Analysis of the shear stress-induced transcriptional program during vasculardevelopment

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The vasculature, consisting of a ramified network of endothelial tubes, is the first organ system to form during development. It is important for the distribution of oxygen and nutrients to all parts of the body. The initial formation of the vasculature is governed by hard-wired genetic factors and several signaling pathways influencing the migration and proliferation of endothelial cells have been identified. At later stages, physiological feedback mechanisms, such as changes in blood flow, and thus shear stress, strongly influence endothelial biology. For instance, shear stress contributes to proper blood vessel remodeling, thereby improving blood flow patterns. In addition, several disease conditions are associated with changes in shear stress. Atherosclerotic plaque formation preferentially occurs at blood vessel branch points, which exhibit disturbed flow patterns. Aberrant blood flow furthermore occurs in regions of arterio-venous malformations. In this setting, a direct connection between a large artery and a vein bypasses the normal capillary bed, leading to impaired tissue perfusion. Therefore, several key processes regulating vascular development and homeostasis are controlled by endothelial cell responses to shear stress. Despite the importance of this regulation, our understanding of the mechanisms by which endothelial cells perceive and react to shear stress is only very limited. We aim at answering two fundamental questions pertaining to endothelial cell responses to shear stress. We will investigate how the shear stress induced transcriptional program is regulated. In order to do so, we will identify shear stress controlled enhancer elements in endothelial cells using Chromatin Immunoprecipitation followed by sequencing (ChIP-Seq) and associate them with the genes they regulate. We will then identify the transcription factors binding to these enhancer elements and elucidate their functions during vascular remodeling. We will furthermore specifically delete flow responsive enhancer elements and interrogate the effect on gene expression and endothelial biology. Together, these studies will provide major insights into the regulation of endothelial cell responses to shear stress. In a second aim, we will investigate the phenotype of zebrafish endoglin mutants, which present with arterio-venous malformations. Endoglin is a member of the transforming growth factor beta (TGF-beta) pathway and associated with the human genetic disorder hereditary hemorrhagic telangiectasia (HHT), which is also characterized by arterio-venous malformations. We could show that blood flow strongly influenced arterio-venous malformations in endoglin mutant fish. We will investigate how endoglin determines proper endothelial cell responses to shear stress and thereby contribute to our understanding of HHT aetiology.