PI3K signalling in the maintenance of epithelial cell structure: Analysis of E-cadherin-based adhesion and cell height

Jeanes, Angela (2011). PI3K signalling in the maintenance of epithelial cell structure: Analysis of E-cadherin-based adhesion and cell height PhD Thesis, Institute for Molecular Bioscience, The University of Queensland.

       
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Author Jeanes, Angela
Thesis Title PI3K signalling in the maintenance of epithelial cell structure: Analysis of E-cadherin-based adhesion and cell height
School, Centre or Institute Institute for Molecular Bioscience
Institution The University of Queensland
Publication date 2011
Thesis type PhD Thesis
Supervisor John Hancock
Total pages 176
Total colour pages 21
Total black and white pages 155
Subjects 11 Medical and Health Sciences
Abstract/Summary Epithelia are the interface between the outside environment and us, and form very important tissue structures within organs. As such, the maintenance of the epithelial barrier and epithelial structural integrity is of the utmost importance. This requires epithelia to maintain their overall shape and size, the appropriate cell-cell and cell-matrix adhesions, as well as inter- and intra-cellular communication. This project investigates the relative contributions of E-cadherin-based cell-cell adhesion, signalling by the lipid kinase, phosphoinositide-3-OH-kinase (PI3K), and the actin cytoskeleton in epithelial morphogenesis. E-cadherin is a critical determinant of epithelial morphogenesis, by facilitating cell-cell adhesion, co-ordinating actin cytoskeletal organisation, and activating intracellular signalling pathways. One such pathway that E-cadherin is known to activate is that of PI3K. The major product of class I PI3K is the phosphoinositide PtdIns(3,4,5)P3 (PIP3). In the current study, PIP3 was identified at both nascent and mature epithelial cell-cell contacts, sites of E-cadherin adhesion. PIP3 was readily depleted from sites of cell-cell contact upon inhibition of PI3K activity, demonstrating that this pool of PIP3 is dynamic. Previous work in the Yap laboratory has shown that PI3K signalling was required for E-cadherin adhesive activity in the early stages of epithelial biogenesis. This project extended these findings into more highly differentiated epithelial cell models and in the context of an established monolayer – representing a later stage of epithelial biogenesis. Interestingly, PI3K seems to play a milder role in E-cadherin activity in a matured monolayer, which was reflected by no change in E-cadherin surface or total protein levels, or its adhesive strength, or by its mobility on the cell surface, as measured by FRAP. However, PI3K signalling played an important role in maintaining epithelial cell shape. Inhibition of PI3K-PIP3 signalling decreased cell height with a concomitant increase in cell width. This change in cell shape was independent of cell number and was probably not due to simply a change in cell volume. As a potential regulator of epithelial apico-basal polarity, it was surprising to find that PIP3-induced changes in cell height were not due to altered apico-basal polarity. The actin cytoskeleton is a major regulator of cell shape and PI3K is known to operate in many pathways, which converge on actin cytoskeletal reorganisation. The actin-binding motor protein myosin II has been implicated in epithelial biogenesis and is an important regulator of cell shape in a wide range of systems including migrating cells and epithelial tissues. This made myosin II an attractive candidate as a downstream effector of PI3K-PIP3 signalling in the maintenance of cell shape and height. Surprisingly, by a number of criteria, myosin II appeared to be unaffected by PI3K inhibition. This was evident by myosin II localisation of both the heavy chains and the phosphorylated light chains, and quantification of phospho-myosin protein levels. Direct inhibition of myosin II did not phenocopy the effects of PI3K inhibition on cell height, which suggested that myosin II was not a major contributor to cell height in this cellular system. Finally, the Rho family GTPase Rac1 was investigated as a downstream effector of PI3K for the maintenance of cell height. Rac1 and PI3K are often linked in a common signalling pathway and Rac1 affects actin, particularly through the organisation of branched actin fibres and lamellipodial formation. Inhibition of PI3K-PIP3 signalling did not affect Rac1 localisation; however, direct inhibition of Rac1 did lead to a decrease in cell height, suggesting that Rac1 may play at least some part in epithelial cell height maintenance.
Keyword Epithelia
Cell structure
Phosphoinositide-3-OH-kinase
PI3K
Adhesion
E-cadherin
Actin Cytoskeleton
Myosin II
Rho GTPases

 
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Created: Fri, 17 Jun 2011, 12:46:44 EST by Ms Angela Jeanes on behalf of Library - Information Access Service