Noggin antagonism of BMP4 signaling controls development of the axial skeleton in the mouse

Mesenchyme-dependent BMP signaling directs the timing of mandibular osteogenesis

To identify molecular and cellular mechanisms that determine when bone forms, and to elucidate the role played by osteogenic mesenchyme, we employed an avian chimeric system that draws upon the divergent embryonic maturation rates of quail and duck. Pre-migratory neural crest mesenchyme destined to form bone in the mandible was transplanted from quail to duck. In resulting chimeras, quail donor mesenchyme established significantly faster molecular and histological programs for osteogenesis within the relatively slower-progressing duck host environment. To understand this phenotype, we assayed for changes in the timing of epithelial-mesenchymal interactions required for bone formation and found that such interactions were accelerated in chimeras. In situ hybridization analyses uncovered donor-dependent changes in the spatiotemporal expression of genes, including the osteo-inductive growth factor Bmp4. Mesenchymal expression of Bmp4 correlated with an ability of quail donor cells to form bone precociously without duck host epithelium, and also relied upon epithelial interactions until mesenchyme could form bone independently. Treating control mandibles with exogenous BMP4 recapitulated the capacity of chimeras to express molecular mediators of osteogenesis prematurely and led to the early differentiation of bone. Inhibiting BMP signaling delayed bone formation in a stage-dependent manner that was accelerated in chimeras. Thus, mandibular mesenchyme dictates when bone forms by temporally regulating its interactions with epithelium and its own expression of Bmp4. Our findings offer a developmental mechanism to explain how neural crest-derived mesenchyme and BMP signaling underlie the evolution of species-specific skeletal morphology.

BMP4 is dispensable for skeletogenesis and fracture-healing in the limb

Bone morphogenetic proteins (BMPs) are potent bone-forming agents that show clinical efficacy when used in patients to augment fracture-healing. Molecular profiling of fracture tissues has confirmed that BMPs 2, 3, 4, 5, 6, and 7 are expressed during the healing process, and it has identified a specific temporal pattern of expression for each BMP. Mice engineered to express increased levels of BMP antagonists have fragile bones that are prone to fracture, suggesting that BMPs not only mediate bone formation in the context of repair, but may also have a role in maintaining adult bone. In this study, mice carrying floxed Bmp4 alleles were bred with Prx1-cre transgenic mice to establish limb-specific removal of Bmp4. We compared these mice to mice in which Bmp2 was specifically deleted from the limb, and we then assessed limb skeletogenesis and fracture-healing. Limb skeletogenesis occurs normally in the absence of BMP4, and postnatal skeletal growth was also unaffected when BMP4 was removed. When mice lacking BMP4 were challenged to repair fractures, they were able to mount a successful healing response. We concluded that BMP4 is not required for formation of the limb skeleton and that femur fracture-healing is unaffected by the absence of BMP4. This study demonstrates that BMP4 is not required for bone formation and function in the limb, giving us further insights into the utility of recombinant human BMPs as therapeutic agents.

The Noggin null mouse phenotype is strain dependent and haploinsufficiency leads to skeletal defects

Noggin is a secreted peptide that binds and inactivates Bone Morphogenetic Proteins, members of the transforming growth factor beta superfamily of secreted signaling molecules. In vertebrate limbs, Noggin is expressed in condensing cartilage and immature chondrocytes. Inactivation of the Noggin gene has been reported in an inbred 129X1/SvJ mouse genetic background. The null allele was lethal at 18.5 dpc and resulted in severe hyperplasia of the cartilage together with multiple joint fusions. In order to investigate the effect of the genetic background on the phenotypic manifestation of Noggin inactivation, we crossed the Noggin null allele into the outbred CD1 and inbred DBA1 and C57BL/6 mouse strains. We describe here skeletal phenotypes of Noggin null mice, such as accelerated or delayed mineralization of different bones suggestive of a complex tissue response to the perturbations in BMP balances. Additionally, we found that in the absence of Noggin, early specification of myogenic differentiation was unaffected, whereas terminal stages of myogenesis were delayed. Furthermore, we have discovered Noggin haploinsufficiency leading to carpal and tarsal fusions reminiscent of some phenotypes reported for NOGGIN haploinsufficiency in humans.

