Structure and functions of keratin proteins in simple, stratified, keratinized and cornified epithelia

Abstract

Historically, the term 'keratin' stood for all of the proteins extracted from skin modifications, such as horns, claws and hooves. Subsequently, it was realized that this keratin is actually a mixture of keratins, keratin filament-associated proteins and other proteins, such as enzymes. Keratins were then defined as certain filament-forming proteins with specific physicochemical properties and extracted from the cornified layer of the epidermis, whereas those filamentforming proteins that were extracted from the living layers of the epidermis were grouped as 'prekeratins' or 'cytokeratins'. Currently, the term 'keratin' covers all intermediate filament-forming proteins with specific physicochemical properties and produced in any vertebrate epithelia. Similarly, the nomenclature of epithelia as cornified, keratinized or non-keratinized is based historically on the notion that only the epidermis of skin modifications such as horns, claws and hooves is cornified, that the non-modified epidermis is a keratinized stratified epithelium, and that all other stratified and non-stratified epithelia are non-keratinized epithelia. At this point in time, the concepts of keratins and of keratinized or cornified epithelia need clarification and revision concerning the structure and function of keratin and keratin filaments in various epithelia of different species, as well as of keratin genes and their modifications, in view of recent research, such as the sequencing of keratin proteins and their genes, cell culture, transfection of epithelial cells, immunohistochemistry and immunoblotting. Recently, new functions of keratins and keratin filaments in cell signaling and intracellular vesicle transport have been discovered. It is currently understood that all stratified epithelia are keratinized and that some of these keratinized stratified epithelia cornify by forming a Stratum corneum . The processes of keratinization and cornification in skin modifications are different especially with respect to the keratins that are produced. Future research in keratins will provide a better understanding of the processes of keratinization and cornification of stratified epithelia, including those of skin modifications, of the adaptability of epithelia in general, of skin diseases, and of the changes in structure and function of epithelia in the course of evolution. This review focuses on keratins and keratin filaments in mammalian tissue but keratins in the tissues of some other vertebrates are also considered.

Cytokeratins of the Liver and Intestine Epithelial Cells During Development and Disease

“…In rats, CK8 and CK19 are the major keratins, while CK18 and CK21 are less abundant. CK21 is homologous to human CK20 (Calnek and Quaroni 1993;Bragulla and Homberger 2009). This keratin is present only in differentiated villus cells in both rats and human.…”

Keratins as the main component for the mechanical integrity of keratinocytes

“…Keratins are encoded by a large multigene family of more than 50 genes that are specifically expressed depending on distinct developmental pathways and physiological requirements (10,11). Based on their amino acid sequence, type I keratins display an overall acidic character and differ considerably from the more basic type II keratins (12).…”

Retinoic Acid-Induced Epidermal Transdifferentiation in Skin

“…Cytokeratin, an intermediate filament observed mainly in epithelial cells, is an essential cytoskeleton component involved in the maintenance of cell morphology. The cytoskeleton of epithelia is formed by 20 subtypes of cytokeratins whose expression depends primarily on the epithelial cell type and degree of differentiation [60]. As keratin filaments, e.g., tonofibrils, K1 and K10 are not at all observed throughout the epidermis; but both K4 and MUC4 proteins are expressed in RA-induced transdifferenti ated epithelial cells, but not in control epidermal cells [22].…”