Project Summary/Abstract Proper blood vessel network formation and remodeling during development, disease, and wound healing depend on heterogeneous responses of endothelial cells (EC) to incoming signals, including physiological blood flow. Once mature, most of our vasculature is understood to be in G0, a quiescent and arrested cell cycle state. However, how quiescence is achieved and potentially regulated by flow is not well defined. Our preliminary data suggests that both p27 and DYRK1a, cell cycle inhibitor proteins, are required for the reduction in cell proliferation under flow. Cells that are treated with p27 or DYRK1a knockdown do not experience a decline in cell proliferation under flow, suggesting that these cells are not entering a quiescent state. Bulk RNA-seq data completed in the lab on cells under static or flow conditions also show an upregulation of p27 under flow, highlighting its importance. However, it is likely that other cell cycle inhibitor proteins play a critical role in response to fluid shear stress and we still do not understand if this looks the same across all endothelial cells or if responses are largely heterogeneous. These results have important implications for disease, specifically in regard to atherosclerosis and wound healing. We hypothesize that laminar shear stress induces EC homeostasis via changes in cell cycle inhibitor protein activity. First, we will determine endothelial cell cycle responses to laminar flow in vitro by manipulating p27 and members of the quiescence DREAM complex pathway (aim 1). To test this, we will utilize 2D and 3D microfluidic units with endothelial cell cycle inhibitor knockdown. One challenge about utilizing cell cycle tools, such as antibodies or flow cytometry, is that it only allows a fixed snapshot of cell cycle profile. Given that we want to understand how cell cycle phase is changing overtime, we will utilize PIP-FUCCI, a fluorescent cell cycle reporter, that will allow us to determine how cell cycle phases change under flow prior to quiescence. Next, we will determine in vivo if cell cycle inhibitor proteins are required for quiescence response using CRISPR knockout and manipulation of flow in PIP-FUCCI zebrafish models (aim 2). The ability to perform live imaging on transparent fish as well as manipulate flow by chemical inhibition of heart contraction make zebrafish an optimal model organism. Successful completion of these experiments will provide insight on how flow regulates vessel quiescence during physiological angiogenesis and will serve as groundwork towards an improved understanding of atherosclerosis and wound healing.