The Spectrin cytoskeleton regulates the Hippo signalling pathway

Abstract

The Spectrin cytoskeleton is known to be polarised in epithelial cells, yet its role remains poorly understood. Here, we show that the Spectrin cytoskeleton controls Hippo signalling. In the developing Drosophila wing and eye, loss of apical Spectrins (alpha/beta-heavy dimers) produces tissue overgrowth and mis-regulation of Hippo target genes, similar to loss of Crumbs (Crb) or the FERM-domain protein Expanded (Ex). Apical beta-heavy Spectrin binds to Ex and co-localises with it at the apical membrane to antagonise Yki activity. Interestingly, in both the ovarian follicular epithelium and intestinal epithelium of Drosophila, apical Spectrins and Crb are dispensable for repression of Yki, while basolateral Spectrins (alpha/beta dimers) are essential. Finally, the Spectrin cytoskeleton is required to regulate the localisation of the Hippo pathway effector YAP in response to cell density human epithelial cells. Our findings identify both apical and basolateral Spectrins as regulators of Hippo signalling and suggest Spectrins as potential mechanosensors.

Integrin signalling regulates YAP and TAZ to control skin homeostasis

“…Importantly, our model is easily reconciled with a possible role of YAP as a mechanosensor in epithelial tissues in vivo (Dupont et al, 2011). Mechanical force has been proposed to modulate signalling by both the apical Crb-Mer/Ex-Kibra-Sav system in Drosophila (Fletcher et al, 2015;Rauskolb et al, 2014) as well as the basal integrin-Src system in mammalian cell culture (reviewed in Humphrey et al, 2014;Lawson and Burridge, 2014). In the early mouse pre-implantation embryo, cortical tension is higher in outer cells than inner cells, leading to nuclear YAP in outer cells despite the presence of an apical domain (Anani et al, 2014;Kono et al, 2014;Nishioka et al, 2009).…”

“…By contrast, columnar epithelia differentiate an apical domain that induces YAP relocalisation to the cytoplasm via apical CRB3-MER-KIBRA-SAV signals, which are known to activate the MST and LATS kinases to promote YAP phosphorylation and cytoplasmic retention despite contact with the basement membrane (Figs 6,7, (Baumgartner et al, 2010;Chen et al, 2010;Fletcher et al, 2015;Genevet et al, 2010;Hamaratoglu et al, 2006;Ling et al, 2010;Sun et al, 2015;Szymaniak et al, 2015;Varelas et al, 2010;Yin et al, 2013;Yu et al, 2010). The CRB3-MER-KIBRA-SAV complex appears to be absent in squamous epithelia, which never differentiate a true apical domain, leaving basement membrane contact as the sole regulatory mechanism (Fig.…”

“…Thus, columnar cells must employ an additional mechanism to promote YAP localisation to the cytoplasm. An obvious candidate is the expression of a differentiated apical plasma membrane domain in columnar epithelial cells, because apical proteins associated with Crumbs (CRB3) are well known to induce Hippo signalling (MST-LATS signalling) to drive YAP to the cytoplasm (Chen et al, 2010;Fletcher et al, 2015;Ling et al, 2010;Szymaniak et al, 2015;Varelas et al, 2010).…”

Mechanisms of Hippo pathway regulation

“…This subject has been comprehensively reviewed elsewhere (Schroeder and Halder 2012;. Recent studies show that loss of Spectrin, a contractile protein at the cytoskeleton-membrane interface, also generates Hpo mutant-like tissue overgrowth phenotypes in Drosophila wings and eyes, which are likely due to dysregulated cytoskeleton tension Fletcher et al 2015;Wong et al 2015).…”

Yes-associated protein in the liver: Regulation of hepatic development, repair, cell fate determination and tumorigenesis

“…In Drosophila or non-hepatic mammalian cells, multiple upstream inputs including extracellular matrix area and stiffness [72,73], cell density [54], cell detachment [74], Factin cytoskeleton [75], spectrin cytoskeleton [76,77], and cellular energy level [78,79] were shown to regulate YAP nuclear accumulation dependent or independent of the canonical Hippo signaling. Detailed reviews of these upstream YAP regulators can be found elsewhere [80,81].…”