Mechanism of endothelial barrier function and disruption in COVID-19. (A) The alveolar-capillary interface regulates gas exchange, performs essential barrier functions, maintains blood flow and hemostasis, and controls leukocyte trafficking. Noxious stimuli including cell death, leukocyte activation, protease release, thrombosis, and hypoxia induce endothelial inflammation, which promotes fluid and cellular extravasation that when unchecked can lead to pulmonary edema and ventilation/perfusion mismatch. (B) Severe COVID-19 is associated with cleavage of endothelial cell receptors from the cell surface. Proteases such as matrix metalloproteinase (MMPs), disintegrin and metalloproteinases (ADAMs), and serine proteases promote loss of barrier function directly via cleavage of junctional molecules such as VE-cadherin or indirectly by cleaving cytoprotective receptors such as Tie2 or thrombomodulin (TM) from the cell surface. Transendothelial migration of neutrophils and monocytes is a major source of these proteases. Cleavage of the glycocalyx may induce vascular leak via generation of hyaluronic acid fragments which activate CD44 to promote endothelial permeability. (C) Vasoactive molecules upregulated in COVID-19 may compromise endothelial barrier function and dysregulate vascular tone. Thrombin cleavage of PAR1 results in phosphorylation of myosin light chain (MLC), leading to reorganization of the actin cytoskeleton into contractile stress fibers. Endothelial barrier destabilization is further promoted by activation of integrin β₁ by Angpt-2 secreted from Weibel-Palade bodies (WPB). Induction of bradykinin receptor 1 (B1R) during inflammation, combined with excessive kallikrein-kinin activation and the persistence of bradykinin and its breakdown products (eg, Des-Arg[9]-BK) due to ACE2 deficiency may result in excessive vasodilation and edema. (D) Cytokines such as IL-1β, IL-6, and TNFα, DAMPs, PAMPs, and VEGF present in plasma during severe COVID-19 result in reorganization of VE-cadherin away from junctional sites, promoting vascular leak. VEGF induces VE-cadherin phosphorylation, targeting the protein for internalization. Cytokine activation of NF-kB and JAK/STAT3 results in upregulation of leukocyte adhesion proteins such as E-selectin, VCAM-1, and ICAM-1, which mediate leukocyte recruitment and transendothelial migration. (E) Microvascular thrombosis further impairs tissue oxygenation, which activates hypoxia-inducible factor (HIF) to express VEGF, which directly promotes vascular permeability, and Angpt-2 and VE-PTP, which synergize to block Tie2 signaling. Loss of laminar flow and high shear stress from thrombosis or dysregulation of vascular tone inhibits cytoprotective KLF2/4 signaling, which further suppresses the vasodilatory capacity of the endothelium by inhibiting NO synthase and C-natriuretic peptide and decreasing Tie2 mRNA. AT1R, Angiotensin receptor 1; CNP, C-type natriuretic peptide; mRNA, messenger RNA; TEM, transendothelial migration; VEGF, vascular endothelial growth factor.