A Synergy between Genetics and Biochemistry Unravels the Molecular Architecture of the Hippo Signaling Pathway

The Hippo signaling pathway is a central regulator of tissue growth in animal development and adult tissue homeostasis, integrating diverse upstream cues to coordinate proliferation and apoptosis. Its discovery stemmed from innovative genetic approaches in Drosophila, where Flippase recognition target (FRT)/Flippase (FLP)-based mosaic screens identified the tumor suppressors Warts, Salvador, Hippo, and Mats, which together constitute a conserved kinase cascade. Complementary biochemical analyses revealed that this core module suppresses tissue overgrowth by phosphorylating and inhibiting the transcriptional coactivator Yorkie, which partners with the transcriptional enhanced associate domain (TEAD) family transcription factor Scalloped to control target gene expression. Subsequent work uncovered various upstream regulators, including Expanded, Merlin, Fat, Kibra, Crumbs, Pez, Tao-1, and the STRIPAK complex, as well as physiological inputs ranging from cell polarity and cell-cell adhesion to mechanical cues, underscoring the pathway's role in integrating tissue architecture with growth. Conservation of the Hippo pathway in mammals, culminating in the Yes-associated protein (YAP)/transcriptional coactivator with PDZ-binding motif (TAZ)-TEAD transcriptional complex, established its evolutionary importance and biomedical relevance, particularly in cancer biology and regenerative medicine. This review traces the historical milestones that defined the Hippo pathway, highlighting the synergy of genetic and biochemical strategies that transformed an orphan tumor suppressor into a paradigm of growth control, and outlining key challenges for future research.