Notochord-derived BMP antagonists inhibit endothelial cell generation and network formation

A collagen-rich arch in the urochordate notochord coordinates cell shaping and multi-tissue elongation

Cell and tissue reshaping is crucial for coordinating three-dimensional pattern formation, in which the size and shape of the cells must be accurately regulated via signal transport and communication among tissues. However, the identity of signaling and transportation mechanisms in this process remains elusive. In our study, we identified an extracellular matrix (ECM) structure with a vertebra-like shape surrounding the central notochord tissue in the larval tail of the urochordate Ciona. Additionally, we verified that the ECM structure was formed de novo, mainly from collagens secreted by notochord cells. Fluorescence recovery after photobleaching and simulation results revealed that this structure was formed via diffusional collagen flow from a notochord that was restricted and molded in the spaces among tail tissues. We revealed that the collagen structure was essential for notochord cell arrangement and elongation. Furthermore, we observed that the central notochord connects with the epidermis through this ECM structure. The disruption of this structure by collagen knockdown and loss-of-collagen function caused the failure of notochord elongation. More importantly, the epidermis could not elongate proportionally with notochord, indicating that the collagen-rich structure serves as a scaffold to coordinate the concurrent elongation of the tail tissues. These findings provide insights into how the central tissue forms and molds its surrounding ECM structure, by not only regulating its own morphogenesis but also functioning as a scaffold for signal transmission to orchestrate the coordinated morphologic reshaping of the surrounding tissues.

Inducing Angiogenesis in the Nucleus Pulposus

Bone morphogenetic protein (BMP) gene delivery to Lewis rat lumbar intervertebral discs (IVDs) drives bone formation anterior and external to the IVD, suggesting the IVD is inhospitable to osteogenesis. This study was designed to determine if IVD destruction with a proteoglycanase, and/or generating an IVD blood supply by gene delivery of an angiogenic growth factor, could render the IVD permissive to intra-discal BMP-driven osteogenesis and fusion. Surgical intra-discal delivery of naïve or gene-programmed cells (BMP2/BMP7 co-expressing or VEGF165 expressing) +/- purified chondroitinase-ABC (chABC) in all permutations was performed between lumbar 4/5 and L5/6 vertebrae, and radiographic, histology, and biomechanics endpoints were collected. Follow-up anti-sFlt Western blotting was performed. BMP and VEGF/BMP treatments had the highest stiffness, bone production and fusion. Bone was induced anterior to the IVD, and was not intra-discal from any treatment. chABC impaired BMP-driven osteogenesis, decreased histological staining for IVD proteoglycans, and made the IVD permissive to angiogenesis. A soluble fragment of VEGF Receptor-1 (sFlt) was liberated from the IVD matrix by incubation with chABC, suggesting dysregulation of the sFlt matrix attachment is a possible mechanism for the chABC-mediated IVD angiogenesis we observed. Based on these results, the IVD can be manipulated to foster vascular invasion, and by extension, possibly osteogenesis.

Regulation of aortic morphogenesis and VE-cadherin dynamics by VEGF

In amniote vertebrates, the definitive dorsal aorta is formed by the fusion of two primordial aortic endothelial tubes. Formation of the definitive dorsal aorta requires extensive cellular migrations and rearrangements of the primordial tubes in order to generate a single vessel located at the embryonic ventral midline. This study examines the role of VEGF signaling in the generation of the definitive dorsal aorta. Through gain- and loss-of-function studies in vivo in the chick embryo, we document a requirement for VEGF signaling in growth and remodeling of the paired primordia. We find that regions of the aorta are differentially sensitive to levels of VEGF signaling, and present evidence that areas of low blood flow are more sensitive to the loss of VEGF signaling. We also find that VEGF signaling regulates the intracellular distribution between membrane and cytoplasm of the cell-cell adhesion molecule VE-cadherin in aortic endothelial cells in vivo. Together, these finding identify mechanisms that likely contribute to the dynamic behavior of endothelial cells during aorta morphogenesis.

Dorsal aorta polarization and haematopoietic stem cell emergence

Recent studies have highlighted the crucial role of the aorta microenvironment in the generation of the first haematopoietic stem cells (HSCs) from specialized haemogenic endothelial cells (HECs). Despite more than two decades of investigations, we require a better understanding of the cellular and molecular events driving aorta formation and polarization, which will be pivotal to establish the mechanisms that operate during HEC specification and HSC competency. Here, we outline the early mechanisms involved in vertebrate aorta formation by comparing four different species: zebrafish, chicken, mouse and human. We highlight how this process, which is tightly controlled in time and space, requires a coordinated specification of several cell types, in particular endothelial cells originating from distinct mesodermal tissues. We also discuss how molecular signals originating from the aorta environment result in its polarization, creating a unique entity for HSC generation.

