Filopodia are actin-rich plasma-membrane

Filopodia are actin-rich plasma-membrane processes that allow cells to probe their environment. The functions of filopodia are broad in nature; they contribute to wound healing processes, adhesion to the extracellular matrix (ECM), guidance toward chemo attractants, neuronal growth-cone path finding and embryonic development. Their size can range from 10 μm in length (nerve growth-cone filo podia) to 40 μm in length (sea-urchin embryo filopodia). Filo podia, or micro spikes, may also be present in the cell cortex or leading edge. Despite the vital role of filo podia in cell functions, the biological mechanisms that govern filopodial functions are not completely understood. Current studies have only scratched the surface of the numerous roles of filopodia, whether beneficial or pathogenic. An emerging example of filopodia functions in infectious diseases is the recent discovery of the role of filopodia in viral surfing during entry and trafficking. Similarly, virus activated filo podia in dendritic cells have been proposed to transfer the virus with higher efficiency compared to the cell free route.

One long-standing function of filo podia is to probe the environment and promote cell motility. It has receptors for signalling molecules and ECM molecules, which allows it to sense the surroundings and interact with other cells. Along the tip and shaft of filopodia exist two classes of cell adhesion molecules known as integrin’s and cadherin’s. During cell spreading, integrin-containing filo podia form the initial adhesion sites. Then, other components, such as talin and paxillin, are recruited to form the mature focal adhesions. Filo podia also play a role in cell-to-cell adhesion. An example of this interaction can be seen during embryonic development and wound healing. During these processes, there is a fusion of sheets of epithelial cells. Dynamic filo podia are present at the edges of epithelial cells and project to adjacent cells to aid in these processes. Interdigitated filo podia, which protrude from opposing cells, also help the sheets of cells align and adhere together. Embryonic epithelial fusion must occur in a precise fashion such as for dorsal closure in Drosophila. Filo podia facilitate cell-to-cell matching by allowing a cell to search for its match, and then pull misaligned sheets into alignment.

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Alex John
Editorial Assistant
Immunotherapy Open Access