Pax3 and Hippo Signaling Coordinate Melanocyte Gene Expression in Neural Crest

Loss of Pax3, a developmentally regulated transcription factor expressed in pre-migratory neural crest, results in severe developmental defects and embryonic lethality. Although Pax3 mutations produce profound phenotypes, the intrinsic transcriptional activation exhibited by Pax3 is surprisingly modest. We postulated the existence of transcriptional co-activators that function with Pax3 to mediate developmental functions. A high-throughput screen identified the Hippo effector proteins Taz and Yap65 as Pax3 co-activators. Synergistic co-activation of target genes by Pax3Taz/Yap65 requires DNA binding by Pax3, is Tead-independent, and is regulated by Hippo kinases Mst1 and Lats2. In vivo, Pax3 and Yap65 co-localize in the nucleus of neural crest progenitors in the dorsal neural tube. Neural crest deletion of Taz and Yap65 results in embryonic lethal neural crest defects and decreased expression of the Pax3 target gene, Mitf. These results suggest that Pax3 activity is regulated by the Hippo pathway and that Pax factors are Hippo effectors.