VEGF mediates angioblast migration during development of the dorsal aorta in Xenopus

Endothelial cell motility, coordination and pattern formation during vasculogenesis

How vascular networks assemble is a fundamental problem of developmental biology that also has medical importance. To explain the organizational principles behind vascular patterning, we must understand how can tissue level structures be controlled through cell behavior patterns like motility and adhesion that, in turn, are determined by biochemical signal transduction processes? We discuss the various ideas that have been proposed as mechanisms for vascular network assembly: cell motility guided by extracellular matrix alignment (contact guidance), chemotaxis guided by paracrine and autocrine morphogens, and multicellular sprouting guided by cell-cell contacts. All of these processes yield emergent patterns, thus endothelial cells can form an interconnected structure autonomously, without guidance from an external pre-pattern.

Arterial and venous progenitors of the major axial vessels originate at distinct locations

Currently, it remains controversial how vascular endothelial progenitor cells (angioblasts) establish their arterial or venous fates. We show using zebrafish that the arterial progenitors of the major axial vessels originate earlier and closer to the midline than the venous progenitors. Both medial and lateral progenitor populations migrate to distinct arterial and venous positions and not into a common precursor vessel as previously suggested. Overexpression of VEGF or Hedgehog (Hh) homologs results in the partially randomized distribution of arterial and venous progenitors within the axial vessels. We further demonstrate that the function of the Etv2 transcription factor is required at earlier stages for arterial development than for venous. Our results argue that the medial angioblasts undergo arterial differentiation because they receive higher concentration of Vegf and Hh morphogens than the lateral angioblasts. We propose a revised model of arterial-venous differentiation that explains how angioblasts choose between an arterial and venous fate.

VEGFA-dependent and -independent pathways synergise to drive Scl expression and initiate programming of the blood stem cell lineage in Xenopus

The first haematopoietic stem cells share a common origin with the dorsal aorta and derive from putative adult haemangioblasts in the dorsal lateral plate (DLP) mesoderm. Here we show that the transcription factor (TF) stem cell leukaemia (Scl/Tal1) is crucial for development of these adult haemangioblasts in Xenopus and establish the regulatory cascade controlling its expression. We show that VEGFA produced in the somites is required to initiate adult haemangioblast programming in the adjacent DLP by establishing endogenous VEGFA signalling. This response depends on expression of the VEGF receptor Flk1, driven by Fli1 and Gata2. Scl activation requires synergy between this VEGFA-controlled pathway and a VEGFA-independent pathway controlled by Fli1, Gata2 and Etv2/Etsrp/ER71, which also drives expression of the Scl partner Lmo2. Thus, the two ETS factors Fli1 and Etv6, which drives the VEGFA expression in both somites and the DLP, sit at the top of the adult haemangioblast gene regulatory network (GRN). Furthermore, Gata2 is initially activated by Fli1 but later maintained by another ETS factor, Etv2. We also establish that Flk1 and Etv2 act independently in the two pathways to Scl activation. Thus, detailed temporal, epistatic measurements of key TFs and VEGFA plus its receptor have enabled us to build a Xenopus adult haemangioblast GRN.

Signalling pathways that control vertebrate haematopoietic stem cell specification

Haematopoietic stem cells (HSCs) are tissue-specific stem cells that replenish all mature blood lineages during the lifetime of an individual. Clinically, HSCs form the foundation of transplantation-based therapies for leukaemias and congenital blood disorders. Researchers have long been interested in understanding the normal signalling mechanisms that specify HSCs in the embryo, in part because recapitulating these requirements in vitro might provide a means to generate immune-compatible HSCs for transplantation. Recent embryological work has demonstrated the existence of previously unknown signalling requirements. Moreover, it is now clear that gene expression in the nearby somite is integrally involved in regulating the transition of the embryonic endothelium to a haemogenic fate. Here, we review current knowledge of the intraembryonic signals required for the specification of HSCs in vertebrates.

Uncoupling VEGFA functions in arteriogenesis and hematopoietic stem cell specification

VEGFA signaling is critical for endothelial and hematopoietic stem cell (HSC) specification. However, blood defects resulting from perturbation of the VEGFA pathway are always accompanied by impaired vascular/arterial development. Because HSCs derive from arterial cells, it is unclear whether VEGFA directly contributes to HSC specification. This is an important question for our understanding of how HSCs are formed and for developing their production in vitro. Through knockdown of the regulator ETO2 in embryogenesis, we report a specific decrease in expression of medium/long Vegfa isoforms in somites. This leads to absence of Notch1 expression and failure of HSC specification in the dorsal aorta (DA), independently of vessel formation and arterial specification. Vegfa hypomorphs and isoform-specific (medium/long) morphants not only recapitulate this phenotype but also demonstrate that VEGFA short isoform is sufficient for DA development. Therefore, sequential, isoform-specific VEGFA signaling successively induces the endothelial, arterial, and HSC programs in the DA.

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.

Embryonic vasculogenesis and hematopoietic specification

Vasculogenesis is the process by which blood vessels are formed de novo. In mammals, vasculogenesis occurs in parallel with hematopoiesis, the formation of blood cells. Thus, it is debated whether vascular endothelial cells and blood cells are derived from a common progenitor. Whether or not this is the case, there certainly is commonality among regulatory factors that control the differentiation and differentiated function of both cell lineages. VEGF is a major regulator of both cell types and plays a critical role, in coordination with other signaling pathways and transcriptional regulators, in controlling the differentiation and behavior of endothelial and blood cells during early embryonic development, as further discussed herein.

