The "Ets" factor: vessel formation in zebrafish--the missing link?

Akt suppression of TGFβ signaling contributes to the maintenance of vascular identity in embryonic stem cell-derived endothelial cells

The ability to generate and maintain stable in vitro cultures of mouse endothelial cells (ECs) has great potential for genetic dissection of the numerous pathologies involving vascular dysfunction as well as therapeutic applications. However, previous efforts at achieving sustained cultures of primary stable murine vascular cells have fallen short, and the cellular requirements for EC maintenance in vitro remain undefined. In this study, we have generated vascular ECs from mouse embryonic stem (ES) cells and show that active Akt is essential to their survival and propagation as homogeneous monolayers in vitro. These cells harbor the phenotypical, biochemical, and functional characteristics of ECs and expand throughout long-term cultures, while maintaining their angiogenic capacity. Moreover, Akt-transduced embryonic ECs form functional perfused vessels in vivo that anastomose with host blood vessels. We provide evidence for a novel function of Akt in stabilizing EC identity, whereby the activated form of the protein protects mouse ES cell-derived ECs from TGFβ-mediated transdifferentiation by downregulating SMAD3. These findings identify a role for Akt in regulating the developmental potential of ES cell-derived ECs and demonstrate that active Akt maintains endothelial identity in embryonic ECs by interfering with active TGFβ-mediated processes that would ordinarily usher these cells to alternate fates.

Etv2/ER71 induces vascular mesoderm from Flk1+PDGFRα+ primitive mesoderm

Etv2 (Ets Variant 2) has been shown to be an indispensable gene for the development of hematopoietic cells (HPCs)/endothelial cells (ECs). However, how Etv2 specifies the mesoderm-generating HPCs/ECs remains incompletely understood. In embryonic stem cell (ESC) differentiation culture and Etv2-null embryos, we show that Etv2 is dispensable for generating primitive Flk-1(+)/PDGFRα(+) mesoderm but is required for the progression of Flk-1(+)/PDGFRα(+) cells into vascular/hematopoietic mesoderm. Etv2-null ESCs and embryonic cells were arrested as Flk-1(+)/PDGFRα(+) and failed to generate Flk-1(+)/PDGFRα(-) mesoderm. Flk-1(+)/Etv2(+) early embryonic cells showed significantly higher hemato-endothelial potential than the Flk-1(+)/Etv2(-) population, suggesting that Etv2 specifies a hemato-endothelial subset of Flk-1(+) mesoderm. Critical hemato-endothelial genes were severely down-regulated in Etv2-null Flk-1(+) cells. Among those genes Scl, Fli1, and GATA2 were expressed simultaneously with Etv2 in early embryos and seemed to be critical targets. Etv2 reexpression in Etv2-null cells restored the development of CD41(+), CD45(+), and VE-cadherin(+) cells. Expression of Scl or Fli1 alone could also restore HPCs/ECs in the Etv2-null background, indicating that these 2 genes are critical downstream targets. Furthermore, VEGF induced Etv2 potently and rapidly in Flk-1(+) mesoderm. We propose that Flk-1(+)/PDGFRα(+) primitive mesoderm is committed into Flk-1(+)/PDGFRα(-) vascular mesoderm through Etv2 and that up-regulation of Etv2 by VEGF promotes this commitment.

A new procedure for determining the genetic basis of a physiological process in a non-model species, illustrated by cold induced angiogenesis in the carp

BACKGROUND

Physiological processes occur in many species for which there is yet no sequenced genome and for which we would like to identify the genetic basis. For example, some species increase their vascular network to minimise the effects of reduced oxygen diffusion and increased blood viscosity associated with low temperatures. Since many angiogenic and endothelial genes have been discovered in man, functional homolog relationships between carp, zebrafish and human were used to predict the genetic basis of cold-induced angiogenesis in Cyprinus Carpio (carp). In this work, carp sequences were collected and built into contigs. Human-carp functional homolog relationships were derived via zebrafish using a new Conditional Stepped Reciprocal Best Hit (CSRBH) protocol. Data sources including publications, Gene Ontology and cDNA libraries were then used to predict the identity of known or potential angiogenic genes. Finally, re-analyses of cold carp microarray data identified carp genes up-regulated in response to low temperatures in heart and muscle.

