Homeostatic control of Hippo signaling activity revealed by an endogenous activating mutation in YAP

Abstract

The Hippo signaling pathway converges on YAP to regulate growth, differentiation, and regeneration. Previous studies with overexpressed proteins have shown that YAP is phosphorylated by its upstream kinase, Lats1/2, on multiple sites, including an evolutionarily conserved 14-3-3-binding site whose phosphorylation is believed to inhibit YAP by excluding it from the nucleus. Indeed, nuclear localization of YAP or decreased YAP phosphorylation at this site (S168 in Drosophila, S127 in humans, and S112 in mice) is widely used in current literature as a surrogate of YAP activation even though the physiological importance of this phosphorylation event in regulating endogenous YAP activity has not been defined. Here we address this question by introducing a Yap S112A knock-in mutation in the endogenous Yap locus. The Yap S112A mice are surprisingly normal despite nuclear localization of the mutant YAP protein in vivo and profound defects in cytoplasmic translocation in vitro. Interestingly, the mutant Yap S112A mice show a compensatory decrease in YAP protein levels due to increased phosphorylation at a mammalian-specific phosphodegron site on YAP. These findings reveal a robust homeostatic mechanism that maintains physiological levels of YAP activity and caution against the assumptive use of YAP localization alone as a surrogate of YAP activity.

Osmotic stress‐induced phosphorylation by NLK at Ser128 activates YAP

“…Inhibition of Ser127 phosphorylation, by mutating YAP serine 127 to alanine (S127A), was shown to abolish 14-3-3 binding and increase nuclear localization [15]. Consistently, in vivo study also supports the notion that S127 (S112 in mouse YAP) phosphorylation is critical for YAP cytoplasmic localization as a more prominent nuclear YAP is found in the liver and the colon of YAP S112A knock-in mice [19]. Therefore, YAP Ser127 phosphorylation has been widely used as an indicator of YAP inactivation.…”

“…Phosphorylation of YAP Ser127 results in YAP-14-3-3 binding and therefore cytoplasmic retention. This phosphorylationdependent cytoplasmic localization is highly conserved in Drosophila (Yki Ser 168) and mice (Yap S112) [17,19]. Furthermore, phosphorylation on Ser397 (Ser381 in mice) by LATS leads to phosphodegron-mediated YAP degradation [20].…”

Dysregulated YAP1/TAZ and TGF-β signaling mediate hepatocarcinogenesis in Mob1a/1b -deficient mice

“…However, LATS1/2 had been considered unlikely to play a major role in mammalian liver because liver-specific loss of MST1/2 had no effect on LATS1/2 phosphorylation (18), and neither LATS1 KO nor liver-specific LATS2 KO mice show obvious liver phenotypes. More recently, it has been reported that AlbCre;Lats1 flox/flox ;Lats2 flox/flox mice display liver abnormalities that are strikingly similar to those of our LMob1DKO mice (48), indicating that the MST-MOB1-LATS-YAP1 axis is important after all in mammalian liver.…”

Mechanisms of Hippo pathway regulation

“…The interaction between YAP/TAZ and TEAD1-4 dissociates VGLL4 from TEAD1-4 and thereby activates TEAD-mediated gene transcription to promote tissue growth and inhibit apoptosis (Koontz et al 2013). Mouse models with deletion of MST1/2, SAV1, MOB1A/B, NF2, or LATS1/2 or YAP overexpression all exhibit upregulation of TEAD target gene expression, increased expansion of progenitor cells, and tissue overgrowth (Camargo et al 2007;Dong et al 2007;Zhou et al 2009;Cai et al 2010;Lee et al 2010;Lu et al 2010;Song et al 2010;Zhang et al 2010;Nishio et al 2012;Chen et al 2015b), supporting the functional roles of these genes in the Hippo pathway.…”