Hemidesmosomes and Notch signaling regulate epidermal differentiation via delamination

Integrins mediate adhesion of basal keratinocytes to the underlying basement membrane. While high expression of integrins has been correlated with stemness, there is limited direct evidence that integrins mediate keratinocyte retention within the basal layer. Here, we generate mosaic, epidermal-specific loss of integrin-β4 (encoded by Itgb4) or its ligand, laminin-α3β3γ2 (Lama3), in mouse using an in utero lentiviral-mediated approach. Although mutations in these genes cause postnatal skin blistering in mice and humans, we observe no evidence of dermal-epidermal separation embryonically. Despite no obvious alterations to apicobasal polarity, Itgb4-deficient basal cells show mild defects in oriented cell divisions, with increased oblique divisions and altered telophase correction. However, differentiation via delamination, whereby basal keratinocytes lose adhesion to the underlying basement membrane and transit into the suprabasal layer, is elevated upon Itgb4 or Lama3 loss. Notably, hyperactive Notch signaling both decreases integrin-β4 expression and increases delamination, while deletion of the Notch effector Rbpj has the opposite effect. These findings conclusively demonstrate a causal role for hemidesmosomes in regulating epidermal differentiation through both mitotic and non-mitotic mechanisms, and shed additional light on the programs regulating delamination.