RESULTS

The CSRBH approach outperformed all other methods and attained 8,726 carp to human functional homolog relationships for 16,650 contiguous sequences. This represented 3,762 non-redundant genes and 908 of them were predicted to have a role in angiogenesis. The total number of up-regulated differentially expressed genes was 698 and 171 of them were putatively angiogenic. Of these, 5 genes representing the functional homologs NCL, RHOA, MMP9, GRN and MAPK1 are angiogenesis-related genes expressed in response to low temperature.

CONCLUSION

We show that CSRBH functional homologs relationships and re-analyses of gene expression data can be combined in a non-model species to predict genes of biological interest before a genome sequence is fully available. Programs to run these analyses locally are available from http://www.cbrg.ox.ac.uk/~jherbert/.

Sox18 and Sox7 play redundant roles in vascular development

Mutations in SOX18 cause the human hypotrichosis-lymphedema-telangiectasia (HLT) syndrome. Their murine counterparts are the spontaneous ragged mutants, showing combined defects in hair follicle, blood vessel, and lymphatic vessel development. Mice null for Sox18 display only mild coat defects, suggesting a dominant-negative effect of Sox18/ragged mutations and functional redundancy between Sox18 and other Sox-F proteins. We addressed this point in zebrafish. The zebrafish homologs of Sox18 and of Sox7 are expressed in angioblasts and in the endothelial component of nascent blood vessels in embryos. Knockdown of either gene, using moderate doses of specific morpholinos, had minimal effects on vessels. In contrast, simultaneous knockdown of both genes resulted in multiple fusions between the major axial vessels. With combined use of transgenic lines and molecular markers, we could show that endothelial cells are specified, but fail to acquire a correct arteriovenous identity. Venous endothelial cell differentiation was more severely affected than arterial. Thus, sox7 and sox18 play redundant but collectively essential roles in the establishment of proper arteriovenous identity in zebrafish. Our data suggest that a defect in arteriovenous identity could be responsible for the formation of telangiectases in patients with HLT.

Zebrafish Sox7 and Sox18 function together to control arterial-venous identity

Sox7 and Sox18 are members of the F-subgroup of Sox transcription factors family and are mostly expressed in endothelial compartments. In humans, dominant mutations in Sox18 are the underlying cause of the severe hypotrichosis-lymphedema-telangiectasia disorder characterized by vascular defects. However little is known about which vasculogenic processes Sox7 and Sox18 regulate in vivo. We cloned the orthologs of Sox7 and Sox18 in zebrafish, analysed their expression pattern and performed functional analyses. Both genes are expressed in the lateral plate mesoderm during somitogenesis. At later stages, Sox18 is expressed in all axial vessels whereas Sox7 expression is mainly restricted to the dorsal aorta. Knockdown of Sox7 or Sox18 alone failed to reveal any phenotype. In contrast, blocking the two genes simultaneously led to embryos displaying dysmorphogenesis of the proximal aorta and arteriovenous shunts, all of which can account for the lack of circulation observed in the trunk and tail. Gene expression analyses performed with general endothelial markers on double morphants revealed that Sox7 and Sox18 are dispensable for the initial specification and positioning of the major trunk vessels. However, morphants display ectopic expression of the venous Flt4 marker in the dorsal aorta and a concomitant reduction of the artery-specific markers EphrinB2a and Gridlock. The striking similarities between the phenotype of Sox7/Sox18 morphants and Gridlock mutants strongly suggest that Sox7 and Sox18 control arterial-venous identity by regulating Gridlock expression.