Pattern formation during vasculogenesis

Vasculogenesis, the assembly of the first vascular network, is an intriguing developmental process that yields the first functional organ system of the embryo. In addition to being a fundamental part of embryonic development, vasculogenic processes also have medical importance. To explain the organizational principles behind vascular patterning, we must understand how morphogenesis of tissue level structures can be controlled through cell behavior patterns that, in turn, are determined by biochemical signal transduction processes. Mathematical analyses and computer simulations can help conceptualize how to bridge organizational levels and thus help in evaluating hypotheses regarding the formation of vascular networks. Here, we discuss the ideas that have been proposed to explain the formation of the first vascular pattern: cell motility guided by extracellular matrix alignment (contact guidance), chemotaxis guided by paracrine and autocrine morphogens, and sprouting guided by cell-cell contacts.

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.

Microfluidic assay of endothelial cell migration in 3D interpenetrating polymer semi-network HA-Collagen hydrogel

Cell migration through the extracellular matrix (ECM) is one of the key features for physiological and pathological processes such as angiogenesis, cancer metastasis, and wound healing. In particular, the quantitative assay of endothelial cell migration under the well-defined three dimensional (3D) microenvironment is important to analyze the angiogenesis mechanism. In this study, we report a microfluidic assay of endothelial cell sprouting and migration into an interpenetrating polymer semi-network HA-Collagen (SIPNs CH) hydrogel as ECM providing an enhanced in vivo mimicking 3D microenvironment to cells. The microfluidic chip could provide a well-controlled gradient of growth factor to cells, whereas the hydrogel could mimic a well-defined 3D microenvironment in vivo. (In addition/Furthermore, the microfluidic chip gives a well-controlled gradient of growth factor to cells) For this reason, three types of hydrogel, composed of semi-interpenetrating networks of collagen and hyaluronic acid were prepared, and firstly we proved the role of the hydrogel in endothelial cell migration. The diffusion property and swelling ratio of the hydrogel were characterized. It modulated the migration of endothelial cells in quantified manner, also being influenced by additional synthesis of Matrix metalloproteinase(MMP)-sensitive remodeling peptides and Arginine-glycine-lycinee (RGD) cell adhesion peptides. We successfully established a novel cell migration platform by changing major determinants such as ECM material under biochemical synthesis and under growth factor gradients in a microfluidic manner.

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.

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.

Embryonic origin of the adult hematopoietic system: advances and questions

Definitive hematopoietic stem cells (HSCs) lie at the foundation of the adult hematopoietic system and provide an organism throughout its life with all blood cell types. Several tissues demonstrate hematopoietic activity at early stages of embryonic development, but which tissue is the primary source of these important cells and what are the early embryonic ancestors of definitive HSCs? Here, we review recent advances in the field of HSC research that have shed light on such questions, while setting them into a historical context, and discuss key issues currently circulating in this field.

Fli+ etsrp+ hemato-vascular progenitor cells proliferate at the lateral plate mesoderm during vasculogenesis in zebrafish

BACKGROUND

Vasculogenesis, the de novo formation of blood vessels from precursor cells is critical for a developing embryo. However, the signals and events that dictate the formation of primary axial vessels remain poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we use ets-related protein-1 (etsrp), which is essential for vascular development, to analyze the early stages of vasculogenesis in zebrafish. We found etsrp(+) cells of the head, trunk and tail follow distinct developmental sequences. Using a combination of genetic, molecular and chemical approaches, we demonstrate that fli(+)etsrp(+) hemato-vascular progenitors (FEVPs) are proliferating at the lateral plate mesoderm (LPM). The Shh-VEGF-Notch-Hey2 signaling pathway controls the proliferation process, and experimental modulation of single components of this pathway alters etsrp(+) cell numbers at the LPM.

CONCLUSIONS/SIGNIFICANCE

This study for the first time defines factors controlling proliferation, and cell numbers of pre-migratory FEVPs in zebrafish.

A mesoderm-derived precursor for mesenchymal stem and endothelial cells

Among the three embryonic germ layers, the mesoderm is a major source of the mesenchymal precursors giving rise to skeletal and connective tissues, but these precursors have not previously been identified and characterized. Using human embryonic stem cells directed toward mesendodermal differentiation, we show that mesenchymal stem/stromal cells (MSCs) originate from a population of mesodermal cells identified by expression of apelin receptor. In semisolid medium, these precursors form FGF2-dependent compact spheroid colonies containing mesenchymal cells with a transcriptional profile representative of mesoderm-derived embryonic mesenchyme. When transferred to adherent cultures, individual colonies give rise to MSC lines with chondro-, osteo-, and adipogenic differentiation potentials. Although the MSC lines lacked endothelial potential, endothelial cells could be derived from the mesenchymal colonies, suggesting that, similar to hematopoietic cells, MSCs arise from precursors with angiogenic potential. Together, these studies identified a common precursor of mesenchymal and endothelial cells, mesenchymoangioblast, as the source of mesoderm-derived MSCs.

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.

Sphingosine-1-phosphate signaling in vasculogenesis and angiogenesis

Blood vessels either form de novo through the process of vasculogenesis or through angiogenesis that involves the sprouting and proliferation of endothelial cells in pre-existing blood vessels. A complex interactive network of signaling cascades downstream from at least three of the nine known G-protein-coupled sphingosine-1-phosphate (S1P) receptors act as a prime effector of neovascularization that occurs in embryonic development and in association with various pathologies. This review focuses on the current knowledge of the roles of S1P signaling in vasculogenesis and angiogenesis, with particular emphasis on vascular cell adhesion and motility responses.

Dynamic analysis of vascular morphogenesis using transgenic quail embryos

Background

One of the least understood and most central questions confronting biologists is how initially simple clusters or sheet-like cell collectives can assemble into highly complex three-dimensional functional tissues and organs. Due to the limits of oxygen diffusion, blood vessels are an essential and ubiquitous presence in all amniote tissues and organs. Vasculogenesis, the de novo self-assembly of endothelial cell (EC) precursors into endothelial tubes, is the first step in blood vessel formation [1]. Static imaging and in vitro models are wholly inadequate to capture many aspects of vascular pattern formation in vivo, because vasculogenesis involves dynamic changes of the endothelial cells and of the forming blood vessels, in an embryo that is changing size and shape.

