Endothelial β‐arrestins regulate mechanotransduction by the type II bone morphogenetic protein receptor in primary cilia

Rationale Modulation of endothelial cell behavior and phenotype by hemodynamic forces involves many signaling components, including cell surface receptors, intracellular signaling intermediaries, transcription factors, and epigenetic elements. Many of the signaling mechanisms that underlie mechanotransduction by endothelial cells are inadequately defined. Objective We sought to better understand how β-arrestins, intracellular proteins that regulate agonist-mediated desensitization and integration of signaling by transmembrane receptors, may be involved in the endothelial cell response to shear stress. Methods and Results In vitro studies with primary endothelial cells subjected to β-arrestin knockdown, and in vivo studies using mice with endothelial specific deletion of β-arrestin 1 and β-arrestin 2 were conducted. We found that β-arrestins are localized to primary cilia in endothelial cells, which are present in subpopulations of endothelial cells in relatively low shear states. Recruitment of β-arrestins to cilia involved its interaction with IFT81, a component of the flagellar transport protein complex in the cilia. β-arrestin knockdown led to marked reduction in shear stress response, including induction of NOS3 expression. Within the cilia, β-arrestins were found to associate with the type II bone morphogenetic protein receptor (BMPR-II), whose disruption similarly led to an impaired endothelial shear response. β-arrestins also regulated Smad transcription factor phosphorylation by BMPR-II. Mice with endothelial specific deletion of β-arrestin 1 and β-arrestin 2 were found to have impaired retinal angiogenesis. Conclusion We have identified a novel role for endothelial β-arrestins as key transducers of ciliary mechanotransduction that play a central role in shear signaling by BMPR-II and contribute to vascular development. NOVELTY AND SIGNIFICANCE What Is Known? Endothelial cells respond to flow-induced shear stress with biochemical changes, such as phosphorylation of endothelial nitric oxide synthase, that promote morphological changes, such as cell alignment. The endothelial response to shear stress can involve primary cilia, microtubule-based sensory organelles that detect extracellular stimuli and generates intracellular signals. The specific ciliary signaling pathways that regulate endothelial mechanotransduction have not been fully elucidated. What New Information Does This Article Contribute? β-arrestins directly interact with the ciliary protein intraflagellar transport protein 81 (IFT81), which is present in the primary cilia of endothelial cells, and are required for the morphological response to flow-induced shear stress. β-arrestins regulates type II bone morphogenetic protein receptor signaling, which is required for the endothelial response to shear stress, and is required for the phosphorylation of Smad transcription factors. β-arrestins are required for endothelial nitric oxide synthase-mediated flow-induced shear stress response in endothelial cells. Endothelial cell-specific knockout of β-arrestins results in abnormal vascular development, with a loss of vessel length and branchpoints.