Keratin 15 promotes a progenitor cell state in basal keratinocytes of skin epidermis

Abstract

The type I intermediate filament proteins keratin 14 (K14) and keratin 15 (K15) are common to all complex epithelia. K14 is highly expressed by progenitor keratinocytes, in which it provides essential mechanical integrity and gates keratinocyte entry into differentiation by sequestering YAP1, a transcriptional effector of Hippo signaling, to the cytoplasm. K15 has long been used as a marker of hair bulge stem cells, though its specific role in skin epithelia is unknown. Here we show that the lack of two biochemical determinants, a cysteine residue within the stutter motif of the central rod domain and a 14-3-3 binding site in the N-terminal head domain, renders K15 unable to effectively sequester YAP1 in the cytoplasm like K14 does. We combine insight obtained from cell culture and transgenic mouse models with computational analyses of transcriptomics data and propose a model in which a higher K15:K14 ratio promotes a progenitor state and antagonizes differentiation in keratinocytes of the epidermis. SUMMARYThe type I intermediate filament keratin 15 (K15) exhibits preferential expression in a subset of skin keratinocytes with stem-like properties. We report that K15 promotes the progenitor state by counteracting the ability of the related K14 to negatively regulate YAP1, a terminal effector of Hippo signaling.