Avians as a Model System of Vascular Development

For more than 2000 years, the avian embryo has helped scientists understand questions of developmental and cell biology. As early as 350 BC Aristotle described embryonic development inside a chicken egg (Aristotle, Generation of animals. Loeb Classical Library (translated), vol. 8, 1943). In the seventeenth century, Marcello Malpighi, referred to as the father of embryology, first diagramed the microscopic morphogenesis of the chick embryo, including extensive characterization of the cardiovascular system (Pearce Eur Neurol 58(4):253-255, 2007; West, Am J Physiol Lung Cell Mol Physiol 304(6):L383-L390, 2016). The ease of accessibility to the embryo and similarity to mammalian development have made avians a powerful system among model organisms. Currently, a unique combination of classical and modern techniques is employed for investigation of the vascular system in the avian embryo. Here, we will introduce the essential techniques of embryonic manipulation for experimental study in vascular biology.

Non-destructive sex-specific monitoring of early embryonic growth rate in light brown broiler eggs using light transmission and its correlation with hatching time and chick weight

1. Monitoring early embryonic growth rate (EGR) has significant economic and animal welfare benefits. This study focuses on monitoring sex-specific early EGR using light transmission, and correlating this with hatching time and chick weight. For broiler eggs in particular, spectral masking of the light brown eggshells needed to be addressed. This was done using longitudinal visible transmission spectroscopy combined with eggshell colour image analysis. 2. Prior to incubation, colour images of eggs were captured followed by daily measurements of transmission spectra of eggs from days one to nine of incubation. The sex of the eggs was subsequently verified 2 d after hatching. 3. To accurately and sensitively determine sex differences in EGR using light transmission, while minimising interference from eggshell colour and thickness, the ratio of longitudinal transmissions was determined to be most effective at 575 and 610 nm. 3. Embryonic growth was detectable from d 3 (72 h) of incubation, 24 h earlier than previously reported lateral transmission measurements. However, at this time, low blood levels meant that no significant sex-differences (P > 0.05) for the mean T575/T610 ratio were detectable. This may have been due, in part, to spectral masking from the light brown eggshells. At d 7, female embryos had a significantly lower (P < 0.05) mean T575/T610 ratio than males. 4. Although the T575/T610 ratio had low correlations with hatching time and hatch-weight of chicks, this could be a good starting point for further non-destructive investigations for such predictions. 5. In conclusion, the methodology had the sensitivity to differentiate sex-specific early EGR in broiler eggs, even with pigmented eggshells, and has the potential to advance precision hatchery management and poultry research.

Human notochordal cell transcriptome unveils potential regulators of cell function in the developing intervertebral disc

The adult nucleus pulposus originates from the embryonic notochord, but loss of notochordal cells with skeletal maturity in humans is thought to contribute to the onset of intervertebral disc degeneration. Thus, defining the phenotype of human embryonic/fetal notochordal cells is essential for understanding their roles and for development of novel therapies. However, a detailed transcriptomic profiling of human notochordal cells has never been achieved. In this study, the notochord-specific marker CD24 was used to specifically label and isolate (using FACS) notochordal cells from human embryonic and fetal spines (7.5–14 weeks post-conception). Microarray analysis and qPCR validation identified CD24, STMN2, RTN1, PRPH, CXCL12, IGF1, MAP1B, ISL1, CLDN1 and THBS2 as notochord-specific markers. Expression of these markers was confirmed in nucleus pulposus cells from aged and degenerate discs. Ingenuity pathway analysis revealed molecules involved in inhibition of vascularisation (WISP2, Noggin and EDN2) and inflammation (IL1-RN) to be master regulators of notochordal genes. Importantly, this study has, for the first time, defined the human notochordal cell transcriptome and suggests inhibition of inflammation and vascularisation may be key roles for notochordal cells during intervertebral disc development. The molecules and pathways identified in this study have potential for use in developing strategies to retard/prevent disc degeneration, or regenerate tissue.

