Objective To construct the platelet-specific Metrnl gene knockout （Plt-Metrnl^-/-）mice model. Methods Based on the Cre-LoxP system, Metrnl^loxp/loxp mice, previously constructed in our laboratory, were mated with Pf4-Cre mice to generate Plt-Metrnl^-/- mice. The genotypes of the offspring were identified, and tissues of the platelet, other peripheral blood cells, heart, liver, spleen, lung, kidney, brain, colon, and bone marrow were collected. The expression of the Metrnl gene in Plt-Metrnl^-/- mice was investigated by quantitative real-time PCR and western blot. Also, the blood routine index was tested in Plt-Metrnl^-/- mice. Results Compared with wild-type mice, the level of Metrnl protein in platelets was significantly decreased in Plt-Metrnl^-/- mice. There was no significant difference in mRNA and protein levels of other peripheral blood cells and tissues, as well as in blood routine index, growth, and development between Plt-Metrnl^-/- mice and WT mice. Conclusion Platelet-specific Metrnl knockout mice（Plt-Metrnl^-/- mice）model was successfully constructed.

METRNL is a recently identified secreted protein with emerging functions. This study is to find major cellular source of circulating METRNL and to determine METRNL novel function. Here, we show METRNL is abundant in human and mouse vascular endothelium and released by endothelial cells using endoplasmic reticulum-Golgi apparatus pathway. By creating endothelial cell-specific Metrnl knockout mice, combined with bone marrow transplantation to produce bone marrow-specific deletion of Metrnl, we demonstrate that most of circulating METRNL (approximately 75%) originates from the endothelial cells. Both endothelial and circulating METRNL decrease in atherosclerosis mice and patients. By generating endothelial cell-specific Metrnl knockout in apolipoprotein E-deficient mice, combined with bone marrow-specific deletion of Metrnl in apolipoprotein E-deficient mice, we further demonstrate that endothelial METRNL deficiency accelerates atherosclerosis. Mechanically, endothelial METRNL deficiency causes vascular endothelial dysfunction including vasodilation impairment via reducing eNOS phosphorylation at Ser1177 and inflammation activation via enhancing NFκB pathway, which promotes the susceptibility of atherosclerosis. Exogenous METRNL rescues METRNL deficiency induced endothelial dysfunction. These findings reveal that METRNL is a new endothelial substance not only determining the circulating METRNL level but also regulating endothelial function for vascular health and disease. METRNL is a therapeutic target against endothelial dysfunction and atherosclerosis.