Methodology/Principal Findings

We have generated Tie1 transgenic quail lines Tg(tie1:H2B-eYFP) that express H2B-eYFP in all of their endothelial cells which permit investigations into early embryonic vascular morphogenesis with unprecedented clarity and insight. By combining the power of molecular genetics with the elegance of dynamic imaging, we follow the precise patterning of endothelial cells in space and time. We show that during vasculogenesis within the vascular plexus, ECs move independently to form the rudiments of blood vessels, all while collectively moving with gastrulating tissues that flow toward the embryo midline. The aortae are a composite of somatic derived ECs forming its dorsal regions and the splanchnic derived ECs forming its ventral region. The ECs in the dorsal regions of the forming aortae exhibit variable mediolateral motions as they move rostrally; those in more ventral regions show significant lateral-to-medial movement as they course rostrally.

Conclusions/Significance

The present results offer a powerful approach to the major challenge of studying the relative role(s) of the mechanical, molecular, and cellular mechanisms of vascular development. In past studies, the advantages of the molecular genetic tools available in mouse were counterbalanced by the limited experimental accessibility needed for imaging and perturbation studies. Avian embryos provide the needed accessibility, but few genetic resources. The creation of transgenic quail with labeled endothelia builds upon the important roles that avian embryos have played in previous studies of vascular development.

Tel1/ETV6 specifies blood stem cells through the agency of VEGF signaling

The regulation of stem cell ontogeny is poorly understood. We show that the leukemia-associated Ets transcription factor, Tel1/ETV6, specifies the first hematopoietic stem cells (HSCs) in the dorsal aorta (DA). In contrast, Tel1/ETV6 has little effect on embryonic blood formation, further distinguishing the programming of the long- and short-term blood populations. Consistent with the notion of concordance of arterial and HSC programs, we show that Tel1/ETV6 is also required for the specification of the DA as an artery. We further show that Tel1/ETV6 acts by regulating the transcription of VegfA in both the lateral plate mesoderm and also in the somites. Exogenous VEGFA rescues Tel1/ETV6 morphants, and depletion of VEGFA or its receptor, Flk1, largely phenocopies Tel1/ETV6 depletion. Few such links between intrinsic and extrinsic programming of stem cells have been reported previously. Our data place Tel1/ETV6 at the apex of the genetic regulatory cascade leading to HSC production.

RhoA/ROCK signaling is essential for multiple aspects of VEGF-mediated angiogenesis

The small GTPase RhoA and its downstream effectors, ROCK1 and ROCK2, regulate a number of cellular processes, including cell motility, proliferation, survival, and permeability. Pharmacological inhibitors of the Rho pathway reportedly block angiogenesis; however, the molecular details of this inhibition are largely unknown. We demonstrate that vascular endothelial growth factor-A (VEGF) rapidly induces RhoA activation in endothelial cells (ECs). Moreover, the pharmacological inhibition of ROCK1/2 using 10 microM Y-27632 (the IC(50) for this compound in ECs) strongly disrupts vasculogenesis in pluripotent embryonic stem cell cultures, VEGF-mediated regenerative angiogenesis in ex vivo retinal explants, and VEGF-mediated in vitro EC tube formation. Furthermore, using small interfering RNA knockdown and mouse heterozygote knockouts of ROCK1 and ROCK2, we provide data indicating that VEGF-driven angiogenesis is largely mediated through ROCK2. These data demonstrate that Rho/ROCK signaling is an important mediator in a number of angiogenic processes, including EC migration, survival, and cell permeability, and suggest that Rho/ROCK inhibition may prove useful for the treatment of angiogenesis-related disorders.

VEGF receptor signaling in vertebrate development

The secreted glycoprotein vascular endothelial growth factor A (VEGF or VEGFA) affects many different cell types and modifies a wide spectrum of cellular behaviors in tissue culture models, including proliferation, migration, differentiation and survival. The versatility of VEGF signaling is reflected in the complex composition of its cell surface receptors and their ability to activate a variety of different downstream signaling molecules. A major challenge for VEGF research is to determine which of the specific signaling pathways identified in vitro control development and homeostasis of tissues containing VEGF-responsive cell types in vivo.

Endothelial cell-specific chemotaxis receptor (ecscr) promotes angioblast migration during vasculogenesis and enhances VEGF receptor sensitivity

Endothelial cell-specific chemotaxis receptor (ECSCR) is a cell surface protein expressed by blood endothelial cells with roles in endothelial cell migration and signal transduction. We investigated the function of ecscr in the development of the zebrafish vasculature. Zebrafish ecscr is expressed in angioblasts and in axial vessels during angioblast migration and vasculogenesis. Morpholino-directed ecscr knockdown resulted in defective angioblast migration in the posterior lateral plate mesoderm, a process known to depend on vascular endothelial-derived growth factor (VEGF). In cultured cells, transfected ECSCR localized to actin-rich membrane protrusions, colocalizing with kinase insert domain protein receptor (KDR)/VEGF receptor 2 in these regions. ECSCR-silenced cells show reduced VEGF-induced phosphorylation of KDR but not of FMS-like tyrosine kinase 1 (FLT1)/VEGF receptor 1. Finally, chemical inhibition of VEGF receptor activity in zebrafish resulted in angioblast deficiencies that partially overlap with those seen in ecscr morphants. We propose that ecscr promotes migration of zebrafish angioblasts by enhancing endothelial kdr sensitivity to VEGF.

Chemokine signaling guides regional patterning of the first embryonic artery

The aorta traverses the body, yet little is known about how it is patterned in different anatomical locations. Here, we show that the aorta develops from genetically distinct endothelial cells originating from diverse locations within the embryo. Furthermore, chemokine (C-X-C motif) receptor 4a (cxcr4a) is restricted to endothelial cells derived from anterior mesoderm, and is required specifically for formation of the lateral aortae. Cxcl12b, a cxcr4a ligand, is expressed in endoderm underlying the lateral aortae, and loss of cxcl12b phenocopies cxcr4a deficiency. These studies reveal unexpected endothelial diversity within the aorta that is necessary to facilitate its regional patterning by local cues.

