Role of Dll4 signaling in endothelial stability

Abstract

Part I. Dll4 suppresses transcytosis for arterial blood-retina barrier homeostasis The retina with the blood-retina barrier (BRB) maintains low vascular permeability by organizing tight junction (TJ) and limiting transcytosis in endothelial cells (ECs). Although retinas with multiple neuronal layers and complex vasculature are vulnerable to vascular edema, how retinal vessels regulate TJ and transcytosis independently in various anatomic, vascular, and pathological contexts remains elusive. Artery-preferential leakage in patients with hypertensive retinopathy (HR) suggests regulatory mechanisms specialized for arterial BRB integrity. Here, I report that endothelial Dll4-Notch1 signaling contributes to low permeability of arteries in the superficial retinal layer by inhibiting endothelial transcytosis. Both antibody-based and genetic inactivation of Dll4 or Notch1 induce hyperpermeability by increasing transcytosis without junctional destabilization in arterial ECs, leading to non-hemorrhagic leakage predominantly in superficial arteries of the retina. Endothelial Sox17 deletion represses Dll4 in retinal arteries, phenocopying Dll4 blocking-induced vascular leakage. Transcriptomic profiling of retinal ECs (RECs) suggests that Dll4-blocking activates SREBP1 and enriches gene sets favorable for caveolae formation. Indeed, SREBP1 inhibition attenuates Dll4 blocking-induced retinal leakage. In mice, angiotensin II (ATII)-induced hypertension (HT) represses arterial Sox17 and Dll4, followed by transcytosis-mediated non-hemorrhagic retinal edema, which is rescued by a gain of Notch activity. In patients with HR, retinal edema correlates with abnormalities of the superficial vascular layer similar to Dll4-inhibited mouse retina. The results identify Sox17-Dll4-SREBP1 axis as a key pathway for transcytosis regulation in adult retinal arteries and suggest that its dysregulation leads to transcytosis-mediated pathological leakage. Part II. Dll4 blockade promotes angiogenesis in non-healing wounds of Sox7-deficient mice This study aimed to elucidate the role of the proangiogenic transcription factors Sox7 and Sox17 in the wound healing process, and investigated the therapeutic potential of Dll4 blockade, which is an upstream regulator of Sox17, for the treatment of problem wounds. By producing a full-thickness skin defect wound model with endothelial Sox7- and/or Sox17-deficient mice, wound healing rates were measured and histologic analysis was performed. Then, the effects of an anti-Dll4 antibody on wound angiogenesis in Sox7-deficient mice were assessed. Sox7 and/or Sox17 deletion delayed wound healing. Further, the loss of Sox7 and Sox17 inhibited wound angiogenesis, without affecting the expression of each other. Interestingly, through anti-Dll4 antibody treatment, Sox17 levels were increased and the suppression of angiogenesis was alleviated in Sox7-deficient mice. As a result, Dll4 blockade effectively recovered the observed delay in wound healing. This study elucidated the proangiogenic role of transcription factors Sox7 and Sox17 in wound angiogenesis. Furthermore, I suggest a novel method for treating problem wounds, specifically by targeting Dll4–Sox17 axis.