Before the Hippo pathway was

Before the Hippo pathway was delineated in Drosophila, several crucial components of the pathway had already been cloned in mammals (Tapon et al., 2002; Xiao et al., 1991; Sudol, 1994; Creasy and Chernoff, 1995; Tao et al., 1999). The functions of these crucial Hippo pathway components have been well addressed in several reviews on Tead (Pobbati and Hong, 2013), Yap (Wang et al., 2009), and Mst (Matallanas et al., 2008). Several excellent reviews of Hippo/Yap signaling have also been published recently (Pan, 2007; Yu and Guan, 2013; Zhao et al., 2010). This section will focus on the recently defined aspects of mammalian Hippo/Yap signaling (summarized in Fig. 1).
Regulation of Yap in the submembrane compartment In response to extracellular cues and cell–cell interactions, the Hippo kinase cascade restrains cell proliferation and organ growth by phosphorylating and inhibiting Yap. However, the mechanisms by which extracellular cues modulate Hippo kinase activity are incompletely understood. Recent work from Yin et al. partially filled this gap by elucidating how NF2, a protein that links cytoskeletal components with proteins in the cell membrane, controls Hippo pathway activity (Yin et al., 2013). NF2-deficient livers had decreased Lats and Yap phosphorylation. Mst was required for NF2-induced Lats activation, but unexpectedly NF2 deficiency increased Mst activity. Further biochemical studies accounted for these observations by showing that NF2 recruits Lats to the plasma membrane through direct binding. Sav1 similarly recruits Mst to the plasma membrane, where it phosphorylates and activates Lats. Liver specific single knock of either NF2 or Sav1 resulted in a mild increase of liver size, but double knock-out caused massive liver overgrowth. Based on these data, the authors raised a “parallel model” of Hippo pathway signal transduction, in which NF2 and Sav1 act as scaffolds to recruit Lats and Mst, respectively, to the membrane. In the submembrane signal transduction compartment formed by these proteins, Mst phosphorylates and activates Lats, which in turn phosphorylates and inhibits Yap. The extracellular signals and their receptors that regulate the mammalian Hippo/Yap pathway are incompletely understood. Yu et al. found that Yap activity is regulated downstream of certain G-protein coupled receptors (GPCRs) (Yu et al., 2012). Specifically, they showed that the lipids lysophosphatidic purchase Sennoside A (LPA) and sphingosine 1-phosphate (S1P) stimulate Yap activity by binding to specific GPCRs that couple to the Gα12/13 subfamily of heterotrimeric G proteins. Gα12/13 signaling inhibited Lats activity through a signal transduction pathway involving Rho GTPases and actin cytoskeleton organization, and independent of Mst. On the other hand, epinephrine stimulation of the β2-adrenergic receptor, a GPCR coupled to Gαs, enhanced Yap phosphorylation and thereby inhibited Yap activity. The Hippo kinase cascade interacts with and phosphorylates Yap in the submembrane compartment, but Yap exerts its main actions in the nucleus. 14-3-3 proteins, highly conserved phosphoserine binding proteins that often bind transcription factors (Eckardt, 2001), are crucial chaperones that mediate proper Yap subcellular localization. In the absence of bound targets, 14-3-3 proteins reside in the nucleus. Target protein phosphorylation leads to 14-3-3 protein binding and export of the protein complex from the nucleus to the cytoplasm (Brunet et al., 2002). 14-3-3 proteins are required for Yap nuclear export (Dong et al., 2007), but it is not clear whether Lats is responsible for Yap phosphorylation in the nucleus. Yap activity is regulated by cell–cell contacts, as contact inhibition leads to Yap phosphorylation, 14-3-3 binding, and exclusion from the nucleus. Yap interacts with α-catenin and angiomotin (AMOT), components of intercellular junctions which modulate Hippo/Yap signaling in response to cell–cell interactions. α-Catenin links cadherins, calcium-dependent intercellular adhesion molecules, with the actin cytoskeleton at adherens junctions. In keratinocytes, α-catenin directly interacts with 14-3-3 proteins, and the interaction is enhanced in the presence of phospho-Yap. α-Catenin negatively regulates Yap activity by recruiting it to the adherens junctions and the submembrane region (Schlegelmilch et al., 2011). Furthermore, the protein complex formed by α-catenin, 14-3-3 and phospho-Yap inhibits Yap dephosphorylation mediated by the phosphatase PP2Ac (Schlegelmilch et al., 2011). Angiomotin (AMOT) is another intercellular junction protein that regulates Yap activity (Zhao et al., 2011). AMOT binds to Yap, recruiting Yap to the sub-membrane region near intercellular junctions. AMOT also binds to filamentous actin (F-actin), and this interaction is regulated by Lats, making AMOT both a regulator and an effector of Hippo signaling. AMOT phosphorylation dissembles it from filamentous actin and releases it from junction complexes into the cytoplasm (Dai et al., 2013; Chan et al., 2013). Phosphorylated AMOT recruits the ubiquitin E3 ligase atrophin-1 interacting protein 4, leading to degradation of AMOT-bound Yap by the ubiquitin proteasome system (Adler et al., 2013).