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  • hospital settings Recently the spectrin cytoskeleton a

    2022-01-13

    Recently, the spectrin cytoskeleton, a deformable actin-associated network, has been identified as a modulator of Yki function, potentially acting both apically and basally in different cell types [75•, 76•, 77•]. Several mechanisms have been proposed, including regulation via Crb/Ex tethering [], through myosin II and cortical apical acto-myosin regulation [], or by regulating basal actin networks [].
    Compartmentalisation of Hippo signalling: endocytic network and the nucleus The endocytic network remains one of the least understood subcellular localisations involved in Hippo signalling (Figure 3). In Drosophila, Yki is regulated at endosomes by Myopic (Mop), the fly homologue of the endocytic regulator His-domain protein tyrosine phosphatase (HD-PTP). Mop interacts with Yki through a WW domain:PPxY interaction and negatively regulates a specific subset of Yki target genes [78]. A study of the Hippo pathway interactome revealed extensive connections to the endocytosis and vesicle trafficking machineries, and led to the identification of Leash, an α-arrestin molecule that controls lysosome-mediated Yki degradation [79]. However, whether Mop and Leash work concomitantly in the regulation of Yki remains unknown. In MDCK cells, the endosomal membrane protein Endotubin (EDTB) promotes endosomal localisation of AMOT. At low cell density, EDTB inhibits AMOT:YAP binding, thereby promoting YAP function. At high density, EDTB interacts with recycling tight junction proteins, allowing AMOT to sequester YAP in endosomes and to block its activity [80]. Interestingly, AMOT proteins are degraded by the Nedd4 family of ubiquitin E3 ligases, which are prominent endocytosis regulators [81, 82]. A recent report proposed that WASP-interacting protein (WIP) promotes YAP/TAZ stabilisation hospital settings by sequestering the β-catenin destruction complex in multivesicular bodies []. It is yet unclear if, like TOR (Target of Rapamycin), which senses amino hospital settings availability via the lysosome-associated Ragulator [84], Hippo pathway components associate with vesicular components in order to respond to external stimuli, or whether this represents a degradative route to prevent excess signalling. Despite the fact that the primary site of Yki/YAP function is the nucleus, where it associates with TEA domain transcription factors, surprisingly little is known about the dynamics of its nuclear import/export. Yki nuclear localisation is regulated by Wts-mediated phosphorylation, which blocks Yki nuclear localisation by 14-3-3-dependent (S168 phosphorylation, with S127 fulfilling a similar function for human YAP) [85, 86, 87] and 14-3-3-independent mechanisms (S111 and S250 phosphorylation, which have been associated with Yki nuclear export) [88]. However, the effect of Wts/LATS-mediated phosphorylation on nuclear import/export rates has not been directly assessed. Once nuclear, Yki/YAP is still likely subject to regulatory inputs. For instance, depending on its phosphorylation status, NF2 can localise to the nucleus, where it inhibits the CRL4DCAF1 E3 ubiquitin ligase [89, 90, 91]. In the absence of NF2, CRL4DCAF1 ubiquitylates and inhibits LATS, thus preventing YAP inhibitory phosphorylation [90]. Interestingly, cortical actin contractility dissociates NF2 from the cortex, allowing it to promote YAP/TAZ nuclear export via its NES (nuclear export sequence), providing a link between cell mechanics and NF2-mediated YAP/TAZ nuclear exclusion []. As described above, localisation of AMOTs is dynamic and dependent on their phosphorylation status. The AMOT p130 splice variant binds YAP in the nucleus, thereby preventing LATS-mediated phosphorylation and promoting YAP function [34]. AMOT hypophosphorylation promotes its nuclear entry, where it facilitates the YAP/TEAD association, whereas phosphorylation at S176 induces junctional localisation and YAP inhibition [36]. In the mouse heart, AMOTL1 junctional localisation is controlled by FAT4. In the absence of FAT4, AMOTL1 enters the nucleus promoting YAP activity []. Nuclear localisation of NF2, AMOT and LATS raises questions regarding the localisation of other Hippo components. MSTs can translocate to the nucleus in response to caspase cleavage [94, 95]. However, to what extent the activity of the core kinase cascade occurs in the nucleus versus the cell cortex remains an open question.