Keratin network modifications lead to the mechanical stiffening of the hair follicle fiber. Proceedings of the National Academy of Sciences (2016), 113(21), 5940-5945. (original) (raw)
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Keratin network modifications lead to the mechanical stiffening of the hair follicle fiber
Proceedings of the National Academy of Sciences, 2016
The complex mechanical properties of biomaterials such as hair, horn, skin, or bone are determined by the architecture of the underlying fibrous bionetworks. Although much is known about the influence of the cytoskeleton on the mechanics of isolated cells, this has been less studied in tridimensional tissues. We used the hair follicle as a model to link changes in the keratin network composition and architecture to the mechanical properties of the nascent hair. We show using atomic force microscopy that the soft keratinocyte matrix at the base of the follicle stiffens by a factor of ∼360, from 30 kPa to 11 MPa along the first millimeter of the follicle. The early mechanical stiffening is concomitant to an increase in diameter of the keratin macrofibrils, their continuous compaction, and increasingly parallel orientation. The related stiffening of the material follows a power law, typical of the mechanics of nonthermal bending-dominated fiber networks. In addition, we used X-ray diff...
Biophysical Journal, 2002
The mechanical behavior of human hair fibers is determined by the interactions between keratin proteins structured into microfibrils (hard ␣-keratin intermediate filaments), a protein sulfur-rich matrix (intermediate filaments associated proteins), and water molecules. The structure of the microfibril-matrix assembly has already been fully characterized using electron microscopy and small-angle x-ray scattering on unstressed fibers. However, these results give only a static image of this assembly. To observe and characterize the deformation of the microfibrils and of the matrix, we have carried out time-resolved small-angle x-ray microdiffraction experiments on human hair fibers stretched at 45% relative humidity and in water. Three structural parameters were monitored and quantified: the 6.7-nm meridian arc, which is related to an axial separation between groups of molecules along the microfibrils, the microfibril's radius, and the packing distance between microfibrils. Using a surface lattice model of the microfibril, we have described its deformation as a combination of a sliding process and a molecular stretching process. The radial contraction of the matrix is also emphasized, reinforcing the hydrophilic gel nature hypothesis.
Insights on the mechanical behavior of keratin fibrils
International Journal of Biological Macromolecules, 2016
A computational molecular model of a truncated keratin protofibril (8 chains of hair keratin, PDB provided in Supplementary material) was used, to run a series of steered molecular dynamics simulations obtaining strain-stress curves. These results were compared with experimental mechanical data on hair fibers. Our data demonstrate that the molecular dynamics simulations can model hair mechanical properties. Simulations done in vacuum showed a better agreement with experimental Young's Modulus (YM) values. The role of hydrogen bonds and the secondary structure of keratin on the mechanical properties was evaluated in detail. The incubation with a fragment of one surfactant protein, the SPD-2 peptide (QAAFSQ), showed the improvement of YM of the hair keratin either by simulations and experimental data. For the first, our research provides mechanistic insights on mechanical microscopic properties of keratin protofibrils through molecular dynamics simulations.
Journal of Structural Biology, 2003
The abundance and cytoplasmic organization of keratin filaments enables them to contribute to the maintenance of structural integrity in epithelial tissues. Co-polymers of the type II keratin 8 and type I keratin 18 form the major intermediate filament network in simple epithelia. We investigated the mechanical properties of K8-K18 filament suspensions using rheological assays in conjunction with light and electron microscopy. Suspensions of K8-K18 filaments behave like a viscoelastic solid under standard assembly conditions. Bulk elasticity is weakly dependent on deformation frequency but is very sensitive to the concentration (G 0 $ C 1:5 ) and size of individual keratin polymers, in agreement with recent models of semiflexible-polymer physics. K8-K18 filaments can self-organize to form a bundled network that exhibits gel-like mechanical properties. In all cases the mechanical properties of the suspensions correlate with the structural features of individual polymers, as seen under light and electron microscopy. Importantly, these bulk viscoelastic properties of K8-K18 filaments are revealed only when interfacial elastic effects are minimized by the application of phospholipids at the air-liquid interface. Suspensions of K5-K14 and vimentin filaments also exhibit interfacial elasticity, which distorts the interpretation of the viscoelastic moduli as determined by standard rheometry. The potential for modulation of mechanical properties through self-organization may be a general property of keratin polymers and contribute to their organization and function in vivo.
