During blood vessel development, endothelial cells become specified toward arterial or venous fates to generate a circulatory network that provides nutrients and oxygen to, and removes metabolic waste from, all tissues. Arterial-venous specification occurs in conjunction with suppression of endothelial cell cycle progression; however, the mechanistic role of cell cycle state is unknown. Herein, using Cdh5-CreERT2;R26FUCCI2aR reporter mice, we find that venous endothelial cells are enriched for the FUCCI-Negative state (early G1) and BMP signaling, while arterial endothelial cells are enriched for the FUCCI-Red state (late G1) and TGF-β signaling. Furthermore, early G1 state is essential for BMP4-induced venous gene expression, whereas late G1 state is essential for TGF-β1-induced arterial gene expression. Pharmacologically induced cell cycle arrest prevents arterial-venous specification defects in mice with endothelial hyperproliferation. Collectively, our results show that distinct endothelial cell cycle states provide distinct windows of opportunity for the molecular induction of arterial vs. venous fate.

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Repair program(s) in "alternatively activated" macrophages can help with short-term wound responses after sterile damage events. Clot resolution by fibrinolysis and phagocytosis by tissue macrophages can activate anti-inflammatory and pro-fibrotic signals like Transforming Growth Factor- β. It has been hypothesized by the late Andrew Tager and Rachel Chambers that some forms of human pulmonary fibrosis may involve chronic alveolar damage, leakage of blood components, clots, and subsequent pro-fibrotic events that then fail to remodel back to normal alveolar structure. We found evidence of chronic bleeds and hemosiderin-positive macrophages in the lungs of Platelet Endothelial Cell Adhesion Molecule-1 deficient mice that developed a progressive and fatal pulmonary fibrosis. We are testing this hypothesis in vitro using murine broncho-alveolar macrophages exposed to aged clots. Interestingly, fresh blood and clots were not efficiently phagocytosed by the murine myeloid leukemia cell line RAW264 nor by broncho-alveolar macrophages. Clots needed to be aged for two days first. Ingestion also causes the macrophages to become fluorescent, which is a useful way to assess phagocytosis. We are now developing mRNA sequencing protocols to assess the gene programs of macrophages over time.