimulated by chronic iron treatment or BMP6 injection and is blocked by injection of neutralizing antibody against BMP6. Our results indicate that BMP6 and iron not only induce hepcidin expression but also induce TMPRSS6, a negative regulator of hepcidin expression. Modulation of TMPRSS6 expression could serve as a negative feedback inhibitor to avoid excessive hepcidin increases by iron to help maintain tight homeostatic balance of systemic iron levels.

BMP-6 and mesenchymal stem cell differentiation

Bone morphogenetic protein-6 (BMP-6) is produced by bone marrow-mesenchymal (BMSC) and hematopoietic stem cells, which can differentiate into bone, cartilage, adipose, muscle, hematopoietic, synovial and other tissues. Bmp6-/- null mice have low hepcidin serum levels and an iron overload, resembling hereditary hemochromatosis, which may cause a reduced number of pancreatic beta-cells, increased serum glucose and diabetes. BMP-6 circulates in the normal human plasma and is produced by BMSC prior to differentiation into osteoblasts. Moreover, it is also released by osteoclasts as a key bone coupling factor recruiting osteoblasts to the resorption site. Due to unique structural, receptor binding and signaling characteristics much smaller amounts of BMP-6 than BMP-7 are needed in vivo to induce regeneration of bone defects in animals.

Identification of a Key Residue Mediating Bone Morphogenetic Protein (BMP)-6 Resistance to Noggin Inhibition Allows for Engineered BMPs with Superior Agonist Activity

Bone morphogenetic proteins (BMPs) are used clinically to induce new bone formation in spinal fusions and long bone non-union fractures. However, large amounts of BMPs are needed to achieve these effects. BMPs were found to increase the expression of antagonists, which potentially limit their therapeutic efficacy. However, the relative susceptibility of osteoinductive BMPs to different antagonists is not well characterized. Here we show that BMP-6 is more resistant to noggin inhibition and more potent in promoting osteoblast differentiation in vitro and inducing bone regeneration in vivo when compared with its closely related BMP-7 paralog. Noggin was found to play a critical role as a negative feedback regulator of BMP-7 but not BMP-6-induced biological responses. Using BMP-6/7 chimeras, we identified lysine 60 as a key residue conferring noggin resistance within the BMP-6 protein. A remarkable correlation was found between the presence of a lysine at this position and noggin resistance among a panel of osteoinductive BMPs. Introduction of a lysine residue at the corresponding positions of BMP-2 and BMP-7 allowed for molecular engineering of recombinant BMPs with increased resistance to noggin antagonism.

The clinical use of bone morphogenetic proteins revisited: a novel biocompatible carrier device OSTEOGROW for bone healing

PurposeThe purpose of this study was to revise the clinical use of commercial BMP2 (Infuse) and BMP7 (Osigraft) based bone devices and explore the mechanism of action and efficacy of low BMP6 doses in a novel whole blood biocompatible device OSTEOGROW.MethodsComplications from the clinical use of BMP2 and BMP7 have been systemically reviewed in light of their role in bone remodeling. BMP6 function has been assessed in Bmp6-/- mice by μCT and skeletal histology, and has also been examined in mesenchymal stem cells (MSC), hematopoietic stem cells (HSC) and osteoclasts. Safety and efficacy of OSTEOGROW have been assessed in rats and rabbits.ResultsClinical use issues of BMP2 and BMP7 have been ascribed to the limited understanding of their role in bone remodeling at the time of device development for clinical trials. BMP2 and BMP7 in bone devices significantly promote bone resorption leading to osteolysis at the endosteal surfaces, while in parallel stimulating exuberant bone formation in surrounding tissues. Unbound BMP2 and BMP7 in bone devices precipitate on the bovine collagen and cause inflammation and swelling. OSTEOGROW required small amounts of BMP6, applied in a biocompatible blood coagulum carrier, for stimulating differentiation of MSCs and accelerated healing of critical size bone defects in animals, without bone resorption and inflammation. BMP6 decreased the number of osteoclasts derived from HSC, while BMP2 and BMP7 increased their number.ConclusionsCurrent issues and challenges with commercial bone devices may be resolved by using novel BMP6 biocompatible device OSTEOGROW, which will be clinically tested in metaphyseal bone fractures, compartments where BMP2 and BMP7 have not been effective.

Bone morphogenetic protein (BMP)1-3 enhances bone repair

Members of the astacin family of metalloproteinases such as human bone morphogenetic protein 1 (BMP-1) regulate morphogenesis by processing precursors to mature functional extracellular matrix (ECM) proteins and several growth factors including TGFβ, BMP2, BMP4 and GFD8. We have recently discovered that BMP1-3 isoform of the Bmp-1 gene circulates in the human plasma and is significantly increased in patients with acute bone fracture. We hypothesized that circulating BMP1-3 might have an important role in bone repair and serve as a novel bone biomarker. When administered systemically to rats with a long bone fracture and locally to rabbits with a critical size defect of the ulna, recombinant human BMP1-3 enhanced bone healing. In contrast, neutralization of the endogenous BMP1-3 by a specific polyclonal antibody delayed the bone union. Invitro BMP1-3 increased the expression of collagen type I and osteocalcin in MC3T3-E(1) osteoblast like cells, and enhanced the formation of mineralized bone nodules from bone marrow mesenchymal stem cells. We suggest that BMP1-3 is a novel systemic regulator of bone repair.

