Filopodia and their links with membrane traffic and cell adhesion

Filopodia are dynamic, narrow, plasma membrane protrusions filled with bundled actin filaments. Fresh actin monomers are incorporated at the tips furthest away from the cell body, where barbed end polymerases such as formins and Ena/VASP proteins are present in a complex with other regulatory proteins, including Myosin-X. The tightly packed, parallel alignment of actin filaments in the shaft is mediated by bundling, notably by fascin but also fimbrin and some formins. Filopodia often, but need not necessarily, emerge from areas of Arp2/3 complex nucleated F-actin. The membrane surrounding filopodia is tightly curved, where inverse-Bin-Amphiphysin-Rvs proteins deform the membrane, together with Ezrin-Radixin-Moesin proteins. Recent studies have revealed that phosphoinositide lipid metabolism and endophilin-dependent endocytosis are associated with filopodia and that filopodia are a site for virus and exosome internalization. The composition of filopodia can share similarity with the integrin adhesome, and adhesion plays mechanical and tissue matching roles in filopodia function. This review considers how filopodia form and discusses putative mechanistic integration between filopodia, endocytosis and adhesion.