SMAD4 maintains the Fluid Shear Stress set point to protect against Arterial-Venous Malformations

Vascular networks form, remodel and mature under the influence of both fluid shear stress (FSS) and soluble factors. Physiological FSS promotes and maintains vascular stability via synergy with Bone Morphogenic Protein 9 (BMP9) and BMP10. Conversely, mutation of the BMP receptors ALK1, Endoglin or the downstream effector SMAD4 leads to Hereditary Hemorrhagic Telangiectasia (HHT), characterized by fragile and leaky arterial-venous malformations (AVMs). But how endothelial cells (EC) integrate FSS and BMP signals in vascular development and homeostasis, and how mutations give rise to vascular malformations is not well understood. Here, we aimed to elucidate the mechanism of synergy between fluid shear stress and SMAD signaling in vascular stability and its failure in HHT. We have now found that loss of Smad4 increases ECs' sensitivity to flow by lowering the FSS set point with resulting AVMs exhibiting features of excessive flow-mediated morphological responses. Mechanistically, loss of Smad4 disinhibits flow-mediated Klf4-Tie2-PI3K/Akt signaling leading to increased EC proliferation and loss of arterial identity due to Klf4-mediated repression of cyclin dependent Kinase (CDK) inhibitors, CDKN2A and CDKN2B. Thus, AVMs caused by Smad4 deletion are characterized by chronic high flow remodeling with excessive EC proliferation and loss of arterial identity as triggering events. Overall design: Gene expression profiling high throughput sequencing of HUVECS transfected with Control or SMAD4 siRNA in the presence or absence of 12dynes/cm2 fluid shear stress