Circulating Bone Morphogenetic Protein 1–3 Isoform Increases Renal Fibrosis

Bone morphogenetic proteins (BMPs) participate in organ regeneration through autocrine and paracrine actions, but the existence and effects of these proteins in the systemic circulation is unknown. Using liquid chromatography-mass spectrometry, we identified BMP6, GDF15, and the BMP1-3 isoform of the Bmp1 gene in plasma samples from healthy volunteers and patients with CKD. We isolated the endogenous BMP1-3 protein and demonstrated that it circulates as an active enzyme, evidenced by its ability to cleave dentin matrix protein-1 in vitro. In rats with CKD, administration of recombinant BMP1-3 increased renal fibrosis and reduced survival. In contrast, administration of a BMP1-3-neutralizing antibody reduced renal fibrosis, preserved renal function, and increased survival. In addition, treating with the neutralizing antibody was associated with low plasma levels of TGFβ1 and connective tissue growth factor. In HEK293 cells and remnant kidneys, BMP1-3 increased the transcription of collagen type I, TGFβ1, β-catenin, and BMP7 via a BMP- and Wnt-independent mechanism that involved signaling through an integrin β1 subunit. The profibrotic effect of BMP1-3 may, in part, be a result of the accompanied decrease in decorin (DCN) expression. Taken together, inhibition of circulating BMP1-3 reduces renal fibrosis, suggesting that this pathway may be a therapeutic target for CKD.

Bone morphogenetic proteins in inflammation, glucose homeostasis and adipose tissue energy metabolism

Bore morphogenetic proteins (BMPs) are members of the transforming growth factor (TGF)-β superfamily, a group of secreted proteins that regulate embryonic development. This review summarizes the effects of BMPs on physiological processes not exclusively linked to the musculoskeletal system. Specifically, we focus on the involvement of BMPs in inflammatory disorders, e.g. fibrosis, inflammatory bowel disease, anchylosing spondylitis, rheumatoid arthritis. Moreover, we discuss the role of BMPs in the context of vascular disorders, and explore the role of these signalling proteins in iron homeostasis (anaemia, hemochromatosis) and oxidative damage. The second and third parts of this review focus on BMPs in the development of metabolic pathologies such as type-2 diabetes mellitus and obesity. The pancreatic beta cells are the sole source of the hormone insulin and BMPs have recently been implicated in pancreas development as well as control of adult glucose homeostasis. Lastly, we review the recently recognized role of BMPs in brown adipose tissue formation and their consequences for energy expenditure and adiposity. In summary, BMPs play a pivotal role in metabolism beyond their role in skeletal homeostasis. However, increased understanding of these pleiotropic functions also highlights the necessity of tissue-specific strategies when harnessing BMP action as a therapeutic target.

Detection of bone and cartilage-related proteins in plasma of patients with a bone fracture using liquid chromatography–mass spectrometry

Following bone fracture, a large number of growth factors, cytokines, and their cognate receptors involved in the repair process are active at the fracture site. To determine whether they appear in patients’ blood as candidate biomarkers for following the outcome of healing, we analysed the plasma of 25 patients with an acute bone fracture following affinity plasma purification, SDS gel electrophoresis and liquid chromatography - tandem mass spectrometry (LC-MS/MS). Two hundred and thirteen nonredundant proteins were identified in the in-gel analysis of pooled plasma proteins. Gene ontology (GO) analysis indicated that a majority of detected proteins were of extracellular origin, whereas only a small number were of intracellular (cytosol and nucleus) origin. A significant proportion of detected proteins was involved in the cell growth and proliferation, transport and coagulation. Twelve proteins were potentially related to bone and cartilage metabolism, and several have not been previously identified in the plasma, including: TGF-β induced protein IG-H3, cartilage acidic protein 1, procollagen C proteinase enhancer protein and TGF-β receptor III.RésuméAprès une fracture, un grand nombre de facteurs de croissance, cytokines et leurs récepteurs apparentés interviennent dans le processus de réparation des foyers de fracture. Nous avons analysé ces différents facteurs circulants chez 25 patients ayant présenté une fracture après purification du sang, électrophorèses, chromatographie et spectrographie de masse. 213 protéines ont été identifiées. L’analyse génétique de la majorité de ces protéines montre qu’elles sont d’origine extra cellulaires avec un très petit nombre de protéines intra cellulaires provenant notamment du noyau. Une proportion significative des protéines détectées intervient au niveau de la croissance, de la prolifération cellulaire et des phénomènes de coagulation. 12 protéines sont spécifiquement en rapport avec les métabolismes osseux et cartilagineux, plusieurs d’entre-elles n’avaient pas été préalablement identifiées au niveau du plasma comme la TGF-β, la protéine IG-H3, la CAP 1, le procollagène de type C, le TGF-β récepteur III.