Noggin is required for normal lobe patterning and ductal budding in the mouse prostate

Mesenchymal expression of the BMP antagonist NOGGIN during prostate development plays a critical role in pre-natal ventral prostate development and opposes BMP4-mediated inhibition of cell proliferation during postnatal ductal development. Morphologic examination of newborn Noggin-/- male fetuses revealed genitourinary anomalies including cryptorchidism, incomplete separation of the hindgut from the urogenital sinus (UGS), absence of the ventral mesenchymal pad, and a complete loss of ventral prostate (VP) budding. Examination of lobe-specific marker expression in the E14 Noggin-/- UGS rescued by transplantation under the renal capsule of a male nude mouse confirmed a complete loss of VP determination. More modest effects were observed in the other lobes, including decreased number of ductal buds in the dorsal and lateral prostates of newborn Noggin-/- males. BMP4 and BMP7 have been shown to inhibit ductal budding and outgrowth by negatively regulating epithelial cell proliferation. We show here that NOGGIN can neutralize budding inhibition by BMP4 and rescues branching morphogenesis of BMP4-exposed UGS in organ culture and show that the effects of BMP4 and NOGGIN activities converge on P63+ epithelial cells located at nascent duct tips. Together, these studies show that the BMP-NOGGIN axis regulates patterning of the ventral prostate, regulates ductal budding, and controls proliferation of P63+ epithelial cells in the nascent ducts of developing mouse prostate.

Identification of mechanosensitive genes in osteoblasts by comparative microarray studies using the rotating wall vessel and the random positioning machine

Weightlessness or microgravity of spaceflight induces bone loss due in part to decreased bone formation by unknown mechanisms. Due to difficulty in performing experiments in space, several ground-based simulators such as the Rotating Wall Vessel (RWV) and Random Positioning Machine (RPM) have become critical venues to continue studying space biology. However, these simulators have not been systematically compared to each other or to mechanical stimulating models. Here, we hypothesized that exposure to RWV inhibits differentiation and alters gene expression profiles of 2T3 cells, and a subset of these mechanosensitive genes behaves in a manner consistent to the RPM and opposite to the trends incurred by mechanical stimulation of mouse tibiae. Exposure of 2T3 preosteoblast cells to the RWV for 3 days inhibited alkaline phosphatase activity, a marker of differentiation, and downregulated 61 and upregulated 45 genes by more than twofold compared to static 1 g controls, as shown by microarray analysis. The microarray results were confirmed by real-time PCR and/or Western blots for seven separate genes and proteins including osteomodulin, runx2, and osteoglycin. Comparison of the RWV data to the RPM microarray study that we previously published showed 14 mechanosensitive genes that changed in the same direction. Further comparison of the RWV and RPM results to microarray data from mechanically loaded mouse tibiae reported by an independent group revealed that three genes including osteoglycin were upregulated by the loading and downregulated by our simulators. These mechanosensitive genes may provide novel insights into understanding the mechanisms regulating bone formation and potential targets for countermeasures against decreased bone formation during space flight and in pathologies associated with lack of bone formation.

The role of noggin in human mesenchymal stem cell differentiation

Noggin is a secreted protein that inhibits the binding of bone morphogenetic proteins (BMPs) to their cognate receptor. Its role in human mesenchymal stem cell differentiation has not been well studied. Here, we studied the effect of noggin on human mesenchymal stem cell differentiation induced by inflammatory cytokines (activated T-cell conditioned medium (ACTTCM) or the combination of four T-cell cytokines, TNF-alpha, TGF-beta, IFN-gamma, and IL-17 (TTII)), BMPs, or dexamthasone (DEX). HMSC treated with TTII alone rapidly induced alkaline phosphatase (AlkP) activity. Inclusion of noggin resulted in an additive effect. Noggin acted additively with DEX to induce a significantly higher level of AlkP induction than either noggin or DEX alone. Noggin was examined for its ability to inhibit mineralization in long-term cultures of HMSC stimulated with BMP-2, BMP-6, BMP-7, DEX, or TTII. Surprisingly, noggin alone induced mineralization while it did not inhibit mineralization induced by TTII or BMP-2, BMP-6, or BMP-7. Interestingly, when HMSC were treated with both noggin and DEX they acted synergistically to induce mineralization nearly 3-fold over DEX alone and 30-fold over noggin alone. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis showed that T-cell cytokines induced noggin, Runx2, BMP-2, and osteocalcin gene expression, while noggin alone induced BMP-2 and osteocalcin gene expression, but not Runx2, although it increased the expression of ActRII, a receptor for BMP-2. These results suggest that in HMSC, the anabolic action of inflammation on bone formation occurs through the induction of noggin, which then induces BMP-2 receptor and BMP-2 leading to the activation of the differentiation process.

