Cell cell junction proteins were the
Cell-cell junction proteins were the first upstream modules to be characterized as contact sensors through the Hippo pathway. These junctions may be divided in 2 categories: adherens and epithelial polarity junctions. The establishment of intercellular contacts involves the cadherin proteins present at the cell surface. Homotypic binding of cadherins from neighboring RI-1 increases with cell density and activates the Hippo pathway. This results in the phosphorylation of YAP/TAZ and their retention in the cytoplasm, resulting in inhibition of cell proliferation (Nishioka et al., 2009; Hirate et al., 2013).
Upon establishment of cell–cell contacts, epithelial cells uniquely develop baso-apical polarity, a process that implicates the establishment of epithelial-specific intercellular junctions called Tight Junctions (TJs), along with the formation of polarity complexes. As a result, the cytoplasmic membrane of polarized cells becomes compartmentalized, with distinct domains, apical and basolateral, that exhibit heterogeneous composition in lipids and proteins and allows epithelia to serve as a physiological barrier necessary for tissue homeostasis. A number of studies both in Drosophila and mammalian cells have provided ample evidence for a direct role of the apical-basal cell polarity determinants Crumbs complex and aPKC–Par6–Par3 complex in the regulation of the Hippo pathway, via direct interaction with either upstream regulators or with YAP/TAZ (reviewed in (Genevet and Tapon 2011; Piccolo et al., 2014; Meng et al., 2016)). In addition to cell–cell contacts, signals originating from interactions of cells with their surrounding extracellular matrix and transmitting forces to the cytoskeleton lead to specific mechanotransduction signals in which YAP and TAZ are critically involved, in a Hippo-dependent or −independent manner implicated in various physiological and disease-related processes ranging from cell fate determination to tissue repair, fibrosis and cancer (reviewed in (Dupont et al., 2011; Low et al., 2014; Dupont 2016; Zanconato et al., 2016; Panciera et al., 2017).
The broad range of contexts implicating the Hippo pathway largely overlaps that of Transforming Growth Factor-ß, a family of growth factors whose ubiquitous expression plays a critical role in controlling a wide array of cellular functions during embryonic development, adult tissue homeostasis and whose deregulation is implicated in disease and tumorigenesis (reviewed in (Massague 2012; Mauviel et al., 2012; Perrot et al., 2013)). Members of the TGF-ß superfamily (TGF-ßs, activins, BMPs, GDFs…) signal via heteromeric serine/threonine kinase transmembrane receptor complexes. Canonical signaling consists in ligand binding to type II receptor, a constitutively active kinase, recruitment, trans-phosphorylation and activation of the signaling (type I) receptor, resulting in the phosphorylation of receptor-associated members of the SMAD family (Fig. 2). BMP receptors phosphorylate receptor-associated SMADs 1/5/8 while TGF-ß and Activin receptors phosphorylate SMAD2/3 specifically. The latter then accumulate in the nucleus as heteromeric complexes together with SMAD4, a co-SMAD common to all ligands, to regulate the transcription of target genes. Inhibitory SMADs, SMAD6 and SMAD7, interfere with TGF-ß/SMAD signaling by various mechanisms that affect receptor activity and/or receptor-associated SMAD nuclear accumulation (reviewed in (Massague 2012; Perrot et al., 2013)).
Hippo-TGF-ß signaling crosstalks The first evidence for functional interaction between the TGF-ß and Hippo pathway emerged as the Hippo signaling cascade per se had not yet been uncovered. Specifically, following a yeast two-hybrid screen designed to identify new partners of the inhibitory SMAD, SMAD7, we characterized YAP1, known at the time as YAP65, as a SMAD7-interacting protein (Ferrigno et al., 2002). YAP is a prototypic WW domain-containing protein (Chen and Sudol, 1995), a structural motif that binds PPY motifs on partner proteins, and found in SMAD7. Yet, the SMAD7-YAP interaction only partially depends on the WW-PPY domains and implicates additional peptidic sequences in both YAP and SMAD7. A mechanism was uncovered, whereby YAP acts by promoting the association of SMAD7 to activated TGF-ß receptor type I (TßRI), thereby abolishing downstream TGF-ß signaling. The SMAD7-YAP interaction was confirmed independently and shown to be independent from phosphorylation (Aragon et al., 2012).