Influence of rabbit notochordal cells on symptomatic intervertebral disc degeneration: anti-angiogenic capacity on human endothelial cell proliferation under hypoxia

OBJECTIVES

Symptomatic degenerative disc disease (DDD) is associated with neovascularization and nerve ingrowth into intervertebral discs (IVDs). Notochordal cells (NCs) are key cells that may lead to regeneration of IVDs. However, their activities under conditions of hypoxia, the real environment of IVD, are not well known. We hypothesized that NCs may inhibit neovascularization by interacting with endothelial cells (ECs) under hypoxia.

DESIGN

Human IVDs were isolated and cultured to produce nucleus pulposus (NP) cell conditioned medium (NPCM). Immortalized human microvascular ECs were cultured in NPCM with notochordal cell-rich rabbit nucleus pulposus cells (rNC) under hypoxia. Vascular endothelial growth factor (VEGF), vascular cell adhesion molecule (VCAM), and interleukin-8 (IL-8) were analyzed by ELISA. Focal adhesion kinase (FAK), filamentous actin (F-actin), and platelet-derived growth factor (PDGF) were evaluated to investigate EC activity. Wound-healing migration assays were performed to examine EC migration.

RESULTS

The VEGF level of EC cells cultured in NPCM was significantly higher under hypoxia compared to normoxia. VEGF expression was significantly decreased, and FAK, F-actin, PDGF expression were inhibited when ECs were cocultured with rNCs under hypoxia. ECs cocultured with rNC in NPCM showed significantly decreased migratory activity compared to those without rNC under hypoxia.

CONCLUSIONS

The angiogenic capacity of ECs was significantly inhibited by NCs under hypoxia via a VEGF-related pathway. Our results suggest that NCs may play a key role in the development of IVDs by inhibiting vascular growth within the disc, and this may be a promising novel therapeutic strategy for targeting vascular ingrowth in symptomatic DDD.

* Coculture with Partial Digestion Notochordal Cell-Rich Nucleus Pulposus Tissue Activates Degenerative Human Nucleus Pulposus Cells

Recent studies suggested that notochordal cells (NCs) and NC-conditioned medium (NCCM) can stimulate cell viability and matrix production of nucleus pulposus cells (NPCs). However, the potential of notochordal cell-rich nucleus pulposus (NRNP) incorporating the native environment of the intervertebral disc (IVD) has not been evaluated. The objective of this study was to develop an optimal NRNP model and test whether it can allow a significant level of NPC activation in vitro. Rabbit NRNP explants were divided into three groups according to different digestion time: digestion NRNP of 8 h, partial digestion NRNP of 2 h, and natural NRNP. Cell viability and NC phenotype were compared between these groups after 14 days of incubation. The products of the selected partial digestion NRNP group were then cocultured with human degenerated NPCs for 14 days. NPC viability, cell proliferation and senescence, the production of glycosaminoglycan (GAG) found in extracellular matrix, and NP matrix production by NPCs were assessed. The results showed that coculturing with partial digestion NRNP significantly improved the cell proliferation, cell senescence, and disc matrix gene expression of NPCs compared with those in the monoculture group. In addition, GAG/DNA ratio in the coculture group increased significantly, while the level of collagen II protein remained unchanged. In this study, we demonstrated that partial digestion NRNP may show a promising potential for NPC regeneration in IVD tissue engineering.

Fibronectin signals through integrin α5β1 to regulate cardiovascular development in a cell type-specific manner

Fibronectin (Fn1) is an evolutionarily conserved extracellular matrix glycoprotein essential for embryonic development. Global deletion of Fn1 leads to mid-gestation lethality from cardiovascular defects. However, severe morphogenetic defects that occur early in embryogenesis in these embryos precluded assigning a direct role for Fn1 in cardiovascular development. We noticed that Fn1 is expressed in strikingly non-uniform patterns during mouse embryogenesis, and that its expression is particularly enriched in the pharyngeal region corresponding with the pharyngeal arches 3, 4, and 6. This region bears a special importance for the developing cardiovascular system, and we hypothesized that the localized enrichment of Fn1 in the pharyngeal region may be essential for cardiovascular morphogenesis. To test this hypothesis, we ablated Fn1 using the Isl1(Cre) knock-in strain of mice. Deletion of Fn1 using the Isl1(Cre) strain resulted in defective formation of the 4th pharyngeal arch arteries (PAAs), aberrant development of the cardiac outflow tract (OFT), and ventricular septum defects. To determine the cell types responding to Fn1 signaling during cardiovascular development, we deleted a major Fn1 receptor, integrin α5 using the Isl1(Cre) strain, and observed the same spectrum of abnormalities seen in the Fn1 conditional mutants. Additional conditional mutagenesis studies designed to ablate integrin α5 in distinct cell types within the Isl1(+) tissues and their derivatives, suggested that the expression of integrin α5 in the pharyngeal arch mesoderm, endothelium, surface ectoderm and the neural crest were not required for PAA formation. Our studies suggest that an (as yet unknown) integrin α5-dependent signal extrinsic to the pharyngeal endothelium mediates the formation of the 4th PAAs.

