Islet vascularization is regulated by primary endothelial cilia via VEGF-A dependent signaling

Abstract

RationaleAccumulating evidence point to a role for primary cilia in endothelial cell function. Islet vascularization is an important determinant of islet function and glucose homeostasis. We have previously shown that β-cell cilia directly regulate insulin secretion. However, it is unclear whether primary cilia are also implicated in islet vascularization and thus contribute to glucose homeostasis.ObjectiveTo characterize the role of primary cilia in islet vascularization.Methods and ResultsAt four weeks, Bbs4−/− islets show markedly lower intra-islet capillary density with enlarged diameters. We transplanted islets into the anterior chamber (ACE) of mouse eyes for longitudinal and non-invasive in vivo monitoring of vascular morphology. Bbs4−/− islets exhibited significantly delayed re-vascularization and enlarged vessels during engraftment. Similar vascular phenotypes were observed in two other ciliopathy models. By shifting the relative contributions of host versus donor endothelial cells in islet revascularization, we found that primary cilia on endothelial cells is essential for this process. Electron microscopy analysis further revealed a lack of fenestration in engrafted Bbs4−/− islets, partially impairing vascular permeability and glucose delivery to β-cells. Finally, we identified that Vascular endothelial cell growth factor A (VEGF-A)/VEGF receptor 2 (VEGFR2) signalling is involved in islet vascularization, islet function and vascular fenestration. In vitro silencing of two different ciliary genes in endothelial cells disrupts VEGF-A/ VEGFR2 internalization and phospho-activation of downstream signalling components. Consequently, key features of angiogenesis including proliferation, migration and tube formation are attenuated in BBS4 silenced endothelial cells.ConclusionsEndothelial cell primary cilia regulate islet vascularization and vascular barrier function via VEGF-A/ VEGFR2 signaling pathway. Islet vascularization is impaired in four weeks old Bbs4−/− mice. Long-time monitoring of re-vascularization of WT and Bbs4−/− islets recapitulates the phenotype and demonstrates a role for cilia in islet vascularization and vascular barrier function. VEGF-A/ VEGFR2-dependent signalling is regulated by endothelial primary cilia.