RIPK4 promotes epidermal differentiation through phase separation and activation of LATS1/2

The basal layer progenitors establish a stratified epidermis through asymmetric division and differentiation. Inactivating mutations of Receptor-interacting serine/threonine kinase 4 (RIPK4) cause human developmental syndromes characterized by defective epidermal differentiation. While the Hippo pathway is crucial in limiting organ size, emerging evidence suggests that it also plays additional roles in differentiation. In this study, we identify RIPK4 as an alternative upstream kinase of LATS1/2 in the Hippo pathway through screening a kinome library. Ripk4 knockout in mice results in activation of Hippo pathway effectors Yap/Taz in the granular layer, which subsequently represses cholesterol biosynthesis. Furthermore, the ablation of Yap/Taz partially rescues skin barrier defects. Mechanistically, RIPK4 directly phosphorylates LATS1/2 after recruiting them into liquid condensates. Disease-derived RIPK4 mutants exhibit defects in LATS1/2 activation either due to impaired kinase activity or disrupted phase separation. Our findings demonstrate that a RIPK4-initiated noncanonical Hippo pathway plays a specific role in epidermal differentiation.