Soluble factors from the notochordal-rich intervertebral disc inhibit endothelial cell invasion and vessel formation in the presence and absence of pro-inflammatory cytokines

BACKGROUND

Chronic low back pain can be associated with the pathological ingrowth of blood vessels and nerves into intervertebral discs (IVDs). The notochord patterns the IVD during development and is a source of anti-angiogenic soluble factors such as Noggin and Chondroitin sulfate (CS). These factors may form the basis for a new minimally invasive strategy to target angiogenesis in the IVD.

OBJECTIVE

To examine the anti-angiogenic potential of soluble factors from notochordal cells (NCs) and candidates Noggin and CS under healthy culture conditions and in the presence of pro-inflammatory mediators.

DESIGN

NC conditioned media (NCCM) was generated from porcine NC-rich nucleus pulposus tissue. To assess the effects of NCCM, CS and Noggin on angiogenesis, cell invasion and tubular formation assays were performed using human umbilical vein endothelial cells (HUVECs) ± tumor necrosis factor alpha (TNFα [10 ng/ml]). vascular endothelial growth factor (VEGF)-A, MMP-7, interleukin-6 (IL-6) and IL-8 mRNA levels were assessed using qRT-PCR.

RESULTS

NCCM (10 & 100%), CS (10 and 100 μg) and Noggin (10 and 100 ng) significantly decreased cell invasion of HUVECs with and without TNFα. NCCM 10% and Noggin 10 ng inhibited tubular formation with and without TNFα and CS 100 μg inhibited tubules in Basal conditions whereas CS 10 μg inhibited tubules with TNFα. NCCM significantly decreased VEGF-A, MMP-7 and IL-6 mRNA levels in HUVECs with and without TNFα. CS and Noggin had no effects on gene expression.

CONCLUSIONS

We provide the first evidence that soluble factors from NCs can inhibit angiogenesis by suppressing VEGF signaling. Notochordal-derived ligands are a promising minimally invasive strategy targeting neurovascular ingrowth and pain in the degenerated IVD.

Avians as a model system of vascular development

For more than 2,000 years, philosophers and scientists have turned to the avian embryo with questions of how life begins (Aristotle and Peck Generations of Animals. Loeb Classics, vol. XIII. Harvard University Press, Cambridge, 1943; Needham, A history of embryology. Abelard-Schuman, New York, 1959). Then, as now, the unique accessibility of the embryo both in terms of acquisition of eggs from domesticated fowl and ease at which the embryo can be visualized by simply opening the shell has made avians an appealing and powerful model system for the study of development. Thus, as the field of embryology has evolved through observational, comparative, and experimental embryology into its current iteration as the cellular and molecular biology of development, avians have remained a useful and practical system of study.

Diversity is in my veins: role of bone morphogenetic protein signaling during venous morphogenesis in zebrafish illustrates the heterogeneity within endothelial cells

Endothelial cells are a highly diverse group of cells which display distinct cellular responses to exogenous stimuli. Although the aptly named vascular endothelial growth factor-A signaling pathway is hailed as the most important signaling input for endothelial cells, additional factors also participate in regulating diverse aspects of endothelial behaviors and functions. Given this heterogeneity, these additional factors seem to play a critical role in creating a custom-tailored environment to regulate behaviors and functions of distinct subgroups of endothelial cells. For instance, molecular cues that modulate morphogenesis of arterial vascular beds can be distinct from those that govern morphogenesis of venous vascular beds. Recently, we have found that bone morphogenetic protein signaling selectively promotes angiogenesis from venous vascular beds without eliciting similar responses from arterial vascular beds in zebrafish, indicating that bone morphogenetic protein signaling functions as a context-dependent regulator during vascular morphogenesis. In this review, we will provide an overview of the molecular mechanisms that underlie proangiogenic effects of bone morphogenetic protein signaling on venous vascular beds in the context of endothelial heterogeneity and suggest a more comprehensive picture of the molecular mechanisms of vascular morphogenesis during development.

