Alk2/ACVR1 and Alk3/BMPR1A Provide Essential Function for Bone Morphogenetic Protein-Induced Retinal Angiogenesis.

OBJECTIVE: Increasing evidence suggests that bone morphogenetic protein (BMP) signaling regulates angiogenesis. Here, we aimed to define the function of BMP receptors in regulating early postnatal angiogenesis by analysis of inducible, endothelial-specific deletion of the BMP receptor components Bmpr2 (BMP type 2 receptor), Alk1 (activin receptor-like kinase 1), Alk2, and Alk3 in mouse retinal vessels. APPROACH AND RESULTS: Expression analysis of several BMP ligands showed that proangiogenic BMP ligands are highly expressed in postnatal retinas. Consistently, BMP receptors are also strongly expressed in retina with a distinct pattern. To assess the function of BMP signaling in retinal angiogenesis, we first generated mice carrying an endothelial-specific inducible deletion of Bmpr2. Postnatal deletion of Bmpr2 in endothelial cells substantially decreased the number of angiogenic sprouts at the vascular front and branch points behind the front, leading to attenuated radial expansion. To identify critical BMPR1s (BMP type 1 receptors) associated with BMPR2 in retinal angiogenesis, we generated endothelial-specific inducible deletion of 3 BMPR1s abundantly expressed in endothelial cells and analyzed the respective phenotypes. Among these, endothelial-specific deletion of either Alk2/acvr1 or Alk3/Bmpr1a caused a delay in radial expansion, reminiscent of vascular defects associated with postnatal endothelial-specific deletion of BMPR2, suggesting that ALK2/ACVR1 and ALK3/BMPR1A are likely to be the critical BMPR1s necessary for proangiogenic BMP signaling in retinal vessels. CONCLUSIONS: Our data identify BMP signaling mediated by coordination of ALK2/ACVR1, ALK3/BMPR1A, and BMPR2 as an essential proangiogenic cue for retinal vessels.

TiF1-gamma plays an essential role in murine hematopoiesis and regulates transcriptional elongation of erythroid genes.

Transcriptional regulators play critical roles in the regulation of cell fate during hematopoiesis. Previous studies in zebrafish have identified an essential role for the transcriptional intermediary factor TIF1γ in erythropoiesis by regulating the transcription elongation of erythroid genes. To study if TIF1γ plays a similar role in murine erythropoiesis and to assess its function in other blood lineages, we generated mouse models with hematopoietic deletion of TIF1γ. Our results showed a block in erythroid maturation in the bone marrow following tif1γ deletion that was compensated with enhanced spleen erythropoiesis. Further analyses revealed a defect in transcription elongation of erythroid genes in the bone marrow. In addition, loss of TIF1γ resulted in defects in other blood compartments, including a profound loss of B cells, a dramatic expansion of granulocytes and decreased HSC function. TIF1γ exerts its functions in a cell-autonomous manner as revealed by competitive transplantation experiments. Our study therefore demonstrates that TIF1γ plays essential roles in multiple murine blood lineages and that its function in transcription elongation is evolutionally conserved.

Control of craniofacial development by the collagen receptor, discoidin domain receptor 2.

Development of the craniofacial skeleton requires interactions between progenitor cells and the collagen-rich extracellular matrix (ECM). The mediators of these interactions are not well-defined. Mutations in the discoidin domain receptor 2 gene (DDR2), which encodes a non-integrin collagen receptor, are associated with human craniofacial abnormalities, such as midface hypoplasia and open fontanels. However, the exact role of this gene in craniofacial morphogenesis is not known. As will be shown, Ddr2-deficient mice exhibit defects in craniofacial bones including impaired calvarial growth and frontal suture formation, cranial base hypoplasia due to aberrant chondrogenesis and delayed ossification at growth plate synchondroses. These defects were associated with abnormal collagen fibril organization, chondrocyte proliferation and polarization. As established by localization and lineage-tracing studies, Ddr2 is expressed in progenitor cell-enriched craniofacial regions including sutures and synchondrosis resting zone cartilage, overlapping with GLI1 + cells, and contributing to chondrogenic and osteogenic lineages during skull growth. Tissue-specific knockouts further established the requirement for Ddr2 in GLI +skeletal progenitors and chondrocytes. These studies establish a cellular basis for regulation of craniofacial morphogenesis by this understudied collagen receptor and suggest that DDR2 is necessary for proper collagen organization, chondrocyte proliferation, and orientation.