The role of bone morphogenetic protein 4 in inner ear development and function

Bone Morphogenetic Protein 4 (BMP4) is a member of the TGF-beta superfamily and is known to be important for the normal development of many tissues and organs, including the inner ear. Bmp4 homozygous null mice die as embryos, but Bmp4 heterozygous null (Bmp4(+/-)) mice are viable and some adults exhibit a circling phenotype, suggestive of an inner ear defect. To understand the role of BMP4 in inner ear development and function, we have begun to study C57BL/6 Bmp4(+/-) mice. Quantitative testing of the vestibulo-collic reflex, which helps maintain head stability, demonstrated that Bmp4(+/-) mice that exhibit circling behavior have a poor response in the yaw axis, consistent with semicircular canal dysfunction. Although the hair cells of the ampullae were grossly normal, the stereocilia were greatly reduced in number. Auditory brainstem responses showed that Bmp4(+/-) mice have elevated hearing thresholds and immunohistochemical staining demonstrated decreased numbers of neuronal processes in the organ of Corti. Thus Bmp4(+/-) mice have structural and functional deficits in the inner ear.

BMP signaling regulates murine enteric nervous system precursor migration, neurite fasciculation, and patterning via altered Ncam1 polysialic acid addition

The enteric nervous system (ENS) forms from migrating neural crest-derived precursors that differentiate into neurons and glia, aggregate into ganglion cell clusters, and extend neuronal processes to form a complex interacting network that controls many aspects of intestinal function. Bone morphogenetic proteins (BMPs) have diverse roles in development and influence the differentiation, proliferation, and survival of ENS precursors. We hypothesized that BMP signaling might also be important for the ENS precursor migration, ganglion cell aggregation, and neurite fasciculation necessary to form the enteric nervous system. We now demonstrate that BMP signaling restricts murine ENS precursors to the outer bowel wall during migration. In addition, blocking BMP signaling causes faster colonization of the murine colon, reduces ganglion cell aggregation, and reduces neurite fasciculation. BMP signaling also influences patterns of neurite extension within the developing bowel wall. These effects on ENS precursor migration and neurite fasciculation appear to be mediated at least in part by increased polysialic acid addition to neural cell adhesion molecule (Ncam1) in response to BMP. Removing PSA enzymatically reverses the BMP effects on ENS precursor migration and neurite fasciculation. These studies demonstrate several novel roles for BMP signaling and highlight new functions for sialyltransferases in the developing ENS.

Bone morphogenetic proteins and their antagonists

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

The BMP antagonist Noggin promotes cranial and spinal neurulation by distinct mechanisms

Here we characterize the consequences of elevated bone morphogenetic protein (BMP) signaling on neural tube morphogenesis by analyzing mice lacking the BMP antagonist, Noggin. Noggin is expressed dorsally in the closing neural folds and ventrally in the notochord and somites. All Noggin-/- pups are born with lumbar spina bifida; depending on genetic background, they may also have exencephaly. The exencephaly is due to a primary failure of neurulation, resulting from a lack of mid/hindbrain dorsolateral hinge point (DLHP) formation. Thus, as previously shown for Shh signaling at spinal levels, BMP activity may inhibit cranial DLHP morphogenesis. However, the increased BMP signaling observed in the Noggin-/- dorsal neural tube is not sufficient to cause exencephaly; it appears to also depend on the action of a genetic modifier, which may act to increase dorsal Shh signaling. The spinal neural tube defect results from a different mechanism: increased BMP signaling in the mesoderm between the limb buds leads to abnormal somite differentiation and axial skeletal malformation. The resulting lack of mechanical support for the neural tube causes spina bifida. We show that this defect is due to elevated BMP4 signaling. Thus, Noggin is required for mammalian neurulation in two contexts, dependent on position along the rostrocaudal axis.