Shape and position of the node and notochord along the bilateral plane of symmetry are regulated by cell-extracellular matrix interactions

The node and notochord (and their equivalents in other species) are essential signaling centers, positioned along the plane of bilateral symmetry in developing vertebrate embryos. However, genes and mechanisms regulating morphogenesis of these structures and their placement along the embryonic midline are not well understood. In this work, we provide the first evidence that the position of the node and the notochord along the bilateral plane of symmetry are under genetic control and are regulated by integrin α5β1 and fibronectin in mice. We found that the shape of the node is often inverted in integrin α5-null and fibronectin-null mutants, and that the positioning of node and the notochord is often skewed away from the perceived plane of embryonic bilateral of symmetry. Our studies also show that the shape and position of the notochord are dependent on the shape and embryonic placement of the node. Our studies suggest that fibronectin regulates the shape of the node by affecting apico-basal polarity of the nodal cells. Taken together, our data indicate that cell–extracellular matrix interactions mediated by integrin α5β1 and fibronectin regulate the geometry of the node as well as the placement of the node and notochord along the plane of bilateral symmetry in the mammalian embryo.

Molecular and cellular mechanisms of bone morphogenetic proteins and activins in the skin: potential benefits for wound healing

Bone morphogenetic proteins (BMPs) and activins are phylogenetically conserved proteins, belonging to the transforming growth factor-β superfamily, that signal through the phosphorylation of receptor-regulated Smad proteins, activating different cell responses. They are involved in various steps of skin morphogenesis and wound repair, as can be evidenced by the fact that their expression is increased in skin injuries. BMPs play not only a role in bone regeneration but are also involved in cartilage, tendon-like tissue and epithelial regeneration, maintain vascular integrity, capillary sprouting, proliferation/migration of endothelial cells and angiogenesis, promote neuron and dendrite formation, alter neuropeptide levels and are involved in immune response modulation, at least in animal models. On the other hand, activins are involved in wound repair through the regulation of skin and immune cell migration and differentiation, re-epithelialization and granulation tissue formation, and also promote the expression of collagens by fibroblasts and modulate scar formation. This review aims at enunciating the effects of BMPs and activins in the skin, namely in skin development, as well as in crucial phases of skin wound healing, such as inflammation, angiogenesis and repair, and will focus on the effects of these proteins on skin cells and their signaling pathways, exploring the potential therapeutic approach of the application of BMP-2, BMP-6 and activin A in chronic wounds, particularly diabetic foot ulcerations.

Vascular development in the zebrafish

The zebrafish has emerged as an excellent vertebrate model system for studying blood and lymphatic vascular development. The small size, external and rapid development, and optical transparency of zebrafish embryos are some of the advantages the zebrafish model system offers. Multiple well-established techniques have been developed for imaging and functionally manipulating vascular tissues in zebrafish embryos, expanding on and amplifying these basic advantages and accelerating use of this model system for studying vascular development. In the past decade, studies performed using zebrafish as a model system have provided many novel insights into vascular development. In this article we discuss the amenability of this model system for studying blood vessel development and review contributions made by this system to our understanding of vascular development.

Notochordal cell-derived therapeutic strategies for discogenic back pain

An understanding of the processes that occur during development of the intervertebral disk can help inform therapeutic strategies for discogenic pain. This article reviews the literature to identify candidates that are found in or derived from the notochord or notochordal cells and evaluates the theory that such factors could be isolated and used as biologics to target the structural disruption, inflammation, and neurovascular ingrowth often associated with discogenic back pain. A systematic review using PubMed was performed with a primary search using keywords "(notochordal OR notochord) And (nerves OR blood vessels OR SHH OR chondroitin sulfate OR notch OR CTGF) NOT chordoma." Secondary searches involved keywords associated with the intervertebral disk and pain. Several potential therapeutic candidates from the notochord and their possible targets were identified. Studies are needed to further identify candidates, explore mechanisms for effect, and to validate the theory that these candidates can promote structural restoration and limit or inhibit neurovascular ingrowth using in vivo studies.

