CBFβ initiates the hematopoietic stem cell program without obligatory binding to RUNX

SUMMARY Hematopoietic stem cells (HSCs) emerge from hemogenic endothelium (HE) localised in the embryonic dorsal aorta (DA). Here we show that Runx1, a transcription factor essential for HSC emergence, controls HE establishment in the absence of its non-DNA-binding partner, CBFβ, and that a CBFβ-binding-deficient Runx1 mutant form can activate the HE program in the DA. Nevertheless, CBFβ is also essential for HSC emergence by regulating the specification of definitive hemangioblasts (DHs), the precursors of the DA and HE, in the lateral plate mesoderm where it mediates VEGFA induction by BMP signalling. Surprisingly, no Runx gene is expressed in DHs and the pharmacological inhibition of CBFβ binding to Runx is not detrimental for DH, confirming that CBFβ functions independently of Runx. Thus, we have uncovered, for the first time, that CBFβ regulates gene expression without Runx, breaking the dogma in which CBFβ ‘s gene regulatory functions are strictly dependent on its binding to Runx. HIGHLIGHTS <jats:list list-type="bullet"><jats:list-item> Runx1 and CBFβ play independent roles in the establishment of the HSC lineage <jats:list-item> Runx1 binding to CBFβ is not required for HE establishment <jats:list-item> CBFβ is downstream of BMP and regulates endogenous VEGFA expression in DH <jats:list-item> Binding to Runx is not obligatory for CBFβ functio

Etv6 activates vegfa expression through positive and negative transcriptional regulatory networks in Xenopus embryos

How vegfa expression is transcriptionally regulated in vivo is unclear. Here, the authors demonstrate that the ETS transcription factor Etv6 acts as a repressor and an activator of two direct regulators of vegfa expression (foxo3 and klf4, respectively) to control blood formation in Xenopus

New methods for computational decomposition of whole-mount in situ images enable effective curation of a large, highly redundant collection of Xenopus images.

The precise anatomical location of gene expression is an essential component of the study of gene function. For most model organisms this task is usually undertaken via visual inspection of gene expression images by interested researchers. Computational analysis of gene expression has been developed in several model organisms, notably in Drosophila which exhibits a uniform shape and outline in the early stages of development. Here we address the challenge of computational analysis of gene expression in Xenopus, where the range of developmental stages of interest encompasses a wide range of embryo size and shape. Embryos may have different orientation across images, and, in addition, embryos have a pigmented epidermis that can mask or confuse underlying gene expression. Here we report the development of a set of computational tools capable of processing large image sets with variable characteristics. These tools efficiently separate the Xenopus embryo from the background, separately identify both histochemically stained and naturally pigmented regions within the embryo, and can sort images from the same gene and developmental stage according to similarity of gene expression patterns without information about relative orientation. We tested these methods on a large, but highly redundant, collection of 33,289 in situ hybridization images, allowing us to select representative images of expression patterns at different embryo orientations. This has allowed us to put a much smaller subset of these images into the public domain in an effective manner. The 'isimage' module and the scripts developed are implemented in Python and freely available on https://pypi.python.org/pypi/isimage/

The Republican Journal: Vol. 93, No. 8 - February 24,1921

https://digitalmaine.com/rj_1921/1007/thumbnail.jp

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

Establishing the transcriptional programme for blood: the SCL stem cell enhancer is regulated by a multiprotein complex containing Ets and GATA factors

Stem cells are a central feature of metazoan biology. Haematopoietic stem cells (HSCs) represent the best-characterized example of this phenomenon, but the molecular mechanisms responsible for their formation remain obscure. The stem cell leukaemia (SCL) gene encodes a basic helix–loop–helix (bHLH) transcription factor with an essential role in specifying HSCs. Here we have addressed the transcriptional hierarchy responsible for HSC formation by characterizing an SCL 3′ enhancer that targets expression to HSCs and endothelium and their bipotential precursors, the haemangioblast. We have identified three critical motifs, which are essential for enhancer function and bind GATA-2, Fli-1 and Elf-1 in vivo. Our results suggest that these transcription factors are key components of an enhanceosome responsible for activating SCL transcription and establishing the transcriptional programme required for HSC formation