Filopodia (
sg.: filopodium) are slender
cytoplasmic projections that extend beyond the leading edge of
lamellipodia in
migrating cells.[1] Within the lamellipodium, actin ribs are known as microspikes, and when they extend beyond the lamellipodia, they're known as filopodia.[2] They contain
microfilaments (also called actin filaments) cross-linked into bundles by actin-bundling proteins,[3] such as
fascin and
fimbrin.[4] Filopodia form focal adhesions with the substratum, linking them to the cell surface.[5] Many types of migrating
cells display filopodia, which are thought to be involved in both sensation of chemotropic cues, and resulting changes in directed locomotion.
Activation of the
Rho family of GTPases, particularly
Cdc42 and their downstream intermediates, results in the polymerization of actin fibers by
Ena/Vasp homology proteins.[6] Growth factors bind to receptor tyrosine kinases resulting in the
polymerization of
actin filaments, which, when cross-linked, make up the supporting
cytoskeletal elements of filopodia. Rho activity also results in activation by phosphorylation of
ezrin-moesin-radixin family proteins that link
actin filaments to the filopodia membrane.[6]
Filopodia have roles in sensing, migration, neurite outgrowth, and cell-cell interaction.[1][further explanation needed] To close a wound in vertebrates, growth factors stimulate the formation of filopodia in
fibroblasts to direct
fibroblast migration and
wound closure.[7] In
macrophages, filopodia act as phagocytic tentacles, pulling bound objects towards the cell for
phagocytosis.[8]
In infections
Filopodia are also used for movement of bacteria between cells, so as to evade the host immune system. The intracellular bacteria Ehrlichia are transported between cells through the host cell filopodia induced by the pathogen during initial stages of infection.[9] Filopodia are the initial contact that human retinal pigment epithelial (RPE) cells make with elementary bodies of Chlamydia trachomatis, the bacteria that causes Chlamydia.[10]
Viruses have been shown to be transported along filopodia toward the cell body, leading to cell infection.[11] Directed transport of receptor-bound
epidermal growth factor (EGF) along filopodia has also been described, supporting the proposed sensing function of filopodia.[12]
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In developing
neurons, filopodia extend from the
growth cone at the leading edge. In neurons deprived of filopodia by partial inhibition of
actin filaments polymerization, growth cone extension continues as normal, but direction of growth is disrupted and highly irregular.[7] Filopodia-like projections have also been linked to
dendrite creation when new
synapses are formed in the brain.[14][15]
^Small JV, Stradal T, Vignal E, Rottner K (March 2002). "The lamellipodium: where motility begins". Trends in Cell Biology. 12 (3): 112–120.
doi:
10.1016/S0962-8924(01)02237-1.
PMID11859023.
^Lodish H, Berk A, Matsudaira P, Kaiser CA, Krieger M, Scott MP, Zipursky SL, Darnell J, eds. (2004). Molecular Cell Biology (fifth ed.). W.H. Freeman and Company. pp. 821, 823.