br Conclusion br Acknowledgements This
Acknowledgements This study was supported by the China Postdoctoral Science Foundation (No.2015M581817), the National Research Council of Science and Technology Support Program (2015BAD03B05-06), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (No. 280100745113). The authors also sincerely thank all the members of Feng Wang's laboratory who contributed to sample collection and technical assistance during the preparation of the study.
The Hippo core kinase cassette: Side A Genetic screens in Drosophila led to the identification of the main players involved in Hippo signalling and its role in tissue growth, regeneration and stem cell biology . Together with elegant genetic and biochemical studies in mammalian systems, a picture emerged of the Hippo pathway core machinery, consisting of the serine/threonine kinases Hippo (Hpo; mammalian MST1/2) and Warts (Wts; LATS1/2) and their respective adaptor proteins Salvador (Sav; SAV1/WW45) and Mob as tumour suppressor (Mats; MOB1A/1B) . Hpo and Wts function in a kinase cascade, which transduces multiple upstream signals (reviewed in [1, 2]), ultimately leading to phosphorylation and inhibition of the pro-growth transcriptional co-activator Yorkie (Yki; YAP and TAZ in mammals)  (Figure 1a,b and Box 1). In both Drosophila and mammals, Wts/LATS-mediated phosphorylation leads to Yki/YAP cytoplasmic retention by 14-3-3 proteins and subsequent downregulation of downstream targets. In mammals, LATS phosphorylation can also prime YAP/TAZ for CK1δ/ɛ-mediated phosphorylation and β-TrCP-induced protein degradation [3, 4]. Yki/YAP/TAZ target 2-Methoxyestradiol globally promote tissue growth and pluripotency (reviewed in ) and, for this reason, Yki and YAP/TAZ are tightly regulated by a complex machinery acting at multiple subcellular localisations. Here, we review new advances to our understanding of how this regulation is achieved.
Compartmentalisation of Hippo signalling: the apical membrane The importance of apical–basal polarity and tissue architecture in Hippo signalling has been a long-standing paradigm in the field  (Figure 2). Recent Drosophila studies have confirmed that the sub-apical domain is a critical site for Yki inhibition and Hpo/Wts activation . Several upstream components of the Drosophila Hippo pathway are recruited to the apical cortex by transmembrane proteins such as the apical polarity determinant Crumbs (Crb), which recruits and controls the stability of the FERM domain protein Expanded (Ex) [8, 9, 10, 11, 12], Echinoid (Ed), which recruits Sav , and the giant cadherin Dachsous, which recruits the atypical myosin Dachs [14, 15]. Thus, key Hippo signalling events occur at the apical membrane. Firstly, Sav is required for efficient membrane recruitment of Hpo . Secondly, the apically-localised FERM domain proteins Merlin (Mer) and Ex recruit Wts, which is activated through Hpo-mediated phosphorylation [7••, 16•] and allosterically by its adaptor Mats . Thirdly, the WW domain protein Kibra recruits Mer, Sav and Hpo to a medial apical pool distinct from the sub-apical cortex where most of Kibra and Mer are localised . This Kibra:Mer medial pool acts independently of Ex and is negatively regulated by Crb . However, the relative importance of the medial and cortical pools for Hpo/Wts activation remains to be addressed . The apical membrane is also a key site of Wts repression, either via the LIM domain protein Ajuba, which traps Wts in an inactive complex at the adherens junction/apical domain boundary [7••, 19], or by Dachs-induced and Zyxin-induced Wts degradation . Finally, the apical domain has been proposed as a site of kinase-independent Yki activation, where Ex-mediated sequestration of Yki can be antagonised by Zyxin [21, 22, 23]. In mammals, CRB3 localises to the tight junctions, forming a complex containing the Hippo regulators AMOT, NF2/Merlin, Kibra and FRMD6 (an Ex-related protein) [24, 25, 26]. The angiomotin family (including AMOT and AMOTL1/2) can inhibit YAP function through direct binding [25, 27, 28, 29], and by promoting LATS/NF2 association . CRB3 promotes the AMOT:YAP interaction at junctions , as well as LATS-mediated YAP phosphorylation , thereby inhibiting YAP nuclear localisation. While initial observations showed that cytoplasmic AMOTs inhibit YAP function, subsequent reports have also proposed that AMOTs can positively regulate YAP in the nucleus [32, 33, 34]. Interestingly, post-translational modifications of AMOT influence its effect on YAP. AMOT is phosphorylated at S176 by LATS, resulting in its translocation to junctions, association with NF2 and YAP, and inhibition of YAP activity [35, 36].