Among the 26 human type II keratins, K78 is the only one that has not yet been explored with regard to its expression characteristics. Here, we show that, at both the transcriptional and translational levels, K78 is strongly expressed in the basal and parabasal cell layers with decreasing intensity in the lower suprabasal cells of keratinising and non-keratinising squamous epithelia and keratinocyte cultures. The same pattern has been detected at the transcriptional level in the corresponding mouse epithelia. Murine K78 protein, which contains an extraordinary large extension of its tail domain, which is unique among all known keratins, is not detectable by the antibody used. Concomitant studies in human epithelia have confirmed K78 co-expression with the classical basal keratins K5 and K14. Similarly, K78 co-expression with the differentiation-related type I keratins K10 (epidermis) and K13 (non-keratinising epithelia) occurs in the parabasal cell layer, whereas that of the corresponding type II keratins K1 (epidermis) and K4 (non-keratinising epithelia) unequivocally starts subsequent to the respective type I keratins. Our data concerning K78 expression modify the classical concept of keratin pair K5/K14 representing the basal compartment and keratin pairs K1/K10 or K4/K13 defining the differentiating compartment of stratified epithelia. Moreover, the K78 expression pattern and the decoupled K1/K10 and K4/K13 expression define the existence of a hitherto unperceived early differentiation stage in the parabasal layer characterized by K78/K10 or K78/K13 expression.
Keywords: Cytoskeleton; Differentiation; Epidermis; Gene expression; Human; Intermediate filaments; Mouse.