Evolution of epidermal cornification proteins

The epidermis and the skin appendages of terrestrial vertebrates are highly resistant to mechanical and chemical stress. This resistance is established by epidermal cell cornification which, at the molecular level, is driven by the cross-linking of proteins. The formation of disulfide bonds between cysteine residues is an important mechanism of cornification and, accordingly, distinct sets of cysteine-rich proteins have been identified as the constituents of hair, claws, feathers, and scales. However, the evolution of the molecular architecture of many skin appendages of amniotes (mammals, reptiles and birds) has remained incompletely understood. In previous studies we have demonstrated that several components of cornified keratinocytes have originated in a common ancestor of amniotes whereas others have evolved specifically in mammals, birds or reptiles. In the present study, the evolution of previously uncharacterized cysteine-rich epidermal cornification proteins will be investigated by comparative genomics, molecular phylogenetics and gene expression analyses in a diverse set of amniote species. Based on our preliminary results, proteins encoded by genes of the epidermal differentiation complex (EDC) and intermediate filament proteins of the keratin family have been selected for in-depth studies. This project will contribute to the comprehensive characterization of skin evolution in reptiles and birds. Moreover, the results of this study will improve the understanding of the function of the skin as a barrier to the environment and of the morphogenesis of important skin appendages such as scales, claws and feathers.

The outermost layer of the skin, the epidermis, protects mammals, reptiles and birds against various types of stress and contributes to growth of skin appendages, such as hair, scales and feathers. The mechanical resilience of epidermal structures depends on special proteins that are cross-linked in a process called cornification. The aim of this project was to determine the molecular identity and the evolutionary history of cornification proteins. By using advanced methods of comparative genomics and gene expression analysis, we identified a large number of genes that control different types of cornification. We found that keratin intermediate filament proteins with high contents of the amino acid cysteine have evolved independently as components of hair in mammals and as components of feathers in birds. The localization of cysteine-rich proteins in feathers supports the concept that cysteine-dependent protein cross-linking is crucial for the formation of these skin appendages. In mammals cysteine-rich proteins are confined to hair and nails whereas two different groups of keratins form the protective surface layer of the skin under resting and regenerating conditions. We found that the keratins of the homeostatic epidermis were lost during the evolutionary transition of whale-related mammals from terrestrial to aquatic life whereas regeneration-associated keratins were conserved and became constitutive cytoskeletal proteins of an exceptionally thick epidermis in whales. Finally, we showed that filaggrin-type proteins which are required for an efficient skin barrier in humans, have already evolved in the amphibious ancestors of all terrestrial vertebrates. The new insights into the structure and evolution of cornification proteins help to understand the molecular architecture and function of the skin in reptiles, birds and mammals.