Angiomotin-Like Protein 1 Controls Endothelial Polarity and Junction Stability During Sprouting Angiogenesis

Rationale: We have previously shown that angiomotin (Amot) is essential for endothelial cell migration during mouse embryogenesis. However, ≈5% of Amot knockout mice survived without any detectable vascular defects. Angiomotin-like protein 1 (AmotL1) potentially compensates for the absence of Amot as it is 62% homologous to Amot and exhibits similar expression pattern in endothelial cells.

Objective: Here, we report the identification of a novel isoform of AmotL1 that controls endothelial cell polarization and directional migration.

Methods and Results: Small interfering RNA–mediated silencing of AmotL1 in mouse aortic endothelial cells caused a significant reduction in migration. In confluent mouse pancreatic islet endothelial cells (MS-1), AmotL1 colocalized with Amot to tight junctions. Small interfering RNA knockdown of both Amot and AmotL1 in MS-1 cells exhibited an additive effect on increasing paracellular permeability compared to that of knocking down either Amot or AmotL1, indicating both proteins were required for proper tight junction activity. Moreover, as visualized using high-resolution 2-photon microscopy, the morpholino-mediated knockdown of amotl1 during zebrafish embryogenesis resulted in vascular migratory defect of intersegmental vessels with strikingly decreased junction stability between the stalk cells and the aorta. However, the phenotype was quite distinct from that of amot knockdown which affected polarization of the tip cells of intersegmental vessels. Double knockdown resulted in an additive phenotype of depolarized tip cells with no or decreased connection of the stalk cells to the dorsal aorta.

Conclusions: These results cumulatively validate that Amot and AmotL1 have similar effects on endothelial migration and tight junction formation in vitro. However, in vivo Amot appears to control the polarity of vascular tip cells whereas AmotL1 mainly affects the stability of cell–cell junctions of the stalk cells.

An in vivo chemical library screen in Xenopus tadpoles reveals novel pathways involved in angiogenesis and lymphangiogenesis

Angiogenesis and lymphangiogenesis are essential for organogenesis but also play important roles in tissue regeneration, chronic inflammation, and tumor progression. Here we applied in vivo forward chemical genetics to identify novel compounds and biologic mechanisms involved in (lymph)angiogenesis in Xenopus tadpoles. A novel 2-step screening strategy involving a simple phenotypic read-out (edema formation or larval lethality) followed by semiautomated in situ hybridization was devised and used to screen an annotated chemical library of 1280 bioactive compounds. We identified 32 active compounds interfering with blood vascular and/or lymphatic development in Xenopus. Selected compounds were also tested for activities in a variety of endothelial in vitro assays. Finally, in a proof-of-principle study, the adenosine A1 receptor antagonist 7-chloro-4-hydroxy-2-phenyl-1,8-naphthyridine, an inhibitor of blood vascular and lymphatic development in Xenopus, was shown to act also as a potent antagonist of VEGFA-induced adult neovascularization in mice. Taken together, the present chemical library screening strategy in Xenopus tadpoles represents a rapid and highly efficient approach to identify novel pathways involved in (lymph)angiogenesis. In addition, the recovered compounds represent a rich resource for in-depth analysis, and their drug-like features will facilitate further evaluation in preclinical models of inflammation and cancer metastasis.

Distribution of EphB4 and EphrinB2 in normal and malignant urogenital tissue

OBJECTIVE

Ephrin (Eph) receptors are receptor tyrosine kinases; both EphrinB2, as a ligand, and EphB4, as a receptor, are involved in angiogenesis. EphrinB2 is expressed on arteries and EphB4, a specific receptor for EphrinB2, is expressed on veins. It is unknown whether involvement of arteries and veins in tumor angiogenesis is distinctive. Here we investigated their distribution in normal and malignant tissue of the urogenital tract.

MATERIALS AND METHODS

Five-micrometer-thick paraffin sections from nontumoral and tumoral tissues of kidney (n = 12), bladder (n = 33), and prostate (n = 20) were immunoreacted with antisera against EphB4 and EphrinB2 using the avidin-biotin-peroxidase complex technique. Comparisons of EphB4 and EphrinB2 stained arterial and venous vessels in the nontumoral and tumoral sections were evaluated in a semiquantitative analysis as frequency of the vessels in a predetermined tumor area counted under light microscopy.

RESULTS

Expression of EphrinB2 in arterial and EphB4 in venous endothelium was significantly greater in tumoral sections compared with nontumoral sections. No statistically significant correlation in comparing the labeling patterns for EphrinB2 with the labeling patterns for EphB4 was observed in nontumoral as well as tumoral sections.

CONCLUSIONS

The high expression of EphrinB2 in arterial and EphB4 in venous endothelium of urogenital tract tumors might contribute to their involvement in the progression of tumor angiogenesis. The relation between arteries and veins in the normal and tumor tissues is unchanged.

Essential role for the alpha 1 chain of type VIII collagen in zebrafish notochord formation

Several zebrafish mutants identified in large-scale forward genetic screens exhibit notochord distortion. We now report the cloning and further characterization of one such mutant, gulliver(m208) (gul(m208)). The notochord defect in gul(m208) mutants is exacerbated under conditions of copper depletion or lysyl oxidase cuproenzyme inhibition that are without a notochord effect on wild-type embryos. The gul(m208) phenotype results from a missense mutation in the gene encoding Col8a1, a lysyl oxidase substrate, and morpholino knockdown of col8a1 recapitulates the notochord distortion observed in gul(m208) mutants. Of interest, the amino acid mutated in gul(m208) Col8a1 is highly conserved, and the equivalent substitution in a closely related human protein, COL10A1, causes Schmid metaphyseal chondrodysplasia. Taken together, the data identify a new protein essential for notochord morphogenesis, extend our understanding of gene-nutrient interactions in early development, and suggest that human mutations in COL8A1 may cause structural birth defects.

