Canonical and Non-canonical Hippo signaling in <i>C. elegans</i>

Hippo is the namesake component of a conserved transduction cascade/regulator of tissue homeostasis and development across metazoans. The Ste20 family kinase Hippo/MST activates the NDR family kinase Warts/LATS to inhibit the transcriptional coactivator Yorkie/YAP/TAZ and its transcription factor partner Scalloped/TEAD. In Caenorhabditis elegans, cell lineages and organ sizes are largely invariant, and classical Hippo phenotypes such as tissue overgrowth are absent. Nevertheless, WTS-1, YAP-1, and the TEAD-like transcription factor EGL-44 form a conserved core module required for larval development past the L2 stage. Crucially, a direct role for Hippo signaling remains unestablished. To address this question, we generated a fluorescently tagged endogenous YAP-1 as a live biomarker of pathway activity. Upon WTS-1 loss, endogenous YAP-1 translocated from cytosol to nucleus in epithelium and intestine. Tissue-specific depletion revealed that intestinal but not epithelial WTS-1 is essential for progression past L2. The duplicated Hippo-related kinases CST-1 and CST-2 repressed YAP-1 nuclear localization in the epithelium but not intestine, indicating that intestinal WTS-1 functions without CST-1/2. The Ste20 kinase MIG-15, orthologous to Drosophila Misshapen and mammalian MAP4K4/6/7, was redundant with CST-1/2 for larval progression. Yet deficient MIG-15 uniquely increased YAP-1 abundance without driving nuclear localization. By contrast, the Ste20 kinase GCK-2, orthologous to Drosophila Happyhour and mammalian MAP4K1/2/3/5, had no detectable role. Our findings establish C. elegans as a model for Hippo signaling, with Hippo-dependent and -independent cascades controlling WTS-1 in epithelia and intestine, respectively. In this context, YAP-1/EGL-44 outputs are repurposed from the conventional association with growth control to non-proliferative developmental functions.