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Osteoblast-derived VEGF is important for bone development and postnatal bone homeostasis. Previous studies have demonstrated that VEGF affects bone repair and regeneration; however, the cellular mechanisms by which it works are not fully understood. In this study, we investigated the functions of osteoblast-derived VEGF in healing of a bone defect. The results indicate that osteoblast-derived VEGF plays critical roles at several stages in the repair process. Using transgenic mice with osteoblast-specific deletion of Vegfa, we demonstrated that VEGF promoted macrophage recruitment and angiogenic responses in the inflammation phase, and optimal levels of VEGF were required for coupling of angiogenesis and osteogenesis in areas where repair occurs by intramembranous ossification. VEGF likely functions as a paracrine factor in this process because deletion of Vegfr2 in osteoblastic lineage cells enhanced osteoblastic maturation and mineralization. Furthermore, osteoblast- and hypertrophic chondrocyte-derived VEGF stimulated recruitment of blood vessels and osteoclasts and promoted cartilage resorption at the repair site during the periosteal endochondral ossification stage. Finally, osteoblast-derived VEGF stimulated osteoclast formation in the final remodeling phase of the repair process. These findings provide a basis for clinical strategies to improve bone regeneration and treat defects in bone healing., Introduction Bone repair following injury is usually a rapid and efficient process. However, it can be compromised in certain clinical circumstances. For example, delayed union or nonunion of bone fragments [...]

Osteoporotic bones have reduced spongy bone mass, altered bone architecture, and increased marrow fat. Bone marrow stem cells from osteoporotic patients are more likely to differentiate into adipocytes than control cells, suggesting that adipocyte differentiation may play a role in osteoporosis. VEGF is highly expressed in osteoblastic precursor cells and is known to stimulate bone formation. Here we tested the hypothesis that VEGF is also an important regulator of cell fate, determining whether differentiation gives rise to osteoblasts or adipocytes. Mice with conditional VEGF deficiency in osteoblastic precursor cells exhibited an osteoporo-sis-like phenotype characterized by reduced bone mass and increased bone marrow fat. In addition, reduced VEGF expression in mesenchymal stem cells resulted in reduced osteoblast and increased adipocyte differentiation. Osteoblast differentiation was reduced when VEGF receptor 1 or 2 was knocked down but was unaf-fected by treatment with recombinant VEGF or neutralizing antibodies against VEGF. Our results suggested that VEGF controls differentiation in mesenchymal stem cells by regulating the transcription factors RUNX2 and PPARγ2 as well as through a reciprocal interaction with nuclear envelope proteins lamin A/C. Impor-tantly, our data support a model whereby VEGF regulates differentiation through an intracrine mechanism that is distinct from the role of secreted VEGF and its receptors., Introduction Osteoporosis, skeletal fragility associated with reduced quantity and quality of bone, is a major problem in the aging population. During aging, the balance between bone formation and resorption shifts [...]

Mesenchymal stem cells can give rise to several cell types, but varying results depending on isolation methods and tissue source have led to controversies about their usefulness in clinical medicine. Here we show that vascular endothelial cells can transform into multipotent stem-like cells by an activin-like kinase-2 (ALK2) receptor-dependent mechanism. In lesions from individuals with fibrodysplasia ossificans progressiva (FOP), a disease in which heterotopic ossification occurs as a result of activating ALK2 mutations, or from transgenic mice expressing constitutively active ALK2, chondrocytes and osteoblasts expressed endothelial markers. Lineage tracing of heterotopic ossification in mice using a Tie2-Cre construct also suggested an endothelial origin of these cell types. Expression of constitutively active ALK2 in endothelial cells caused endothelial-to-mesenchymal transition and acquisition of a stem cell-like phenotype. Similar results were obtained by treatment of untransfected endothelial cells with the ligands transforming growth factor-β2 (TGF-β2) or bone morphogenetic protein-4 (BMP4) in an ALK2-dependent manner. These stem-like cells could be triggered to differentiate into osteoblasts, chondrocytes or adipocytes. We suggest that conversion of endothelial cells to stem-like cells may provide a new approach to tissue engineering., Epithelial and endothelial cell plasticity is crucial for embryonic development and disease progression (1). Transformation of epithelial cells into mesenchymal cells (epithelial-to-mesenchymal transition, EMT) regulates gastrulation, neural crest and somite [...]

