טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentLieber Arnon
SubjectThe Dynamic Interplay between Membrane Tension and the
Spatiotemporal Organization of the Cytoskeleton in
Rapidly Moving Cells
DepartmentDepartment of Nanoscience and Nanotechnology
Supervisor Professor Kinneret Keren
Full Thesis textFull thesis text - English Version


Abstract

The ability of cells to move and remodel their shape stands at the center of various biological processes such as embryonic development, the immune response and wound healing. While the main players that participate in the motility process and the basic biochemical mechanisms involved are largely known, the intricate interplay between these biochemical reactions and biophysical processes is still not well understood. In particular, the role of membrane tension in the motility process and its dynamic interplay with the actin cytoskeleton is still unclear. Here we addressed this intricate interplay by an integrative experimental approach which considers both the cytoskeleton and the membrane, using fish keratocytes as a relatively simple model system. We combine measurements of membrane tension in motile keratocytes with various perturbations of their motility machinery. We find that enlargement of the cell surface area has only minor effects on membrane tension and on cell movement. However, modulation of the cytoskeletal forces has a substantial influence on tension: reduction of the actin-pushing forces along the cell’s leading edge leads to a significant decrease in membrane tension, whereas increasing the strength of adhesion and/or decreasing myosin-induced contraction lead to higher tension. These findings suggest that, in contrast to the prevailing views in the field, membrane tension in rapidly moving cells is primarily determined by a mechanical force balance between the cell membrane and cytoskeletal forces rather than regulation of plasma membrane surface area or the membrane-to-cytoskeleton adhesion level. In addition, we characterize the membrane tension distribution in rapidly moving keratocytes, showing that during steady movement a tension gradient forms along the axis of motion, whereby the tension at the leading edge is ~20% higher than at the rear.  This tension gradient is likely due to frictional forces due to relative movement between the membrane and cytoskeleton-attached membrane proteins. We perform similar measurements on lamellipodial fragments of keratocytes, which consist of little else than a treadmilling lamellipodial actin network enclosed by a membrane, yet move with similar speed and persistence to whole cells. These fragments show membrane tension values and a front-to-rear tension gradient similar to that of whole cells. Finally, we present some examples illustrating the dynamic nature of membrane tension and how it can change in response to variations in the motility machinery of the cell or its environment. Together, our results highlight the role of membrane tension as a dynamic global regulator of cell behavior, which integrates mechanical inputs across the membrane and regulates cell boundary dynamics.