Characterizing role of Nf2/Merlin in tip cell induction during sprouting angiogenesis

Abstract

In endothelial cells (ECs), differential vascular endothelial growth factor receptor 2 (VEGFR2) signaling in a nascent sprout yields two distinct cellular phenotypes, tip ECs and stalk cells, in a stochastic pattern. However, the precise mechanisms underlying how intracellular VEGFR2 signaling is higher in one EC compared with its neighbour and acquires the tip EC phenotype under a similar external cue are elusive. Here, we show that Merlin, encoded by the neurofibromatosis type-2 (NF2) gene, suppresses VEGFR2 internalization depending on vascular endothelial cadherin (VE-cadherin) density and inhibits tip EC induction. Accordingly, endothelial Nf2 depletion significantly promotes tip EC induction with excessive filopodia by enhancing VEGFR2 internalization not only in the growing vessels but also in the fully matured vessels. Mechanistically, Merlin physically binds to the VEGFR2–VE-cadherin complex at cell–cell junctions and reduces the activity of VEGFR2 internalization–induced downstream signaling pathways during tip EC induction. As a consequence, non-functional excessive sprouting largely occurs during tumor angiogenesis in EC-specific Nf2-deleted mice, leading to delayed tumor growth. Together, our findings show that Nf2/Merlin is a crucial molecular gatekeeper for tip EC induction, capillary integrity, and proper tumor angiogenesis by suppressing VEGFR2 internalization.