Brachyury, SOX-9, and podoplanin, new markers in the skull base chordoma vs chondrosarcoma differential: a tissue microarray-based comparative analysis

The distinction between chondrosarcoma and chordoma of the skull base/head and neck is prognostically important; however, both have sufficient morphologic overlap to make delineation difficult. As a result of gene expression studies, additional candidate markers have been proposed to help in separating those entities. We sought to evaluate the performance of new markers: brachyury, SOX-9, and podoplanin alongside the more traditional markers glial fibrillary acid protein, carcinoembryonic antigen, CD24, and epithelial membrane antigen. Paraffin blocks from 103 skull base/head and neck chondroid tumors from 70 patients were retrieved (1969-2007). Diagnoses were made based on morphology and/or whole-section immunohistochemistry for cytokeratin and S100 protein yielding 79 chordomas (comprising 45 chondroid chordomas and 34 conventional chordomas), and 24 chondrosarcomas. A tissue microarray containing 0.6 mm cores of each tumor in triplicate was constructed using a manual array (MTA-1; Beecher Instruments). For visualization of staining, the ImmPRESS detection system (Vector Laboratories) with 2-diaminobenzidine substrate was used. Sensitivities and specificities were calculated for each marker. Core loss from the microarray ranged from 25 to 29% yielding 66-78 viable cases per stain. The classic marker, cytokeratin, still has the best performance characteristics. When combined with brachyury, accuracy improves slightly (sensitivity and specificity for detection of chordoma 98 and 100%, respectively). Positivity for both epithelial membrane antigen and AE1/AE3 had a sensitivity of 90% and a specificity of 100% for detecting chordoma in this study. SOX-9 is apparently common to both notochordal and cartilaginous differentiation, and is not useful in the chordoma-chondrosarcoma differential diagnosis. Glial fibrillary acid protein, carcinoembryonic antigen, CD24, and epithelial membrane antigen did not outperform other markers, and are less useful in the diagnosis of chordoma vs chondrosarcoma. Podoplanin still remains the only positive marker for chondrosarcoma, though its accuracy is less than previously reported.

Expression of the fras1/frem gene family during zebrafish development and fin morphogenesis

Mouse studies have highlighted the requirement of the extracellular matrix Fras and Frem proteins for embryonic epidermal adhesion. Mutations of the genes encoding some of these proteins underlie the blebs mouse mutants, whereas mutations in human FRAS1 and FREM2 cause Fraser syndrome, a congenital disorder characterized by embryonic blistering and renal defects. We have cloned the zebrafish homologues of these genes and characterized their evolutionary diversification and expression during development. The fish gene complement includes fras1, frem1a, frem1b, frem2a, frem2b, and frem3, which display complex overlapping and complementary expression patterns in developing tissues including the pharyngeal arches, hypochord, musculature, and otic vesicle. Expression during fin development delineates distinct populations of epidermal cells which have previously only been described at a morphological level. We detect relatively little gene expression in epidermis or pronephros, suggesting that the essential role of these proteins in mediating their development in humans and mice is recently evolved.

Essential role for fibrillin-2 in zebrafish notochord and vascular morphogenesis

Recent studies demonstrate that lysyl oxidase cuproenzymes are critical for zebrafish notochord formation, but the molecular mechanisms of copper-dependent notochord morphogenesis are incompletely understood. We, therefore, conducted a forward genetic screen for zebrafish mutants that exhibit notochord sensitivity to lysyl oxidase inhibition, yielding a mutant with defects in notochord and vascular morphogenesis, puff daddygw1 (pfdgw1). Meiotic mapping and cloning reveal that the pfdgw1 phenotype results from disruption of the gene encoding the extracellular matrix protein fibrillin-2, and the spatiotemporal expression of fibrillin-2 is consistent with the pfdgw1 phenotype. Furthermore, each aspect of the pfdgw1 phenotype is recapitulated by morpholino knockdown of fibrillin-2. Taken together, the data reveal a genetic interaction between fibrillin-2 and the lysyl oxidases in notochord formation and demonstrate the importance of fibrillin-2 in specific early developmental processes in zebrafish.

Role of crosstalk between phosphatidylinositol 3-kinase and extracellular signal-regulated kinase/mitogen-activated protein kinase pathways in artery-vein specification

Functional and structural differences between arteries and veins lie at the core of the circulatory system, both in health and disease. Therefore, understanding how artery and vein cell identities are established is a fundamental biological challenge with significant clinical implications. Molecular genetic studies in zebrafish and other vertebrates in the past decade have begun to reveal in detail the complex network of molecular pathways that specify artery and vein cell fates during embryonic development. Recently, a chemical genetic approach has revealed evidence that artery-vein specification is governed by cross talk between phosphoinositide 3-kinase and extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) signaling in artery-vein specification. We discuss recent findings on the signaling pathways involved in artery-vein specification during zebrafish development and compare and contrast these results to those from mammalian systems. It is anticipated that the complementary approaches of genetics and chemical biology, involving a variety of model organisms and systems, will lead to a better understanding of artery-vein specification and possibly to novel therapeutic approaches to treat vascular diseases.