Dorsal aorta formation: separate origins, lateral-to-medial migration, and remodeling

Blood vessel formation is a highly dynamic tissue-remodeling event that can be observed from early development in vertebrate embryos. Dorsal aortae, the first functional intra-embryonic blood vessels, arise as two separate bilateral vessels in the trunk and undergo lateral-to-medial translocation, eventually fusing into a single large vessel at the midline. After this dramatic remodeling, the dorsal aorta generates hematopoietic stem cells. The dorsal aorta is a good model to use to increase our understanding of the mechanisms controlling the establishment and remodeling of larger blood vessels in vivo. Because of the easy accessibility to the developing circulatory system, quail and chick embryos have been widely used for studies on blood vessel formation. In particular, the mapping of endothelial cell origins has been performed using quail-chick chimera analysis, revealing endothelial, vascular smooth muscle, and hematopoietic cell progenitors of the dorsal aorta. The avian embryo model also allows conditional gene activation/inactivation and direct observation of cell behaviors during dorsal aorta formation. This allows a better understanding of the molecular mechanisms underlying specific morphogenetic events during dynamic dorsal aorta formation from a cell behavior perspective.

Head and neck lymphatic tumors and bony abnormalities: a clinical and molecular review

Lymphatic malformations and lymphatic-derived tumors commonly involve the head and neck, where they may be associated with bony abnormalities and other systemic symptoms. The reasons for the association between these disorders and local skeletal changes are largely unknown, but such changes may cause significant disease-related morbidity. Ongoing work in molecular and developmental biology is beginning to uncover potential reasons for the bony abnormalities found in head and neck lymphatic disease; this article summarizes current knowledge on possible mechanisms underlying this association.

Nonintegrating knockdown and customized scaffold design enhances human adipose-derived stem cells in skeletal repair

An urgent need exists in clinical medicine for suitable alternatives to available techniques for bone tissue repair. Human adipose-derived stem cells (hASCs) represent a readily available, autogenous cell source with well-documented in vivo osteogenic potential. In this article, we manipulated Noggin expression levels in hASCs using lentiviral and nonintegrating minicircle short hairpin ribonucleic acid (shRNA) methodologies in vitro and in vivo to enhance hASC osteogenesis. Human ASCs with Noggin knockdown showed significantly increased bone morphogenetic protein (BMP) signaling and osteogenic differentiation both in vitro and in vivo, and when placed onto a BMP-releasing scaffold embedded with lentiviral Noggin shRNA particles, hASCs more rapidly healed mouse calvarial defects. This study therefore suggests that genetic targeting of hASCs combined with custom scaffold design can optimize hASCs for skeletal regenerative medicine.

Enhancement of human adipose-derived stromal cell angiogenesis through knockdown of a BMP-2 inhibitor

BACKGROUND

Previous studies have demonstrated the role of noggin, a bone morphogenetic protein-2 inhibitor, in vascular development and angiogenesis. The authors hypothesized that noggin suppression in human adipose-derived stromal cells would enhance vascular endothelial growth factor secretion and angiogenesis in vitro and in vivo to a greater extent than bone morphogenetic protein-2 alone.

METHODS

Human adipose-derived stromal cells were isolated from human lipoaspirate (n = 6) noggin was knocked down using lentiviral techniques. Knockdown was confirmed and angiogenesis was assessed by tubule formation and quantitative real-time polymerase chain reaction. Cells were seeded onto scaffolds and implanted into a 4-mm critical size calvarial defect. In vivo angiogenic signaling was assessed by immunofluorescence and immunohistochemistry.

RESULTS

Human adipose-derived stromal cells with noggin suppression secreted significantly higher amounts of angiogenic proteins, expressed higher levels of angiogenic genes, and formed more tubules in vitro. In vivo, calvarial defects seeded with noggin shRNA human adipose-derived stromal cells exhibited a significantly higher number of vessels in the defect site than controls by immunohistochemistry (p < 0.05). In addition, bone morphogenetic protein-2-releasing scaffolds significantly enhanced vascular signaling in the defect site.

CONCLUSIONS

Human adipose-derived stromal cells demonstrate significant increases in angiogenesis in vitro and in vivo with both noggin suppression and BMP-2 supplementation. By creating a cell with noggin suppressed and by using a scaffold with increased bone morphogenetic protein-2 signaling, a more angiogenic niche can be created.

