A keratin scaffold regulates epidermal barrier formation, mitochondrial lipid composition, and activity.

Keratin intermediate filaments (KIFs) protect the epidermis against mechanical force, support strong adhesion, help barrier formation, and regulate growth. The mechanisms by which type I and II keratins contribute to these functions remain incompletely understood. Here, we report that mice lacking all type I or type II keratins display severe barrier defects and fragile skin, leading to perinatal mortality with full penetrance. Comparative proteomics of cornified envelopes (CEs) from prenatal KtyI(-/-) and KtyII(-/-)(K8) mice demonstrates that absence of KIF causes dysregulation of many CE constituents, including downregulation of desmoglein 1. Despite persistence of loricrin expression and upregulation of many Nrf2 targets, including CE components Sprr2d and Sprr2h, extensive barrier defects persist, identifying keratins as essential CE scaffolds. Furthermore, we show that KIFs control mitochondrial lipid composition and activity in a cell-intrinsic manner. Therefore, our study explains the complexity of keratinopathies accompanied by barrier disorders by linking keratin scaffolds to mitochondria, adhesion, and CE formation.

Keratin 1 maintains skin integrity and participates in an inflammatory network in skin through interleukin-18.

Keratin 1 (KRT1) and its heterodimer partner keratin 10 (KRT10) are major constituents of the intermediate filament cytoskeleton in suprabasal epidermis. KRT1 mutations cause epidermolytic ichthyosis in humans, characterized by loss of barrier integrity and recurrent erythema. In search of the largely unknown pathomechanisms and the role of keratins in barrier formation and inflammation control, we show here that Krt1 is crucial for maintenance of skin integrity and participates in an inflammatory network in murine keratinocytes. Absence of Krt1 caused a prenatal increase in interleukin-18 (IL-18) and the S100A8 and S100A9 proteins, accompanied by a barrier defect and perinatal lethality. Depletion of IL-18 partially rescued Krt1(-/-) mice. IL-18 release was keratinocyte-autonomous, KRT1 and caspase-1 dependent, supporting an upstream role of KRT1 in the pathology. Finally, transcriptome profiling revealed a Krt1-mediated gene expression signature similar to atopic eczema and psoriasis, but different from Krt5 deficiency and epidermolysis bullosa simplex. Our data suggest a functional link between KRT1 and human inflammatory skin diseases.

Genes encoding cytoplasmic intermediate filament proteins of vertebrates revisited: identification of a cytoplasmic intermediate filament protein in the sea anemone Nematostella.

The cytoskeleton is crucial in determining cell architecture, division, motility, transport processes and in local control of signal transduction. Relatives of actin and tubulin are expressed in all phyla, underlining the fundamental importance of conserved cytoskeletal functions. Intermediate filament proteins have evolved in parallel with tissue diversity in the animal kingdom, likely from the demand to adapt one class of cytoskeletal proteins to cell type-restricted functions. Up to now, the evolutionary origin of cytoplasmic intermediate filament proteins remains unknown. Using a known gene encoding a cytoplasmic intermediate filament protein from the hemichordate Saccoglossus kowalevskii, we have identified the first corresponding gene in the sea anemone Nematostella, tentatively named cytovec. Our data reveal a relationship of cytovec with Hydra vulgaris nematocilins A and B that also lack a CAAX box. In light of additional recent findings, our data show that cytoplasmic intermediate filament genes are present in the common ancestor of Cnidaria and Bilateria.

Keratin function and regulation in tissue homeostasis and pathogenesis.

Abstract Epithelial tissues act as hubs in metabolism and communication and protect the organism against dehydration, infections, pharmacological and physical stress. Keratin intermediate filament proteins are well established as major cytoskeletal players in maintaining epithelial integrity. More recently, an involvement of keratins in growth control and organelle functions has emerged. Disruption of the keratin cytoskeleton by mutations or its reorganization following posttranslational modifications can render epithelia susceptible to tissue damage and various stresses, while loss of keratin expression is a hallmark of epithelial-mesenchymal transition (EMT). To understand the molecular mechanisms by which keratins perform their functions remains a formidable challenge. Based on selected examples, we will discuss how cell-specific expression of keratin isotypes affects cytoarchitecture and cell behavior. Further, we ask how posttranslational modifications alter keratin organization and interactions during signaling. Next, we discuss pathomechanisms of epidermal keratin disorders in the light of novel data. Finally, we raise open questions and point out future directives.