The limits of filopodium stability

TL;DR

This paper investigates the stability of filopodia, revealing conditions under which they can be arbitrarily long and stable, and predicting a helical shape for filaments in such structures, challenging traditional buckling limits.

Contribution

It provides a theoretical and computational analysis showing that filopodia stability is not limited by Euler buckling and predicts a helical filament shape in long stable filopodia.

Findings

Filopodia can be stable at arbitrary lengths under certain conditions.

Filaments in stable filopodia may adopt a helical shape.

Experimental observations support the theoretical predictions.

Abstract

Filopodia are long, finger-like membrane tubes supported by cytoskeletal filaments. Their shape is determined by the stiffness of the actin filament bundles found inside them and by the interplay between the surface tension and bending rigidity of the membrane. Although one might expect the Euler buckling instability to limit the length of filopodia, we show through simple energetic considerations that this is in general not the case. By further analyzing the statics of filaments inside membrane tubes, and through computer simulations that capture membrane and filament fluctuations, we show under which conditions filopodia of arbitrary lengths are stable. We discuss several in vitro experiments where this kind of stability has already been observed. Furthermore, we predict that the filaments in long, stable filopodia adopt a helical shape.