Kampyli, Charis; (2020) Investigation into the cell biological roles of class II PI3K-C2β. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The family of PI3Ks generate PI3P, PI(3,4)P2 or PI(3,4,5)P3. These lipids control the function and the localisation of several downstream effectors which regulate many intracellular processes including signalling and membrane traffic (Vanhaesebroeck et al., 2010; Falasca and Maffucci, 2012; Vanhaesebroeck et al., 2012; Maffucci and Falasca, 2014; Bilanges et al., 2019). In mammals, there are eight PI3K isoforms which are classified into class I, II and III PI3Ks (Vanhaesebroeck et al., 2010; Banfic et al., 2009; Maffucci and Falasca, 2014; Falasca and Maffucci, 2012; Bilanges et al., 2019). Among them, class I PI3Ks have been most broadly studied and are often mutated in cancer. Class II and III PI3Ks produce PI3P in intracellular membranes and, in the case of class II PI3Ks, also PI(3,4)P2. These PI3Ks have emerged as regulators of intracellular vesicular traffic (Posor et al., 2013; Marat and Haucke, 2016; Franco et al., 2014; Yoshioka et al., 2012). However, the understanding of their roles in vivo is still limited. The aim of this study was to increase the currently incomplete knowledge on the biological roles and molecular mechanisms of action of the class II PI3K-C2β isoform. For this purpose, siRNA-mediated depletion of PI3K-C2β in HeLa cells and MEFs derived from a PI3K-C2β kinase-dead knock-in (KI) mouse model engineered by the host laboratory prior to this study were used. The latter strategy allows the assessment of the kinase-dependent functions rather than possible scaffolding functions of the targeted PI3K, without affecting the expression levels of the targeted or non-targeted PI3K isoforms (Alliouachene et al., 2015). Here I show that PI3K-C2β regulates focal adhesion (FA) dynamics and cell migration. PI3K-C2β depletion or inactivation leads to an increased number of FAs, delayed FA disassembly and impaired cell migration. Additionally, in live cells PI3K-C2β accumulates at disassembling FAs upon blebbistatin treatment, a tool to induce FA dismantling. Moreover, a yeast-two hybrid screen identified DEPBC1B, a promoter of FA disassembly (Marchesi et al., 2014) as a potential PI3K-C2β interactor. Immunoprecipitation assays confirmed this interaction. I also provide evidence that PI3K-C2β interacts with talin, a well-characterised FA component. The mechanistic characterisation of a potential interplay between PI3K-C2β, talin and DEPDC1B in the regulation of FA dynamics requires further investigation.

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