Integration of repulsive guidance cues generates avascular zones that shape mammalian blood vessels

RATIONALE

Positive signals, such as vascular endothelial growth factor, direct endothelial cells (ECs) to specific locations during blood vessel formation. Less is known about repulsive signal contribution to shaping vessels. Recently, "neuronal guidance cues" have been shown to influence EC behavior, particularly in directing sprouting angiogenesis by repelling ECs. However, their role during de novo blood vessel formation remains unexplored.

OBJECTIVE

To identify signals that guide and pattern the first mammalian blood vessels.

METHODS AND RESULTS

Using genetic mouse models, we show that blood vessels are sculpted through the generation of stereotyped avascular zones by EC-repulsive cues. We demonstrate that Semaphorin3E (Sema3E) is a key factor that shapes the paired dorsal aortae in mouse, as sema3E(-/-) embryos develop an abnormally branched aortic plexus with a markedly narrowed avascular midline. In vitro cultures and avian grafting experiments show strong repulsion of ECs by Sema3E-expressing cells. We further identify the mouse notochord as a rich source of multiple redundant neuronal guidance cues. Mouse embryos that lack notochords fail to form cohesive aortic vessels because of loss of the avascular midline, yet maintain lateral avascular zones. We demonstrate that lateral avascular zones are directly generated by the lateral plate mesoderm, a critical source of Sema3E.

CONCLUSIONS

These findings demonstrate that Sema3E-generated avascular zones are critical regulators of mammalian cardiovascular patterning and are the first to identify a repulsive role for the lateral plate mesoderm. Integration of multiple, and in some cases redundant, repulsive cues from various tissues is critical to patterning the first embryonic blood vessels.

Isolation and characterization of node/notochord-like cells from mouse embryonic stem cells

The homeobox gene Noto is expressed in the node and its derivative the notochord. Here we use a targeted Noto-GFP reporter to isolate and characterize node/notochord-like cells derived from mouse embryonic stem cells. We find very few Noto-expressing cells after spontaneous differentiation. However, the number of Noto-expressing cells was increased when using Activin A to induce a Foxa2- and Brachyury-expressing progenitor population, whose further differentiation into Noto-expressing cells was improved by simultaneous inhibition of BMP, Wnt, and retinoic acid signaling. Noto-GFP(+) cells expressed the node/notochord markers Noto, Foxa2, Shh, Noggin, Chordin, Foxj1, and Brachyury; showed a vacuolarization characteristic of notochord cells; and can integrate into midline structures when grafted into Hensen's node of gastrulating chicken embryos. The ability to generate node/notochord-like cells in vitro will aid the biochemical characterization of these developmentally important structures.

Early arterial differentiation and patterning in the avian embryo model

Of the many models to study vascular biology the avian embryo remains an informative and powerful model system that has provided important insights into endothelial cell recruitment, assembly and remodeling during development of the circulatory system. This review highlights several discoveries in the avian system that show how arterial patterning is regulated using the model of dorsal aortae development along the embryo midline during gastrulation and neurulation. These discoveries were made possible through spatially and temporally controlled gain-of-function experiments that provided direct evidence that BMP signaling plays a pivotal role in vascular recruitment, patterning and remodeling and that Notch-signaling recruits vascular precursor cells to the dorsal aortae. Importantly, BMP ligands are broadly expressed throughout embryos but BMP signaling activation region is spatially defined by precisely regulated expression of BMP antagonists. These discoveries provide insight into how signaling, both positive and negative, regulate vascular patterning. This review also illustrates similarities of early arterial patterning along the embryonic midline in amniotes both avian and mammalians including human, evolutionarily specialized from non-amniotes such as fish and frog.

Bone Morphogenetic Protein functions as a context-dependent angiogenic cue in vertebrates

Bone Morphogenetic Protein (BMP) signaling has been implicated in diverse biological processes. Although how BMP signaling regulates behaviors of endothelial cells during angiogenesis are not fully understood, increasing evidence indicate functions of BMP signaling components are essential in developmental and pathological angiogenesis. Here we review recent advances in delineating the functions of BMP signaling during angiogenesis. In addition, we discuss downstream pathways that transduce BMP signaling in endothelial cells, and factors that modulate BMP signaling response in endothelial cells. Finally, we provide recent insight on how BMP signaling functions as a context dependent angiogenic cue.

