The prevailing model of the mechanical function of intermediate
filaments in cells assumes that these 10 nm diameter filaments make up
networks that behave as entropic gels, with individual intermediate
filaments never experiencing direct loading in tension. However, recent
work has shown that single intermediate filaments and bundles are
remarkably extensible and elastic in vitro, and therefore well-suited
to bearing tensional loads. Here we tested the hypothesis that the
intermediate filament network in keratinocytes is extensible and
elastic as predicted by the available in vitro data. To do this, we
monitored the morphology of fluorescently-tagged intermediate filament
networks in cultured human keratinocytes as they were subjected to
uniaxial cell strains as high as 133%. We found that keratinocytes not
only survived these high strains, but their intermediate filament
networks sustained only minor damage at cell strains as high as 100%.
Electron microscopy of stretched cells suggests that intermediate
filaments are straightened at high cell strains, and therefore likely
to be loaded in tension. Furthermore, the buckling behavior of
intermediate filament bundles in cells after stretching is consistent
with the emerging view that intermediate filaments are far less stiff
than the two other major cytoskeletal components F-actin and
microtubules. These insights into the mechanical behavior of
keratinocytes and the cytokeratin network provide important baseline
information for current attempts to understand the biophysical basis of
genetic diseases caused by mutations in intermediate filament genes.