LPA1 is essential for lymphatic vessel development in zebrafish

Lysophosphatidic acid (LPA) has long been implicated in regulating vascular development via endothelial cell-expressed G protein-coupled receptors. However, because of a lack of notable vascular defects reported in LPA receptor knockout mouse studies, the regulation of vasculature by LPA receptors in vivo is still uncertain. Using zebrafish as a model, we studied the gene expression patterns and functions of an LPA receptor, LPA(1), during embryonic development, in particular, vascular formation. Whole-mount in situ hybridization experiments revealed that zebrafish lpa(1) (zlpa(1)) was ubiquitously expressed early in development, and its expression domains were later localized to the head region and the vicinity of the dorsal aorta. The expression of zlpa(1) surrounding the dorsal aorta suggests its role in vasculature development. Knocking down of zLPA(1) by injecting morpholino (MO) oligonucleotides at 0.625-1.25 ng per embryo resulted in the absence of thoracic duct and edema in pericardial sac and trunk in a dose-dependent manner. These zlpa(1)-MO-resulted defects could be specifically rescued by ectopic expression of zlpa(1). In addition, overexpression of vegf-c, a well-known lymphangiogenic factor, also partially ameliorated the inhibition of thoracic duct development. Taken together, these results demonstrate that LPA(1) is necessary for lymphatic vessel formation during embryonic development in zebrafish.

XRASGRP2 expression during early development of Xenopus embryos

Previously, we described the DNA microarray screening of vascular endothelial cells that were formed by treatment of aggregates prepared from Xenopus animal cap cells with activin and angiopoietin-2. One of the genes identified in this screening showed homology to human RASGRP2 which plays a role in the regulation of GTP-GDP exchange of the Ras and Rap proteins, and was named XRASGRP2. In the present study, we analyzed the expression pattern of xrasgrp2 during Xenopus embryogenesis. The xrasgrp2 mRNA was expressed after stage 24, as assessed by stage PCR analysis. Whole-mount in situ hybridization showed that xrasgrp2 mRNA was located in the vascular region of the embryo. Loss-of-function analysis revealed that the formation of blood and endothelial cells in the explants transplanted into Xenopus embryos was inhibited by antisense morpholino oligonucleotides that block xrasgrp2 translation. These results suggest that XRASGRP2 plays a role in angiogenesis in Xenopus embryos.

Two deltaC splice-variants have distinct signaling abilities during somitogenesis and midline patterning

Notch signaling is required for many developmental processes, yet differences in the signaling abilities of various Notch ligands are poorly understood. Here, we have isolated a splice variant of the zebrafish Notch ligand deltaC in which the inclusion of the last intron leads to a truncation of the C-terminal 39 amino acids (deltaC(tv2)). We show that, unlike deltaC(tv1), deltaC(tv2) cannot function effectively in somitogenesis but has an enhanced ability to signal during midline development. Additionally, over-expression of deltaC(tv2) preferentially affects anterior midline development, while another Notch ligand, deltaD, shows a posterior bias. Using chimeric Deltas we show that the intracellular domain is responsible for the strength of signal in midline development, while the extracellular domain influences the anterior-posterior bias of the effect. Together our data show that different deltas can signal in biologically distinct ways in both midline formation and somitogenesis. Moreover, it illustrates the importance of cell-type-dependent modifiers of Notch signaling in providing ligand specificity.

How to create the vascular tree? (Latest) help from the zebrafish

The cardiovascular system provides oxygen, nutrients and hormones to organs, it directs traffic of metabolites and it maintains tissue homeostasis. It is one of the first organs assembled during vertebrate development and it is essential to life from early stages to adult. For these reasons, the process of vessel formation has being studied for more than a century, but it is only in the late eighties that there has been an explosion of research in the field with the employment of various in vitro and in vivo model systems. The zebrafish (Danio rerio) offers several advantages for in vivo studies; it played a fundamental role in new discoveries and helped to refine our knowledge of the vascular system. This review recapitulates the zebrafish data on vasculogenesis and angiogenesis, including the specification of the haemangioblasts from the mesoderm, their migration to form the vascular cord followed by axial vessels specification, the primary and secondary sprouting of intersomitic vessels, the formation of the lumen, the arterial versus venous specification and patterning. To emphasize the strengths of the zebrafish system in the vascular field, we summarize main tools, such as gene expression and mutagenesis screens, knock down technologies, transgenic lines and imaging, which played a major role in the development of the field and allowed significant discoveries, for instance the recent visualization of the lymphatic system in zebrafish. This information contributes to the prospective of drug discovery to cure human diseases linked to angiogenesis, not last tumours.

A flk-1 promoter/enhancer reporter transgenic Xenopus laevis generated using the Sleeping Beauty transposon system: an in vivo model for vascular studies

We have used the Sleeping Beauty (SB) transposable element to generate transgenic Xenopus laevis with expression of green fluorescent protein (GFP) in vascular endothelial cells using the frog flk-1 promoter. This is the first characterization of a SB-generated transgenic Xenopus that has tissue-restricted expression. We demonstrate that the transgene integrated into single genomic loci in two independent founder lines and is transmitted through the germline at the expected Mendelian frequencies. Transgene integration occurred through a noncanonical transposition process possibly reflecting Xenopus-specific interactions with the SB system. The transgenic animals express GFP in the same spatial and temporal pattern as the endogenous flk-1 gene throughout development and into adulthood. Overexpression of xVEGF122 in the transgenic animals disrupts vascular development that is visualized by fluorescent microscopy. These studies demonstrate the convenience of the SB system for generating transgenic animals and the utility of the xflk-1:GFP transgenic line for in vivo studies of vascular development.

Multicellular sprouting during vasculogenesis

Living organisms, from bacteria to vertebrates, are well known to generate sophisticated multicellular patterns. Numerous recent interdisciplinary studies have focused on the formation and regulation of these structures. Advances in automatized microscopy allow the time-resolved tracking of embryonic development at cellular resolution over an extended area covering most of the embryo. The resulting images yield simultaneous information on the motion of multiple tissue components-both cells and extracellular matrix (ECM) fibers. Recent studies on ECM displacements in bird embryos resulted in a method to distinguish tissue deformation and cell-autonomous motion. Patterning of the primary vascular plexus results from a collective action of primordial endothelial cells. The emerging "polygonal" vascular structure is shown to be formed by cell-cell and cell-ECM interactions: adhesion and protrusive activity (sprouting). Utilizing avb3 integrins, multicellular sprouts invade rapidly into avascular areas. Sprout elongation, in turn, depends on a continuous supply of endothelial cells. Endothelial cells migrate along the sprout, towards its tip, in a vascular endothelial (VE) cadherin-dependent process. The observed abundance of multicellular sprout formation in various in vitro and in vivo systems can be explained by a general mechanism based on preferential attraction to elongated structures. Our interacting particle model exhibits robust sprouting dynamics and results in patterns with morphometry similar to native primordial vascular plexuses--without ancillary assumptions involving chemotaxis or mechanochemical signaling.

