Endothelial Exostosin‐1 Is Needed for Glycocalyx Integrity and Inflammatory Resolution in Acute Lung Injury

ABSTRACT The endothelial glycocalyx depends on Exostosin‐1 (Ext1)‐mediated heparan sulfate synthesis; however, its endothelial cell‐autonomous role in acute inflammation remains unclear. Therefore, we examined the role of Ext1 during acute inflammation. In this study, endothelial‐specific Ext1‐deficient mice (Ext1 flox/flox and VE‐cadherin‐Cre) were generated, and their response to endotoxin challenge was investigated using transcriptomics and imaging. Transcriptomic profiling revealed downregulation of heparan sulfate‐ and glycocalyx‐related genes, with upregulation of genes associated with immune activation. Fluorescence imaging demonstrated lectin‐ and heparan sulfate‐defined glycocalyx depletion; dextran extravasation confirmed impaired barrier integrity. Ultrastructural analyses demonstrated a near‐complete absence of the luminal glycocalyx at baseline in Ext1‐deficient lungs. Following lipopolysaccharide administration, Ext1‐deficient mice exhibited altered cytokine kinetics, delayed clearance of neutrophils and CD11b + myeloid cells, increased late‐phase macrophage accumulation, and prolonged pulmonary edema, indicating impaired resolution of inflammation. Gene set enrichment analysis revealed upregulated leukocyte transendothelial migration, chemokine signaling, and vascular endothelial growth factor pathways, consistent with a persistently activated endothelial phenotype. Endothelial Ext1 is indispensable for heparan sulfate biosynthesis, endothelial glycocalyx integrity, and timely inflammation resolution. Loss of Ext1 leads to an intrinsically pro‐inflammatory endothelial state characterized by impaired heparan sulfate–dependent signaling and dysregulated leukocyte–endothelial interactions, heightened vascular permeability, and prolonged tissue injury. Our findings support glycocalyx‐targeted therapy in sepsis, acute respiratory distress syndrome, diabetes‐associated inflammation, and other conditions of endothelial vulnerability.