Keratin 14-dependent disulfides regulate epidermal homeostasis and barrier function via 14-3-3σ and YAP1

Abstract

The intermediate filament protein keratin 14 (K14) provides vital structural support in basal keratinocytes of epidermis. Recent studies evidenced a role for K14-dependent disulfide bonding in the organization and dynamics of keratin IFs in skin keratinocytes. Here we report that knock-in mice harboring a cysteine-to-alanine substitution at Krt14’s codon 373 (C373A) exhibit alterations in disulfide-bonded K14 species and a barrier defect secondary to enhanced proliferation, faster transit time and altered differentiation in epidermis. A proteomics screen identified 14-3-3 as K14 interacting proteins. Follow-up studies showed that YAP1, a transcriptional effector of Hippo signaling regulated by 14-3-3sigma in skin keratinocytes, shows aberrant subcellular partitioning and function in differentiating Krt14 C373A keratinocytes. Residue C373 in K14, which is conserved in a subset of keratins, is revealed as a novel regulator of keratin organization and YAP function in early differentiating keratinocytes, with an impact on cell mechanics, homeostasis and barrier function in epidermis.

From structural resilience to cell specification — Intermediate filaments as regulators of cell fate

“…Keratins are thought to provide cells with mechanical stability and tissues with appropriate architecture, but recently keratins are emerging as signaling platforms or modulators regulating several different cellular processes. [80][81][82] Mutations affecting keratins, cause an array of diseases called keratinopathies that mainly manifest trough disturbances in skin and nails, for example, epidermolysis bullosa simplex, ichthyosis hystrix, epidermolytic hyperkeratosis, and pachyonychia congenita 83 (full list of keratin-related diseases found on inter fil.org). Meesmann epithelial corneal dystrophy 84 and some gastroenterological disorders have also been linked to keratins.…”

“…14-3-3σ is expressed by the differentiated cells of the skin and loss of 14-3-3σ allows the keratinocytes to escape terminal differentiation by maintaining telomerase activity and reducing p16 INK4a expression. 101 14-3-3 specifically binds keratins 82,[102][103][104] and it is tempting to hypothesize that the interaction with keratin would be essential for bringing 14-3-3 into proximity of its other interaction partners. The downstream effector of the Hippo kinase cascade, YAP, interacts with 14-3-3 maintaining its cytoplasmic localization ( Figure 1D).…”

“…The displacement of 14-3-3σ results in enhanced nuclear entry of YAP together with increased proliferation and defected terminal differentiation. 82 YAP signaling has also been linked to Notch suppression in the epidermis, which would further counteract keratinocyte terminal differentiation. 105 When the epidermis is subjected to damage it requires efficient repair and regeneration to minimize blood loss and recreate the tissue barrier to the outside.…”

“…201 Inter-keratin disulfide bonding is essential for the assembly and organization of the keratin network 202,203 and disruption of these bonds not only affects the structure of the keratin network, but also displaces 14-3-3 and YAP in keratinocytes leading to perturbed epidermal homeostasis. 82 The dynamic regulation of IFs in this manner allows context dependent cell fate choices. For example, shear stress experienced by endothelial and epithelial cells induces phosphorylation of both keratins and vimentin.…”

“…Upon stress, epithelial cells induce expression of specific stress resilient keratins (K6, K16, & K17), which possess‐specific characteristics beneficial, for example, wound healing and inflammation. Keratins are thought to provide cells with mechanical stability and tissues with appropriate architecture, but recently keratins are emerging as signaling platforms or modulators regulating several different cellular processes 80‐82 . Mutations affecting keratins, cause an array of diseases called keratinopathies that mainly manifest trough disturbances in skin and nails, for example, epidermolysis bullosa simplex, ichthyosis hystrix, epidermolytic hyperkeratosis, and pachyonychia congenita 83 (full list of keratin‐related diseases found on http://interfil.org).…”