Abnormal venous and arterial patterning in Chordin mutants

Classic dye injection methods yielded amazingly detailed images of normal and pathological development of the cardiovascular system. However, because these methods rely on the beating heart of diffuse the dyes, the vessels visualized have been limited to the arterial tree, and our knowledge of vein development is lagging. In order to solve this problem, we injected pigmented methylsalicylate resins in mouse embryos after they were fixed and made transparent. This new technique allowed us to image the venous system and prompted the discovery of multiple venous anomalies in Chord-/- mutant mice. Genetic inactivation of Chordin, an inhibitor of the Bone Morphogenetic Protein signaling pathway, results in neural crest defects affecting heart and neck organs, as seen in DiGeorge syndrome patients. Injection into the descending aorta of Chrd-/- mutants demonstrated how a very severe early phenotype of the aortic arches develops into persistent truncus arteriosus. In addition, injection into the atrium revealed several patterning defects of the anterior cardinal veins and their tributaries, including absence of segments, looping and midline defects. The signals that govern the development of the individual cephalic veins are unknown, but our results show that the Bone Morphogenetic Protein pathway is necessary for the process.

A role of D domain-related proteins in differentiation and migration of embryonic cells in Xenopus laevis

We have characterized a cDNA clone, rdd (repeated D domain-like), that encodes for a secretory protein consisting of repeated domains of cysteine-rich sequence. Whole-mount in situ hybridization analysis revealed that rdd2, rdd3 and rdd4 are transiently expressed in the ventral and lateral mesoderm and the overlying ectoderm at the late gastrula and tailbud stages. Morpholino oligonucleotide (MO) was used to inhibit the translation of endogenous rdd3 and rdd4, and we found that the circulation of red blood cells completely disappears in the MO-injected tadpoles. Histological analysis showed that formation of the ventral aorta, dorsal aorta and posterior cardinal vein in the trunk region was severely disorganized in these animals. Injection of MO affected the expression of alpha-globin, a terminal differentiation marker of red blood cells, but did not affect the expression of scl, flk-1 or tie-2, suggesting that angiopoietic and hematopoietic precursor cells differentiate normally in the rdd-depleted embryo. The transplantation of labeled tissues followed by tracing of the donor cells revealed a role of rdds in migration of the embryonic angioblasts and myeloid cells. These observations first demonstrate the role of the novel cysteine-rich proteins in migration of the embryonic cells.

Endothelial Precursor Cell Migration During Vasculogenesis

Migration of endothelial precursor cells (so-called "angioblasts" in embryos and "endothelial progenitor cells" in adults) during vasculogenesis is a requirement for the formation of a primary vascular plexus. The migration is initiated by the change of endothelial precursors to their migratory phenotype. The endothelial precursor cells are then guided to the position where the primary vascular plexus is formed. Migration is stopped by the reversion of the cells to their nonmigratory phenotype. A combination of regulatory mechanisms and factors controls this process. These include gradients of soluble factors, extracellular matrix-cell interaction and cell-cell interaction. In this review, we give an overview of the regulation of angioblast migration during embryonic vasculogenesis and its relationship to the migration of endothelial progenitors during postnatal vascular development.

Essential role of lysyl oxidases in notochord development

Recent studies reveal a critical role for copper in the development of the zebrafish notochord, suggesting that specific cuproenzymes are required for the structural integrity of the notochord sheath. We now demonstrate that beta-aminopropionitrile, a known inhibitor of the copper-dependent lysyl oxidases, causes notochord distortion in the zebrafish embryo identical to that seen in copper deficiency. Characterization of the zebrafish lysyl oxidase genes reveals eight unique sequences, several of which are expressed in the developing notochord. Specific gene knockdown demonstrates that loss of loxl1 results in notochord distortion, and that loxl1 and loxl5b have overlapping roles in notochord formation. Interestingly, while notochord abnormalities are not observed following partial knockdown of loxl1 or loxl5b alone, in each case this markedly sensitizes developing embryos to notochord distortion if copper availability is diminished. Likewise, partial knockdown of the lysyl oxidase substrate col2a1 results in notochord distortion when combined with reduced copper availability or partial knockdown of loxl1 or loxl5b. These data reveal a complex interplay of gene expression and nutrient availability critical to notochord development. They also provide insight into specific genetic and nutritional factors that may play a role in the pathogenesis of structural birth defects of the axial skeleton.

Census of vertebrate Wnt genes: isolation and developmental expression of Xenopus Wnt2, Wnt3, Wnt9a, Wnt9b, Wnt10a, and Wnt16

The Wnt family of growth factors regulate many different aspects of embryonic development. Assembly of the complete mouse and human genome sequences, plus expressed sequence tag surveys have established the existence of 19 Wnt genes in mammalian genomes. However, despite the importance of model vertebrates for studies in developmental biology, the complete complement of Wnt genes has not been established for nonmammalian genomes. Using genome sequences for chicken (Gallus gallus), frog (Xenopus tropicalis), and fish (Danio rerio and Tetraodon nigroviridis), we have analyzed gene synteny to identify the orthologues of all 19 human Wnt genes in these species. We find that, in addition to the 19 Wnts observed in humans, chicken contained an additional Wnt gene, Wnt11b, which is orthologous to frog and zebrafish Wnt11 (silberblick). Frog and fish genomes contained orthologues of the 19 mammalian Wnt genes, plus Wnt11b and several duplicated Wnt genes. Specifically, the Xenopus tropicalis genome contained 24 Wnt genes, including additional copies of Wnt7-related genes (Wnt7c) and 3 recent Wnt duplications (Wnt3, Wnt9b, and Wnt11). The Danio rerio genome contained 27 Wnt genes with additional copies of Wnt2, Wnt2b, Wnt4b, Wnt6, Wnt7a, and Wnt8a. The presence of the additional Wnt11 sequence (Wnt11b) in the genomes of all ancestral vertebrates suggests that this gene has been lost during mammalian evolution. Through these studies, we identified the frog orthologues of the previously uncharacterized Wnt2, Wnt3, Wnt9a, Wnt9b, Wnt10a, and Wnt16 genes and their expression has been characterized during early Xenopus development.

