Mechanical stress induces profound remodelling of keratin filaments and cell junctions inepidermolysis bullosa simplexkeratinocytes

Abstract

IntroductionAs the flexible outer covering of the body, the properties of skin must accommodate continuous stretching forces. The skin must be strong enough to endure physical trauma yet be flexible enough to allow a large range of movement. A large contribution to the physical resilience of epidermal skin cells comes from the keratin intermediate filaments that they express, as was shown with the discoveries that the inherited skin fragility disorder epidermolysis bullosa simplex (EBS) is caused by mutations in keratin genes (Bonifas et al., 1991;Coulombe et al., 1991;Lane et al., 1992). In EBS, the basal layer cells of the epidermis exhibit fragility under mild physical trauma, resulting in blistering; in severe cases, electron microscopy characteristically reveals electron-dense keratin aggregates in basal keratinocytes of patients (AntonLamprecht and Schnyder, 1982).Keratins belong to the superfamily of intermediate filament proteins. Heterodimeric filaments of type I and type II keratins are expressed in a tissue-and cell-specific manner in epithelial cells (reviewed by Fuchs and Weber, 1994). Stratified epithelia such as the epidermis express K5/K14 in the basal layer of cells and K1/K10 in the suprabasal layers. The keratins form a cytoplasmic network of filaments that are connected to the plasma membrane at complex cell-cell junctions (desmosomes) and cell-substrate junctions (hemidesmosomes). Tissue integrity in the epidermis is maintained by a complex cytoskeleton interaction between the keratin filaments and desmosomes and hemidesmosomes. Keratins are linked into desmosomes through desmoplakin and into hemidesmosomes via plectin; desmosomes are connected from cell to cell by the transmembrane cadherins (desmogleins and desmocollins), and hemidesmosomes are connected to the extracellular matrix substrate by integrins (usually α 6 β 4 ) and BP180. Other proteins found in these zones include plakoglobin within the plaques of both junctions, plakophilin in desmosomes and BP230 in hemidesmosomes. The exact nature of interactions between all these components is not yet clear (for reviews see Borradori and Sonnenberg, 1996;Jones et al., 1998).The mutations that cause the severe (Dowling-Meara) type of EBS are mostly found in the highly conserved ends of the α-helical rod domains of keratins K5 or K14. These regions are particularly important in filament assembly (Herrmann and Aebi, 1998). There is a particularly mutagenic codon at residue 125 of K14, in the helix initiation motif, leading to the substitution of the arginine residue at this position (Coulombe et al., 1991); mutations at this residue account for approximately 70% of Dowling-Meara EBS cases (Porter and Lane, 2003). Morphological effects of most keratin mutations are hardly detectable in unstressed cultured keratinocytes but spontaneous keratin aggregates are characteristically seen in cells containing a K14-R125 mutation; this is in keeping with the theory that the helix initiation motif is crucial in normal filament assembly (Herrmann and Aebi...

Hyper-adhesion in desmosomes: its regulation in wound healing and possible relationship to cadherin crystal structure

“…When the intermediate filaments are disrupted, as in the human genetic disease epidermolysis bullosa and transgenic mice that mimic it, the epidermis is greatly weakened (Fuchs, 1996;Lane and McLean, 2004). Weakening or disrupting the interaction between desmosomes and intermediate filaments also diminishes the strength of epithelia (Huen et al, 2002;Russell et al, 2004). Scaffolding, like a chain, is only as strong as its weakest link.…”

Desmosomes and Intermediate Filaments: Their Consequences for Tissue Mechanics

“…Of note, even mutations causing severe disease do not prevent formation of long keratin intermediate filaments (KIFs) in vitro (Herrmann et al 2002), suggesting that mutations and physical stress act at the level of keratin bundling, network organization, dynamics, or by affecting association with other proteins. Indeed, the most frequent KRT14 Arg125 mutation compromises desmosome adhesion (Russell et al 2004;Homberg et al 2015).…”

“…The resulting perinuclear keratin reorganization affects the viscoelasticity of metastatic cancer cells and promotes tumor invasion (Beil et al 2003). Mechanically stretching of keratinocytes expressing EBS-like keratin mutations triggered a progressive disassembly of desmosomes and weakened intercellular adhesion (Russell et al 2004;Homberg et al 2015). Thus, keratin reorganization on posttranslational modifications might contribute to the invasive properties of metastatic tumor cells and to altered adhesion in skin disorders (Beil et al 2003;Russell et al 2004;Homberg et al 2015;Loschke et al 2016).…”

E1^E4-mediated keratin phosphorylation and ubiquitylation: a mechanism for keratin depletion in HPV16-infected epithelium

“…Studies in both stressed cells and cells expressing keratin mutants, have shown that altered keratin IF network dynamics and keratin reorganisation are accompanied by an elevation in keratin phosphorylation Russell et al, 2004;Werner et al, 2004). Since the association of 16E1^E4 with keratin results in altered keratin dynamics, we investigated the phosphorylation status of this malfunctioning 16E1^E4-associated keratin network.…”