Filopodium

In eukaryotic cells, a filopodium (plural: filopodia) is a protrusion and the protrusion of the cell membrane in eukaryotic cells. Filopodia protrude from the cell in the shape of a finger and are used in particular for cell migration.

Filopodia are formed by the actin cytoskeleton, which occurs in parallel bundles, in contrast to the related lamellipodia. Filopodia stretch in the direction of migration and form a cell adhesion point with the substrate at their end. The cell body can now be moved in the corresponding direction by contraction in the filopodium.

Filopodia are very mobile and are used, for example, for exploratory growth during axon guidance. The formation of filopodia is controlled by growth factors and the protein family of Rho-GTPases. Filopodia carry receptors on their membrane surface that can interact specifically with the guidance molecules. This interaction is, for example, decisive for the speed and direction of axonal growth. The stability of the filopodia is ensured by a skeleton of microfilaments (actin filament). Certain growth factors also mediate the migration of fibroblasts into wounds by means of filopodia.

A dendrite grows by sprouting filopodia, some of which mature into stable dendrite branches that carry synapses and sprout filopodia themselves. Recent work has shown that a filopodium begins to develop into a stable branch within 1 minute of contact with a presynaptic partner. There are 3 possible mechanisms for this: neurotransmitter receptor activity, signaling by adhesion proteins, and increased membrane tension when the filopodium attempts to retract but is held in place by adhesive contacts with the target. Membrane tension-induced signaling is particularly interesting because it serves as a general reporter of binding, regardless of which specific adhesion molecules are used (Heiman MG et al. (2010).

1. Brose N (1999) Synaptic cell adhesion proteins and synaptogenesis in the mammalian central nervous system. Nature Sciences 86:516-524.
2. Heiman MG et al. (2010) Twigs into branches: how a filopodium becomes a dendrite. Curr Opin Neurobiol 20:86-91.