Regulation of hard α-keratin mechanics via control of intermediate filament hydration: matrix squeeze revisited

Abstract

Mammalian hard a-keratins are fibre-reinforced biomaterials that consist of 10 nm intermediate filaments (IFs) embedded in an elastomeric protein matrix. Recent work suggests that the mechanical properties of IFs are highly sensitive to hydration, whereas hard a-keratins such as wool, hair and nail are relatively hydration insensitive. This raises the question of how mammalian keratins remain stiff in water. The matrix squeeze hypothesis states that the IFs in hard a-keratins are stiffened during an air-drying step during keratinization, and subsequently locked into a dehydrated state via the oxidation and cross-linking of the keratin matrix around them. The result is that even when hard a-keratins are immersed in water, their constituent IFs remain essentially 'dry' and therefore stiff. This hypothesis makes several predictions about the effects of matrix abundance and function on hard a-keratin mechanics and swelling behaviour. Specifically, it predicts that high matrix keratins in water will swell less, and have a higher tensile modulus, a higher yield stress and a lower dry-to-wet modulus ratio. It also predicts that disruption of the keratin matrix in water should lead to additional swelling, and a drop in modulus and yield stress. Our results are consistent with these predictions and suggest that the keratin matrix plays a critical role in governing the mechanical properties of mammalian keratins via control of IF hydration.

How do baleen whales stow their filter? A comparative biomechanical analysis of baleen bending

“…Our findings support the conclusions of previously published research, especially the pronounced difference between dried and hydrated baleen tissue (Werth et al, 2016a) and flow-induced entanglement of fringes to create a mat-like mesh (Werth, 2013). However, the bending of baleen plates has not previously been the subject of focused biomechanical investigation or experiment, despite published speculation (Gray, 1877). Our integrative study of baleen bending revealed a complex story, not only because multiple factors independently and collectively govern plate flexibility and mobility (including dorsoventral, mediolateral and anteroposterior positions) but also because additional ( potentially confounding or complicating) factors directly or indirectly contribute to stiffness/ flexibility.…”

“…1). This dilemma has long been recognized (Gray, 1877) but never investigated. Although balaenid plates presumably bend during gape closure, this has not been observed in live whales, even with close scrutiny of video-recorded feeding sequences.…”

Sediment Effects on the Preservation of Burgess Shale-Type Compression Fossils

“…Visschers and de Jongh 2005). If so, then localized availability of free oxygen might be a critical factor in fossil preservation, just as it is for the (primary) crosslinking of hard keratins (Greenberg and Fudge 2013).…”

High‐pressure freezing followed by freeze substitution of a complex and variable density miniorgan: the wool follicle

“…Wool fibres contain heterogeneous regions of hydrophobicity. Although the exterior wool fibre is hydrophobic, the KIFs contain a hydrophobic core surrounded by hydrophobic matrix, and at a tissue level, the fibre cortex is hydrophilic (Greenberg & Fudge, 2013). Therefore, the shrinkage observed in prefixed HPF-FS follicles could partially be due to this difference in hydrophobicity.…”