Intermediate Filaments in Small Configuration Spaces

Intermediate filaments play a key role in cell mechanics. Apart from their great importance from a biomedical point of view, they also act as a very suitable micrometer-sized model system for semiflexible polymers. We perform a statistical analysis of the thermal fluctuations of individual filaments confined in microchannels. The small channel width and the resulting deflections at the walls give rise to a reduction of the configuration space by about 2 orders of magnitude. This circumstance enables us to precisely measure the intrinsic persistence length of vimentin intermediate filaments and to show that they behave as ideal wormlike chains; we observe that small fluctuations in perpendicular planes decouple. Furthermore, the inclusion of results for confined actin filaments demonstrates that the Odijk confinement regime is valid over at least 1 order of magnitude in persistence length.

Figure 1

(a) Sketch of the experimental situation. Fluorescently labeled filaments are confined in microchannels of varying widths $d$ and heights $h$. (b) Fluorescence images of vimentin IFs confined in microchannels with different widths. Channel walls are indicated by white lines. The filaments’ characteristic fluctuations change significantly with the width of the confining channel. (The scale bar indicates $10\mu \mathrm{m}$.)

Figure 2

Tangent correlation functions for filaments in channels of different widths. Each curve shows data averaged from several measured filaments. The data are fitted with the analytical solution for a confined polymer in a parabolic potential. Error bars are indicated in a lighter color.

Figure 3

Pairs of tangent correlation functions of filaments confined in channels of two different heights but equal width show identical behavior. Error bars are indicated in a lighter color.

Figure 4

Vimentin and actin filament properties are compared by using the scaling constant $a$ from Odijk’s scaling law. We find very similar values for the scaling constant for different confinement parameters and also for both vimentin and actin [5] filaments.