Objectives: The project aims were to identify infectious mechanisms responsible for an extreme form of mandibular osteonecrosis and osteomyelitis in West African populations and test the hypothesis that Mycobacterium tuberculosis plays a pivotal role. Materials and Methods: DNA was extracted from mandibular fragments of 9 of 19 patients previously included in a prospective study leading to the mycobacterial hypothesis. Amplified DNAs were used for preparing libraries suitable for next‐generation sequencing. For comparison of the whole‐genome sequencing data of the 9 patients with DNAs of both microbiota and human tissues, DIAMOND v0.9.26 was used to align sequencing reads to NCBI‐nr database and MEGAN 6 for taxonomy binning and identification of Mycobacterium tuberculosis strains. Results: The data show that mandibular bone fragments of all 9 patients not only contain Homo sapiens and Mycobacterium tuberculosis DNAs; they also contain DNAs of Plasmodium ovale wallikeri, Staphylococcus aureus, Staphylococcus hominis, and Prevotella P3‐120/intermedia; as well as large numbers of DNAs from other infectious components. Conclusions: The data obtained provide direct evidence to support the conclusion that combinations of Mycobacterium tuberculosis, Plasmodium ovale wallikeri, and other oral bacteria are involved in this particular type of mandibular destruction in West African individuals of many ages. [ABSTRACT FROM AUTHOR]

Infantile hemangiomas are localized and rapidly growing regions of disorganized angiogenesis. We show that expression of vascular endothelial growth factor receptor-1 (VEGFR1) in hemangioma endothelial cells (hemECs) and hemangioma tissue is markedly reduced compared to controls. Low VEGFR1 expression in hemECs results in VEGF-dependent activation of VEGFR2 and downstream signaling pathways. In hemECs, transcription of the gene encoding VEGFR1 (FLT1) is dependent on nuclear factor of activated T cells (NFAT). Low VEGFR1 expression in hemECs is caused by reduced activity of a pathway involving [beta]1 integrin, the integrin-like receptor tumor endothelial marker-8 (TEM8), VEGFR2 and NFAT. In a subset of individuals with hemangioma, we found missense mutations in the genes encoding VEGFR2 (KDR) and TEM8 (ANTXR1). These mutations result in increased interactions among VEGFR2, TEM8 and [beta]1 integrin proteins and in inhibition of integrin activity. Normalization of the constitutive VEGFR2 signaling in hemECs with soluble VEGFR1 or antibodies that neutralize VEGF or stimulate [beta]1 integrin suggests that local administration of these or similar agents may be effective in hemangioma treatment., Infantile hemangiomas, localized lesions of disorganized angiogenesis, are the most common tumors of infancy (1). They typically appear around the second week of life, proliferate over the course of 6-10 [...]

