Membrane tension homeostasis of mammalian cells

-mechanosensitive study of the area regulation of adherent cells

by Bastian Rouven Brückner
Doctoral thesis
Date of Examination:2016-06-03
Date of issue:2017-04-12
Advisor:Prof. Dr. Andreas Janshoff
Referee:Prof. Dr. Andreas Janshoff
Referee:Prof. Dr. Sarah Köster
Persistent Address: http://dx.doi.org/10.53846/goediss-6246

 

 

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Abstract

English

Although a cell is the smallest unit that is viable on its own, its construction and processes are of great complexity. Many functions, which keep a cell alive, are only poorly understood up to now. Plasma membrane tension is supposed to be a key player in controlling a variety of cellular functions, such as cell migration, division or apoptosis. In order to investigate how membrane tension is controlled by enlargement or reduction of excess surface area, polarised epithelial cell monolayers were studied. Madin-Darby canine kidney cells (MDCK II) serve as a cellular model system to unravel the question how tension-driven membrane surface area regulation is realised to accommodate tension changes. The mechanical behaviour of cells was measured by force-indentation experiments followed by membrane tether extraction. These experiments carried out by atomic force microscopy (AFM) in conjunction with the application of a powerful mechanical model, the tension model, provide a deep insight into the interplay between tension control and excess surface area regulation. The performance of the tension model in reproducing the mechanical behaviour of plasma membranes was investigated. In order to provoke tremendous changes in the mechanical behaviour of cells, the role of the cytoskeleton for tension homeostasis was unravelled. Since the membrane-cytoskeleton attachment dominates membrane tension, the importance of the linker molecule ezrin was studied by ezrin depletion. Cells stabilise each other in a confluent monolayer by forming tight connections to neighbouring cells. Tight junctions mediated by zonula occludens-1 and adherens junctions built up by E-cadherins are important cell-cell connections in MDCK II cells. The role of both proteins for tension homeostasis was clarified by experiments disrupting one or the other connection. Furthermore, the role of endocytosis, a process driven by membrane surface area changes and cytoskeleton rearrangement, was investigated in terms of tension and membrane surface area adjustment. In summary, this study allows us to draw a comprehensive picture of membrane tension homeostasis by surface area regulation. Our data demonstrate the importance of an intact cytoskeleton and an intact membrane-cytoskeleton interface for tension maintenance. Alterations of the cellular morphology of the cell monolayer do not necessarily provoke an altered mechanical behaviour of the cells, as long as the cytoskeleton is unaffected and a stable contact to binding partners is maintained.
Keywords: Atomic Force Microscopy; Cell Mechanics; Tension Homeostasis; Cell Membrane; Epithelial Cells
 
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