Studies of early cell fate

Studies of early cell fate specification in the mouse embryo have identified a role for differential Motin localization in controlling Hippo signaling [88]. At the 32-cell stage, mouse blastomeres are subdivided into inner cells, which form the inner cell mass (ICM), and outer cells, which form the trophectoderm (TE). This subdivision requires Hippo signaling, which is high in the ICM, leading to cytoplasmic YAP, and low in the TE, leading to nuclear YAP [89]. Both cell contacts and cell polarity influence Hippo signaling at this stage. In the ICM, Motins localize to cell–cell junctions and Hippo signaling is active. However, the outer NG25 australia become polarized, causing Motins to localize to the apical domain rather than to cell–cell junctions 86, 90. Evidently, this loss of Motins from cell junctions prevents formation of the Hippo pathway activation complex needed to promote LATS activation (Figure 2B). Thus, while there are some differences in the specific proteins involved between Drosophila and mammals, a common theme has emerged regarding the existence and importance of platforms for Hippo signal transduction at cell junctions.
Regulation of Hippo Signaling by the ECM Hippo signaling is also regulated by attachment to the ECM (Figure 4). For example, the extent of cell–ECM contacts influences Hippo signaling and detachment of cells can result in cell death through activation of the Hippo pathway 91, 92, 93. The requirement for cell–substrate attachment has both biochemical and biomechanical components. One biochemical mechanism involves the modulation of Hippo signaling by Integrin-linked Kinase (ILK), which could inhibit Merlin activation by inhibiting the phosphatase MYPT1 [94]. More recently, a link between integrin and Hippo signaling that depends on focal adhesion kinase (FAK) was identified [95]. Integrins bound to fibronectin stimulate FAK, thereby activating Src, which activates PI3K. The PI3K downstream kinase PDK1 then disrupts the core kinase cassette, resulting in the inhibition of Hippo signaling [96]. As activation of FAK by integrins can be modulated by substrate stiffness [97], regulation of Hippo signaling through FAK could also contribute to influences of the mechanical environment on Hippo signaling. An alternative mechanism by which Src can promote YAP activity, involving direct phosphorylation of tyrosines residues on YAP by Src, has also recently been described 68, 98.
Regulation of Hippo Signaling by F-Actin Levels Indications of the key influence of the cytoskeleton on Hippo signaling first came from observations that mutations in Drosophila that increase F-actin accumulation could be associated with increased Yki activity 99, 100; this also occurs in mammalian cells [101]. Demonstration of the influence of mechanical force on YAP and TAZ activity then came from observations that cell shape and the rigidity of the ECM could influence YAP/TAZ activity [93] and that this influence requires myosin, which generates tension in the actin cytoskeleton. It was also reported that regulation of YAP/TAZ by cell attachment and cell shape occurs independently of LATS [93], but others have reported LATS-dependent effects 91, 92. Modulation of the actin cytoskeleton is also correlated with cell density-dependent effects on Yap activity 101, 102. The influences of cell shape, cell attachment, cell density, and matrix rigidity on Hippo signaling are suppressed by inhibiting the key cytoskeletal regulator Rho 91, 92, 93, 101. This is also true for other upstream inputs of Hippo signaling, such as GPCR pathways, that may influence Hippo signaling through cytoskeleton regulation [103]. In Drosophila, Zyxin may also regulate Hippo signaling in part through modulation of the cytoskeleton 21, 104. Initially, how the actin cytoskeleton influences Hippo signaling was unknown, but a series of recent studies have begun to make progress in identifying molecular mechanisms that link the cytoskeleton to regulation of Hippo signaling.