Osteoporosis results from an imbalance in skeletal remodeling that favors bone resorption over bone formation. Bone matrix is degraded by osteoclasts, which differentiate from myeloid precursors in response to the cytokine RANKL. To gain insight into the transcriptional regulation of bone resorption during growth and disease, we generated a conditional knockout of the transcription factor nuclear factor of activated T cells c1 (Nfatc1). Deletion of Nfatc1 in young mice resulted in osteopetrosis and inhibition of osteoclastogenesis in vivo and in vitro. Transcriptional profiling revealed NFATc1 as a master regulator of the osteoclast transcriptome, promoting the expression of numerous genes needed for bone resorption. In addition, NFATc1 directly repressed osteoclast progenitor expression of osteoprotegerin, a decoy receptor for RANKL previously thought to be an osteoblast-derived inhibitor of bone resorption. "Cherubism mice", which carry a gain-of-function mutation in SH3-domain binding protein 2 (Sh3bp2), develop osteoporosis and widespread inflammation dependent on the proinflammatory cytokine, TNF-[alpha]. Interestingly, deletion of Nfatc1 protected cherubism mice from systemic bone loss but did not inhibit inflammation. Taken together, our study demonstrates that NFATc1 is required for remodeling of the growing and adult skeleton and suggests that NFATc1 may be an effective therapeutic target for osteoporosis associated with inflammatory states., Introduction Osteoporosis, the most common pathologic condition resulting from excessive bone destruction, afflicts 10 million Americans over the age of 50 and results in 1.5 million fractures annually (1), (2). [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2008.07.005 Byline: Elona Kolpakova-Hart (a), Brandeis McBratney-Owen (a), Bo Hou (a), Naomi Fukai (a), Claudia Nicolae (a), Jing Zhou (b), Bjorn R. Olsen (a) Keywords: Craniofacial skeleton; Craniosynostosis; Cranial base; Synchondrosis; Suture; Polycystin-1 Abstract: In vertebrates, coordinated embryonic and postnatal growth of the craniofacial bones and the skull base is essential during the expansion of the rostrum and the brain. Identification of molecules that regulate skull growth is important for understanding the nature of craniofacial defects and for development of non-invasive biologically based diagnostics and therapies. Here we report on spatially restricted growth defects at the skull base and in craniofacial sutures of mice deficient for polycystin-1 (Pkd1). Mutant animals reveal a premature closure of both presphenoid and sphenooccipital synchondroses at the cranial base. Furthermore, knockout mice lacking Pkd1 in neural crest cells are characterized by impaired postnatal growth at the osteogenic fronts in craniofacial sutures that are subjected to tensile forces. Our data suggest that polycystin-1 is required for proliferation of subpopulations of cranial osteochondroprogenitor cells of both mesodermal and neural crest origin during skull growth. However, the Erk1/2 signalling pathway is up-regulated in the Pkd1-deficient skeletal tissue, similarly to that previously reported for polycystic kidney. Author Affiliation: (a) Department of Developmental Biology, Harvard School of Dental Medicine, Boston, Massachusetts 02115, USA (b) Renal Division, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, MA 02115, USA Article History: Received 9 January 2008; Revised 23 June 2008; Accepted 1 July 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2008.01.005 Byline: Donald J. Glotzer (a), Elazar Zelzer (b), Bjorn R. Olsen (a) Keywords: Runx2; Skin; Hair follicles; Mouse; Sonic hedgehog; Development; Gene expression; Hair cycle; Skin allograft; Skin thickness Abstract: The transcription factor, Runx2, is known to play crucial roles in skeletal and tooth morphogenesis. Here we document that Runx2 has a regulatory role in skin and hair follicle development. The expression of Runx2 is restricted to hair follicles and is dynamic, pari passu with follicle development. Follicle maturation is delayed in the absence of Runx2 and overall skin and epidermal thickness of Runx2 null embryos is significantly reduced. The Runx2 null epidermis is hypoplastic, displaying reduced expression of Keratin 14, Keratin 1 and markers of proliferation. The expression pattern of Runx2 in the bulb epithelium of mature hair follicles is asymmetric and strikingly similar to that of Sonic hedgehog. This suggests that Runx2 may be a regulator of hedgehog signaling in skin as it is in bones and teeth. Supporting this possibility, we demonstrate that Sonic hedgehog, Patched1 and Gli1 transcripts are reduced in the skin of Runx2 null embryos. Moreover, we document Patched1 expression in epidermal basal cells and show that the skin of Sonic.sup.+/- embryos is thinner than that of wild-type littermates. These observations suggest that Runx2 and hedgehog signaling are involved in the well known, but unexplained, coupling of skin thickness to hair follicle development. Author Affiliation: (a) Department of Developmental Biology, Harvard School of Dental Medicine, 188 Longwood Avenue, Boston, MA 02115, USA (b) Department of Molecular Genetics, Weizmann Institute of Science, Rehovot, Israel Article History: Received 4 September 2007; Revised 11 December 2007; Accepted 3 January 2008