Distinct signalling pathways regulate sprouting angiogenesis from the dorsal aorta and the axial vein

Angiogenesis, the formation of new blood vessels from pre-existing vessels, is critical to most physiological processes and many pathological conditions. During zebrafish development, angiogenesis expands the axial vessels into a complex vascular network that is necessary for efficient oxygen delivery. Although the dorsal aorta and the axial vein are spatially juxtaposed, the initial angiogenic sprouts from these vessels extend in opposite directions, indicating that distinct cues may regulate angiogenesis of the axial vessels. We found that angiogenic sprouts from the dorsal aorta are dependent on vascular endothelial growth factor A (Vegf-A) signalling, and do not respond to bone morphogenetic protein (Bmp) signals. In contrast, sprouts from the axial vein are regulated by Bmp signalling independently of Vegf-A signals, indicating that Bmp is a vein-specific angiogenic cue during early vascular development. Our results support a paradigm whereby different signals regulate distinct programmes of sprouting angiogenesis from the axial vein and dorsal aorta, and indicate that signalling heterogeneity contributes to the complexity of vascular networks.

Promotion of avian endothelial cell differentiation by GATA transcription factors

In the avian embryo, endothelial cells originate from several sources, including the lateral plate and somite mesoderm. In this study, we show that Gata transcription factors are expressed in the lateral plate and in vasculogenic regions of the avian somite and are able to promote a vascular endothelial fate when ectopically expressed in somite precursors. A fusion of GATA4 to the transcriptional activator VP16 promoted endothelium formation, indicating that GATA transcription factors promote vasculogenesis via activation of downstream targets, while a fusion of GATA4 to the transcriptional repressor engrailed repressed expression of Vascular Endothelial Growth Factor Receptor 2, a marker of endothelial precursors. These findings indicate a role for GATA transcription factors in the differentiation of the endothelium.

An anteroposterior wave of vascular inhibitor downregulation signals aortae fusion along the embryonic midline axis

Paracrine signals, both positive and negative, regulate the positioning and remodeling of embryonic blood vessels. In the embryos of mammals and birds, the first major remodeling event is the fusion of bilateral dorsal aortae at the midline to form the dorsal aorta. Although the original bilaterality of the dorsal aortae occurs as the result of inhibitory factors (antagonists of BMP signaling) secreted from the midline by the notochord, it is unknown how fusion is later signaled. Here, we report that dorsal aortae fusion is tightly regulated by a change in signaling by the notochord along the anteroposterior axis. During aortae fusion, the notochord ceases to exert its negative influence on vessel formation. This is achieved by a transcriptional downregulation of negative regulators while positive regulators are maintained at pre-fusion levels. In particular, Chordin, the most abundant BMP antagonist expressed in the notochord prior to fusion, undergoes a dramatic downregulation in an anterior to posterior wave. With inhibitory signals diminished and sustained expression of the positive factors SHH and VEGF at the midline, fusion of the dorsal aortae is signaled. These results demonstrate a novel mechanism by which major modifications of the vascular pattern can occur through modulation of vascular inhibitors without changes in the levels of positive vascular regulators.

In vitro and In vivo imaging of antivasculogenesis induced by Noggin protein expression in human venous endothelial cells

Noggin protein is a potent bone morphogenetic protein (BMP) antagonist capable of inhibiting vasculogenesis even in the presence of provasculogenic VEGF and FGF-2. We found that human umbilical vein endothelial cells (HUVECs) do not express Noggin in culture and used these cells for modeling of antivasculogenesis. We hypothesized that high-efficiency transduction of HUVECs with bicistronic lentiviral vector encoding Noggin and enhanced green fluorescent protein (EGFP) enables direct visualization of Noggin effects in homogenous primary cell populations in vitro and in vivo. By comparing HUVECs transduced with a control GFP and GFP/Noggin expression cassettes, we showed that constitutive and orthotopic Noggin protein expression did not influence cell proliferation, down-regulated BMP-2 expression, and showed no effect on BMP receptor transcripts. We demonstrated that in contrast to GFP-only control, Noggin expression in endothelial cells abrogated endothelial migration in response to monolayer injury, blocked endothelial transmigration, and caused abrogation of cord formation in vitro. Adding exogenous BMP-4 restored the formation of cords. Imaging experiments in vivo investigated vessel formation in Matrigel implants in athymic mice by utilizing GFP imaging or magnetic resonance imaging of perfusion in the implants. Both approaches demonstrated the lack of functional vessel formation after the adoptive transfer of GFP/Noggin-expressing human endothelial cells in mice.