A transgene-assisted genetic screen identifies essential regulators of vascular development in vertebrate embryos

Formation of a functional vasculature during mammalian development is essential for embryonic survival. In addition, imbalance in blood vessel growth contributes to the pathogenesis of numerous disorders. Most of our understanding of vascular development and blood vessel growth comes from investigating the Vegf signaling pathway as well as the recent observation that molecules involved in axon guidance also regulate vascular patterning. In order to take an unbiased, yet focused, approach to identify novel genes regulating vascular development, we performed a three-step ENU mutagenesis screen in zebrafish. We first screened live embryos visually, evaluating blood flow in the main trunk vessels, which form by vasculogenesis, and the intersomitic vessels, which form by angiogenesis. Embryos that displayed reduced or absent circulation were fixed and stained for endogenous alkaline phosphatase activity to reveal blood vessel morphology. All putative mutants were then crossed into the Tg(flk1:EGFP)(s843) transgenic background to facilitate detailed examination of endothelial cells in live and fixed embryos. We screened 4015 genomes and identified 30 mutations affecting various aspects of vascular development. Specifically, we identified 3 genes (or loci) that regulate the specification and/or differentiation of endothelial cells, 8 genes that regulate vascular tube and lumen formation, 8 genes that regulate vascular patterning, and 11 genes that regulate vascular remodeling, integrity and maintenance. Only 4 of these genes had previously been associated with vascular development in zebrafish illustrating the value of this focused screen. The analysis of the newly defined loci should lead to a greater understanding of vascular development and possibly provide new drug targets to treat the numerous pathologies associated with dysregulated blood vessel growth.

The murine allantois: emerging paradigms in development of the mammalian umbilical cord and its relation to the fetus

The fertilized egg of the mammal gives rise to the embryo and its extraembryonic structures, all of which develop in intimate relation with each other. Yet, whilst the past several decades have witnessed a vast number of studies on the embryonic component of the conceptus, study of the extraembryonic tissues and their relation to the fetus have been largely ignored. The allantois, precursor tissue of the mature umbilical cord, is a universal feature of all placental mammals that establishes the vital vascular bridge between the fetus and its mother. The allantois differentiates into the umbilical blood vessels, which become secured onto the chorionic component of the placenta at one end and onto the fetus at the other. In this way, fetal blood is channeled through the umbilical cord for exchange with the mother. Despite the importance of this vascular bridge, little is known about how it is made. The aim of this review is to address current understanding of the biology of the allantois in the mouse and genetic control of its features and functions, and to highlight new paradigms concerning the developmental relationship between the fetus and its umbilical cord.

Ontogeny of the hematopoietic system

Blood cells are constantly produced in the bone marrow (BM) of adult mammals. This constant turnover ultimately depends on a rare population of progenitors that displays self-renewal and multilineage differentiation potential, the hematopoietic stem cells (HSCs). It is generally accepted that HSCs are generated during embryonic development and sequentially colonize the fetal liver, the spleen, and finally the BM. Here we discuss the experimental evidence that argues for the extrinsic origin of HSCs and the potential locations where HSC generation might occur. The identification of the cellular components playing a role in the generation process, in these precise locations, will be important in understanding the molecular mechanisms involved in HSC production from undifferentiated mesoderm.

The anuran Bauplan: a review of the adaptive, developmental, and genetic underpinnings of frog and tadpole morphology

Anurans (frogs, toads, and their larvae) are among the most morphologically derived of vertebrates. While tightly conserved across the order, the anuran Bauplan (body plan) diverges widely from that of other vertebrates, particularly with respect to the skeleton. Here we address the adaptive, ontogenetic, and genetic bases of three such hallmark anuran features: (1) the absence of discrete caudal vertebrae, (2) a truncated axial skeleton, and (3) elongate hind limbs. We review the functional significance of each as it relates to the anuran lifestyle, which includes locomotor adaptations to both aquatic and terrestrial environments. We then shift our focus to the proximal origins of each feature, namely, ontogeny and its molecular regulation. Drawing on relatively limited data, we detail the development of each character and then, by extrapolating from comparative vertebrate data, propose molecular bases for these processes. Cast in this light, the divergent morphology of anurans emerges as a product of evolutionary modulation of the generalised vertebrate developmental machinery. Specifically, we hypothesise that: (1) the formation of caudal vertebrae is precluded due to a failure of sclerotomes to form cartilaginous condensations, perhaps resulting from altered expression of a suite of genes, including Pax1, Pax9, Msx1, Uncx-4.1, Sonic hedgehog, and noggin; (2) anteriorised Hox gene expression in the paraxial mesoderm has led to a rostral shift of morphological boundaries of the vertebral column; and, (3) spatial and temporal shifts in Hox expression may underlie the expanded tarsal elements of the anuran hind limb. Technology is currently in place to investigate each of these scenarios in the African clawed frog Xenopus. Experimental corroboration will further our understanding of the molecular regulation of the anuran Bauplan and provide insight into the origin of vertebrate morphological diversity as well as the role of development in evolution.