Keratin 9 regulates mechanosensitive YAP1 signalling in palmoplantar epidermis

The palmoplantar epidermis, in adapting to the exceptional mechanical strain it bears during locomotion, is histologically and molecularly distinct from other skin body sites. The mechanisms specifying and maintaining its unique identity remain incompletely defined. Here we identify the type I keratin 9 (KRT9/K9), a protein uniquely expressed in palmoplantar epidermis, as a key modulator of mechanosensitive YAP1 signalling in response to postnatal mechanical compression. K9 loss-of-function variants driving KRT9-pEDD, a palmoplantar epidermal differentiation disorder, result in aberrant YAP1 subcellular partitioning and elevated levels of the stress-induced keratin 16 (KRT16/K16). Krt9 null mice recapitulate these molecular phenotypes as early as postnatal day 3. We further identify dynamic, YAP1-dependent regulation of Krt16/K16, upstream of and necessary for Krt9/K9 expression, during early postnatal development in situ and in response to mechanical compression of keratinocytes ex vivo, highlighting the role of mechanical stress in epidermal specification. Mechanistically, K9 interacts with the YAP1 binding protein 14-3-3σ and sequesters YAP1 in the cytoplasm, inhibiting its transcriptional activity; these functions are disrupted by KRT9-pEDD-causing variants in K9. Finally, we show that genetic or pharmacological inhibition of YAP1 ameliorates palmoplantar keratoderma in Krt9 null mice. These findings reveal a role for mechanical cues in specification and maintenance of palmoplantar skin and suggest new therapeutic interventions for inherited palmoplantar epidermal differentiation disorders.