A 'hot-spot' mutation alters the mechanical properties of keratin filament networks
Nature Cell Biology, 2001
Keratins 5 and 14 polymerize to form the intermediate filament network in the progenitor basal cells of many stratified epithelia including epidermis, where it provides crucial mechanical support. Inherited mutations in K5 or K14 result in epidermolysis bullosa simplex (EBS), a skinfragility disorder 1 . The impact that such mutations exert on the intrinsic mechanical properties of K5/K14 filaments is unknown. Here we show, by using differential interference contrast microscopy, that a 'hot-spot' mutation in K14 greatly reduces the ability of reconstituted mutant filaments to bundle under crosslinking conditions. Rheological assays measure similar small-deformation mechanical responses for crosslinked solutions of wildtype and mutant keratins. The mutation, however, markedly reduces the resilience of crosslinked networks against large deformations. Single-particle tracking, which probes the local organization of filament networks, shows that the mutant polymer exhibits highly heterogeneous structures compared to those of wild-type filaments. Our results indicate that the fragility of epithelial cells expressing mutant keratin may result from an impaired ability of keratin polymers to be crosslinked into a functional network. W e previously showed 2 that suspensions of in vitro-assembled, wild-type K5/K14 filaments feature properties akin to those of visco-elastic solids when tested by rheological methods. In support of this, the elastic modulus, G′, measured for a K5/K14 sample at 0.5-1.0 mg ml -1 is typically 30-50 dynes cm -2 , a value that is ten times greater than the viscous modulus, G′′. Accordingly, the phase shift, δ, is very low, usually 8-10°(see Methods for rheological definitions). Pure intermediate filaments (IF) of keratin in suspension are still intrinsically too weak to account for their supportive functions in epithelial cells in vivo 3 . By analogy with Factin 4 , it is likely that filament crosslinking makes a significant contribution to the mechanical properties of keratin IFs in vivo. In this study, we devised in vitro conditions that result in a network of crossbridged keratin filaments with enhanced mechanical resilience. We exploited these conditions to assess the impact that inherited mutations exert on the intrinsic mechanical properties of keratin IFs in suspension. To this end, we studied keratin polymers containing K14R 125 -C, a missense mutation discovered in the K14 gene of a large number of patients who suffer from a severe form of EBS 5 . This arginine residue, located at the beginning of the α-helical rod domain, is highly conserved among type I keratin genes and represents a mutational hot-spot in keratinopathies 6,7 .
Proceedings of the Royal Society B: Biological Sciences, 2012
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.
Molecular Biology of the Cell, 2002
Most type I and II keratin genes are spatially and temporally regulated in a pairwise manner in epithelial tissues, where they represent the major structural proteins. Epithelia can be partitioned into simple (single-layered) and complex (multilayered) types. We compared the structural and mechanical properties of natural keratin polymers occurring in complex (K5-K14) and simple (K8-K18) epithelia. The intrinsic properties of these distantly related keratin filaments, whether dispersed or bundled in vitro, were surprisingly similar in all respects when at high polymer concentration. When type I and II assembly partners were switched to give rise to mismatched polymers (K5-K18; K8-K14), the interfilament interactions, which determine the structural and mechanical properties of keratin polymers, were significantly altered. We also show that a K5-K16 polymer exhibits lesser elasticity than K5-K14, which may help explain the inability of K16 to fully rescue the skin blistering characteristic of K14 null mice. The property of self-interaction exhibited by keratin filaments is likely to assist their function in vivo and may account for the relative paucity of cytoplasmic and keratin-specific cross-linkers. Our findings underscore the fundamental importance of pairwise polymerization and have implications for the functional significance of keratin sequence diversity.
