CDS Lecture Series


Arpita Upadhyaya
Department of Physics
University of Maryland at College Park

Forces generated by actin polymerization: from lipid vesicles to immune cells
A fundamental attribute of living cells is their ability to move and generate forces. The polymerization of the protein, actin, appears to be the source of the propulsive force for eukaryotic cell motion. Actin filaments are semi-flexible polymers that are cross-linked together to form a network. One outstanding question is how the forces generated depend on the structure and mechanical properties of actin networks and the physical properties of the cellular membrane. While the alphabet soup of proteins that initiate and control actin polymerization has been scrupulously characterized, it is not clear how this generates a force to push. We have reconstructed movement using phospholipid vesicles as model cell membranes in order to probe the polymerization forces. Actin polymerization provides the driving force for membrane movement during cell substrate interactions in diverse physiological contexts. One such example is the spreading of T-lymphocytes on antigen-presenting cells resulting in the formation of the immune synapse. While the signaling events of this process have been studied in some detail, the physical mechanisms that determine the kinetics of immune cell spreading are not well understood. We quantitatively characterize the spreading kinetics of T-cells while visualizing actin dynamics and measuring the forces generated during this process. Actin filaments spontaneously organize into a variety of structures including traveling waves and mobile clusters. Membrane deformations induced by wavelike organization of the cytoskeleton may be a general phenomenon that underlies cell-substrate interactions and cell movements.

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