Filopodia are thin, actin-rich cellular protrusions that play an important role in many fundamental cellular functions. They vary in their occurrence, length, and positioning in different cell types, suggesting their diverse roles.
Their main function is to guide migrating cells during normal tissue morphogenesis or cancer metastasis by recognizing and making initial contacts with the extracellular matrix. However, they can also act as stationary cell anchors or help to establish communication between two cells. For example, the formation of filopodial bridges between two adjacent endothelial or epithelial cells is important for the subsequent establishment of adherens junctions. In addition, filopodia can also help the cells to reach and internalize distant targets, such as pathogens.
Filopodia Formation and Disassembly
Actin nucleation, elongation, and bundling are critical for filopodia formation and function. The filopodia core comprises 12-20 actin filaments spaced 12 nm apart. Bundling of these filaments by fascin is crucial to give filopodia the rigidity to maintain its elongated shape.
The disassembly of actin filaments during the retraction phase involves periodic helical and rotational motion of the actin shaft and is regulated by several factors. For instance, capping proteins promote filopodial retraction by shielding the polymerizing ends of the filaments from further elongation. RhoA kinase activity also regulates actin polymerization within filopodia.