The Structure, Functions, and Mechanical Properties of Keratin
JOM, 2012
It is classified into two types: a-helices and b-pleated sheets. Keratinized materials can be considered as fiber-reinforced composites consisting of crystalline intermediate filaments embedded in an amorphous protein matrix. They have a wide variety of morphologies and properties depending on different functions. Here, we review selected keratin-based materials, such as skin, hair, wool, quill, horn, hoof, feather, and beak, focusing on the structure-mechanical property-function relationships and finally give some insights on bioinspired composite design based on keratinized materials.
Nanoscale Strain-Hardening of Keratin Fibres
PLoS ONE, 2012
Mammalian appendages such as hair, quill and wool have a unique structure composed of a cuticle, a cortex and a medulla. The cortex, responsible for the mechanical properties of the fibers, is an assemblage of spindle-shaped keratinized cells bound together by a lipid/protein sandwich called the cell membrane complex. Each cell is itself an assembly of macrofibrils around 300 nm in diameter that are paracrystalline arrays of keratin intermediate filaments embedded in a sulfur-rich protein matrix. Each macrofibril is also attached to its neighbors by a cell membrane complex. In this study, we combined atomic force microscopy based nano-indentation with peak-force imaging to study the nanomechanical properties of macrofibrils perpendicular to their axis. For indentation depths in the 200 to 500 nm range we observed a decrease of the dynamic elastic modulus at 1 Hz with increasing depth. This yielded an estimate of 1.6GPa for the lateral modulus at 1 Hz of porcupine quill's macrofibrils. Using the same data we also estimated the dynamic elastic modulus at 1 Hz of the cell membrane complex surrounding each macrofibril, i.e., 13GPa. A similar estimate was obtained independently through elastic maps of the macrofibrils surface obtained in peak-force mode at 1 kHz. Furthermore, the macrofibrillar texture of the cortical cells was clearly identified on the elasticity maps, with the boundaries between macrofibrils being 40-50% stiffer than the macrofibrils themselves. Elasticity maps after indentation also revealed a local increase in dynamic elastic modulus over time indicative of a relaxation induced strain hardening that could be explained in term of a a-helix to b-sheet transition within the macrofibrils.
Viscoelastic properties of α-keratin fibers in hair
Acta Biomaterialia, 2017
Considerable viscoelasticity and strain-rate sensitivity are a characteristic in α-keratin fibers, which can be considered a biopolymer. The understanding of viscoelasticity is an important part to the knowledge of the overall mechanical properties of these biological materials. Here, horse and human hairs are examined to analyze the sources of this response. The dynamic mechanical response of α-keratin fibers over a range of frequencies and temperatures is analyzed using a dynamic mechanical analyzer. The α-keratin fibers behave more elastically at higher frequencies while they become more viscous at higher temperatures. A glass transition temperature of ~55 o C is identified. The stress relaxation behavior of α-keratin fibers at two strains, 0.02 and 0.25, is established and fit to a constitutive equation based on the Maxwell-Wiechert model. The constitutive equation is further compared to the experimental results within the elastic region and a good agreement is obtained. The two relaxation constants, 14 s and 359 s for horse hair and 11 s and 207 s for human hair, are related to two hierarchical levels of relaxation: the amorphous matrix-intermediate filaments interfaces, for the short term, and the cellular components for the long term. Results of the creep test also provide important knowledge on the uncoiling and phase transformation of the α-helical structure as hair is uniaxially stretched. SEM results show that horse hair not only has a rougher surface morphology and damaged cuticles, but also exhibits a lower strain-rate sensitivity of 0.05 compared to that of 0.11 in human hair. After the horse and human hairs are chemically treated and the disulfide bonds are cleaved, they exhibit a similar strain-rate sensitivity of ~0.05. FTIR data confirms that the human hair is more sensitive to the-S-S-cleavage, resulting in an increase of cysteic acid content. Therefore, the disulfide bonds in the matrix are experimentally identified as one source of the strain-rate sensitivity and viscoelasticity in α-keratin fibers. 2 Statement of significance Hair has outstanding mechanical strength which is equivalent to metals on a densitynormalized basis. It possess, in addition to the strength, a large ductility that is enabled by either the unfolding of the alpha helices and/or the transformation of these helices to beta sheets. We identify the deformation and failure mechanisms and connect them to the hierarchical structure, with emphasis on the significant viscoelasticity of these unique biological materials