Vascular homeostasis is impaired in various diseases thereby contributing to the progression of their underlying pathologies. The endothelial immediate early gene Apolipoprotein L domain-containing 1 (APOLD1) helps regulate endothelial function. However, its precise role in endothelial cell (EC) biology remains unclear. We have immuno-localized APOLD1 to EC cell contacts and to Weibel- Palade bodies (WPB) where it associates with von Willebrand factor (VWF) tubules. Silencing of APOLD1 in primary human ECs disrupted the cell junction-cytoskeletal interface, thereby altering endothelial permeability accompanied by spontaneous release of WPB contents. This resulted in an increased presence of WPB cargoes, notably VWF and ANGPT2 in the extracellular medium. Autophagy flux, previously recognized as an essential mechanism for the regulated release of WPBs, was impaired in the absence of APOLD1. In addition, we report APOLD1 as a candidate gene for a novel inherited bleeding disorder (IBD) across three generations of a large family associating an atypical bleeding diathesis with episodic impaired microcirculation. A dominant heterozygous nonsense APOLD1:p.R49* variant segregated to affected family members. Compromised vascular integrity resulting from an excess of plasma ANGPT2, and locally impaired availability of VWF may explain the unusual clinical profile of APOLD1:p.R49* patients. In summary, our findings identify APOLD1 as an important regulator of vascular homeostasis and raise the need to consider testing of EC function in patients with IBDs without apparent platelet or coagulation defects.
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