Ana Russo

Significance: In this report, we describe how endothelial cells, the cells lining the interior of blood vessels, invade into tissues to form new vessels through sprouting angiogenesis. We found that endothelial cells use a specific lamellipodia-related membrane protrusion for invasion, which we termed dactylopodia. These protrusions have a special morphology, originate from filopodia, are linked to membrane-ruffling activity, and are specialized in invading into avascular extracellular matrix. Our work lays the foundations for drug discovery targeting sprouting angiogenesis.

Sprouting angiogenesis is fundamental for development and contributes to multiple diseases, including cancer, diabetic retinopathy and cardiovascular diseases. Sprouting angiogenesis depends on the invasive properties of endothelial tip cells. However, the current concept assumes that sprouting angiogenesis is based on a universal endothelial tip cell invasive profile. Here, we propose the existence of two modes of sprouting angiogenesis in vivo, filopodia-sensitive and filopodia-insensitive. We disclose that endothelial tip cells use long lamellipodia projections (LLPs) as the main mechanism for invasion, whilst the presence of filopodia is necessary in tissue specific contexts. We further show that LLPs and filopodia protrusions are balanced by myosin-IIA (MIIA) and actin-related protein 2/3 (Arp2/3) activity. Endothelial cell autonomous ablation of MIIA promotes excessive LLPs formation in detriment of filopodia. Conversely, endothelial cell-autonomous ablation of Arp2/3 prevents LLPs development and leads to excessive filopodia formation. We further show that MIIA inhibits integrin-dependent activation of Arp2/3 by regulating Rac1 activity. Our discoveries demonstrate how endothelial tip cells regulate its protrusive activity and will pave the way towards new strategies to block invasive tip cells during sprouting angiogenesis.