Coronary microvascular dysfunction (CMD) is associated with coronary artery diseases (CAD), diabetic cardiomyopathy (DCM), ischemia with the non-obstructive coronary artery (INOCA), and HFpEF (heart failure with preserved ejection fraction). Patients with diabetes exhibit coronary endothelial dysfunction, which is characterized by impaired acetylcholine-induced endothelial- dependent relaxation. Impaired endothelium-dependent vasodilation (EDD) decreases coronary blood flow and myocardium perfusion, leading to myocardial ischemia even without an obstructive coronary artery. However, the underlying mechanism of impaired coronary endothelial dilation in DCM is not fully understood. Our preliminary study finds that NO is the mediator of endothelium- dependent dilation (EDD) in small coronary arteries in healthy mice. However, in diabetic mice, we observe that hydrogen peroxide (H2O2) is the principal endothelial-dependent vasodilator. Such a unique preclinical diabetic model recapitulates a clinical observation of NO to H2O2 in CAD patients. Moreover, we find a deficiency of miR-21 that restores the NO-dependent coronary vasodilation. This application will address the underlying mechanism of how miR-21 regulates the NO to H2O2 switches in diabetes and the pathological consequence of the switch in DCM. We hypothesize that restoring “normal” coronary microvascular function (restoring endothelial dilation) by modulating miR-21can ameliorate diabetic cardiomyopathy (which is thought to be a disease related to impaired coronary microvascular function). We will test our hypothesis by an interdisciplinary approach encompassing a range of methods and disciplines from molecular and cell analyses, vascular biology to physiology and pathophysiology, engendering the study of a novel mechanism of coronary microvascular dysfunction, such as tissue-specific knockouts and lineage tracing with 3D fluorescent imaging, measurement of vasodilation and myocardial blood flow in vivo by contrast echocardiography and cardiac function by echocardiography along with RNA-seq, sc RNA-seq, etc. Completing this project may lead to a new strategy to treat microvascular dysfunction and diabetic cardiomyopathy and improve the cardiovascular prognosis of diabetes.