107 Single-cell transcriptomics of avian skin reveals evolutionarily ancient keratinocyte differentiation genes

Abstract

The genetic program of epidermal keratinocyte differentiation has evolved by adaptation to a dry terrestrial environment. The dynamics of epidermal gene expression is largely conserved between human and mouse, the standard model of biomedical research. However, only little is known about keratinocyte differentiation in non-mammalian vertebrates. Here we performed a single-cell transcriptome study of keratinocyte differentiation in avian skin. Keratinocytes and other skin cells from the leg skin of 3 chicks were analyzed by single-cell RNA-sequencing. We identified two distinct populations of differentiated keratinocytes corresponding to hard scales and soft inter- scale epidermis. The scale keratinocytes contained cysteine-rich keratins, one of which could be immunolocalized in the tissue using a newly raised antibody. By contrast, interscale keratinocytes contained KRT9 and KRT78-like keratins with low cysteine contents. The transcriptome of the latter type of keratinocytes showed many similarities to the transcriptome of keratinocytes in the granular layer of human epidermis, including expression of established differentiation markers, such as loricrin and claudin 1, and as yet incompletely characterized genes, such as POF1B and HOPX. Furthermore, genes involved in lipid metabolism and transport, such as ELOVL4, ABCA12, FABP5 and GLTP were upregulated during both chicken and human keratinocyte differentiation. In conclusion, comparative analysis of chicken and human skin at the single-cell level identifies regulators of epidermal differentiation that have evolved prior to divergence of the phylogenetic lineages leading to birds and mammals.