Cells contain 3D scaffolds of interconnected filament systems referred to as the cytoskeleton. Three major filament systems encompassing actin-based microfilaments, microtubules and intermediate filaments contribute in different ways to the specific biomechanics of distinct cell types. Protecting the body from mechanical stress is a major task of surface-covering epithelial cells. The stress-protective function is in large part determined by keratin intermediate filaments forming networks with cell type-specific spatial arrangements. Intermediate filaments exhibit unique mechanical properties. They are semiflexible polymers with a persistence length of ~1 µm and are highly elastic with an extensibility up to 3.6-fold. They are also mechanosensitive, responding to increasing tension by strain-stiffening.
To understand how the unique 3D network arrangement of the keratin cytoskeleton affects cell mechanics, we are working to reconstruct the entire keratin filament system of single cells. To this end, fluorescent keratin reporters are used to label keratin filaments and high-resolution imaging is used to create digital representation of the network. The cover image for the July 7 issue of the Biophysical Journal shows the result for a single epithelial cell presenting an inside view of the keratin cytoskeleton. Fluorescence recordings that were obtained in 23 focal planes were subjected to image analysis for subsequent filament segmentation. The cover image does not provide information on filament brightness, (i.e., filament bundling) but classifies different filaments by color: red, apical filaments; yellow, lateral filaments; blue, perinuclear filaments; green, basal filaments.
The relationship between keratin filament organization and mechanical properties is key to explaining pathogenic mechanisms of keratin disorders, a group of hereditary epidermal diseases caused by single-point mutations of keratin-encoding genes and characterized by blisters. In addition, time-lapse imaging of living cells has uncovered an unexpected range of dynamic reorganization of the keratin system, which is essential for understanding wound healing and invasion of transformed malignant cells.
Our study in the current issue of the Biophysical Journal is based on recordings of keratin network dynamics in vital cells and proposes a model for bundling keratin intermediate filaments, taking interfilament electrostatic and hydrophobic interactions into account.
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- Ehud Haimov, Reinhard Windoffer, Rudolf Leube, Michael Urbakh, Michael Kozlov