We each have unique facial features that are key to our identities. These features are inherited, but the mechanisms are poorly understood. People with the genetic disease spondylo-meta-epiphyseal dysplasia, or SMED, have characteristic facial and skull abnormalities including a flattened face and shortened skull. SMED is associated with mutations that inactivate the gene encoding a protein called discoidin domain receptor 2 (DDR2), which is a receptor for collagen. Collagen is the major structural protein in the human body, supporting the structure of cells and tissues. It also controls cell behaviors including growth, migration and differentiation, and it helps form tissues such as cartilage or bone. At least some of the effects of collagen on cells depend on its interaction with DDR2. Since the facial and skull abnormalities in mice with mutations that stop DDR2 from working correctly resemble those of SMED patients, these mice can be used to understand the cellular basis for this disease, as well as the role of DDR2 in the embryonic development of the face and skull. Therefore, Mohamed et al. set out to understand how loss of DDR2 causes the characteristic facial and skull defects associated with SMED. Mohamed et al. used mice that had been genetically modified so that DDR2 could be inactivated in skeletal progenitor cells, cartilage cells and bone cells (osteoblasts). Examining these mice, they found that the shortened skulls and flat face characteristic of mice lacking DDR2 are due to bones at the skull base failing to elongate correctly due to defects in the growth centers that depend on cartilage. Mohamed et al. also discovered that the cells that normally produce DDR2 are the progenitors of cartilage and bone-forming cells, which partly explains why lacking this protein leads to issues in growth of these tissues. In addition to shedding light on the causes of SMED, Mohamed et al.'s results also provide general insights into the mechanisms controlling the formation of facial and skull bones that depend on interactions between cells and collagen. This information may help explain how other abnormalities in the face and skull emerge, and provide a basis for how the shape of the skull has changed during human evolution. In the future, it may be possible to manipulate the activity of DDR2 to correct skull defects.

Loss-of-function of ACVR1 in osteoblasts increases bone mass and activates canonical Wnt signaling through suppression of Wnt inhibitors SOST and DKK1.

BMPs (Bone morphogenetic proteins) such as BMP2 and BMP7 have been used about one decade as bone anabolic agents in orthopaedics. The BMP receptor ACVR1, which is a key receptor of BMP7, is expressed in bone. The pathological role of ACVR1 in humans has been reported: a point mutation in ACVR1 can cause fibrodysplasia ossificans progressiva (FOP) in which ectopic ossification occurs in skeletal muscles and deep connective tissues. The physiological function of ACVR1 in bone, however, is totally unknown. The purpose of this study is to investigate the endogenous role of ACVR1 in osteoblasts, one of the most dominant cell-types in bone. We generated Acvr1-null mice in an osteoblast-specific manner using an inducible Cre-loxP system. Surprisingly, we found that bone mass was increased in the Acvr1-null mice. Interestingly, canonical Wnt signaling was increased and expression levels of Wnt inhibitors Sost and Dkk1 were both suppressed in the null bones during the developmental stages. In addition, we confirmed that expression levels of both Sost and Dkk1 were upregulated by BMP7 dose-dependently in vitro. These results suggest that the Acvr1-deficiency can increase bone mass by activating Wnt signaling in which both Sost and Dkk1 expression levels are diminished. This study leads to a new concept of the BMP7-ACVR1-SOST/DKK1 axis in osteoblasts, in which BMP7 signaling through ACVR1 can reduce Wnt signaling via SOST/DKK1 and then inhibits osteogenesis. Although this concept is beyond the current known function of BMP7, it can explain the varied outcomes of BMP7 treatment. We believe BMP signaling can exhibit multifaceted effects by context and cell type.

Epithelial-specific knockout of the Rac1 gene leads to enamel defects.

The Ras-related C3 botulinum toxin substrate 1 (Rac1) gene encodes a 21-kDa GTP-binding protein belonging to the RAS superfamily. RAS members play important roles in controlling focal adhesion complex formation and cytoskeleton contraction, activities with consequences for cell growth, adhesion, migration, and differentiation. To examine the role(s) played by RAC1 protein in cell-matrix interactions and enamel matrix biomineralization, we used the Cre/loxP binary recombination system to characterize the expression of enamel matrix proteins and enamel formation in Rac1 knockout mice (Rac1(-/-)). Mating between mice bearing the floxed Rac1 allele and mice bearing a cytokeratin 14-Cre transgene generated mice in which Rac1 was absent from epithelial organs. Enamel of the Rac1 conditional knockout mouse was characterized by light microscopy, backscattered electron imaging in the scanning electron microscope, microcomputed tomography, and histochemistry. Enamel matrix protein expression was analyzed by western blotting. Major findings showed that the Tomes' processes of Rac1(-/-) ameloblasts lose contact with the forming enamel matrix in unerupted teeth, the amounts of amelogenin and ameloblastin are reduced in Rac1(-/-) ameloblasts, and after eruption, the enamel from Rac1(-/-) mice displays severe structural defects with a complete loss of enamel. These results support an essential role for RAC1 in the dental epithelium involving cell-matrix interactions and matrix biomineralization.