Final Report Summary - LEUKOCYTEFORCES (Cytoskeletal force generation and force transduction of migrating leukocytes) In the project we investigated fundamental principles of cell migration. Our focus was on immune cells, the most motile cells in multicellular organisms. However, the processes we investigated are highly conserved and therefore are equally relevant for cells migrating in the context of embryology, cancer and regeneration. The main focus of the project was: how do the cells generate intracellular forces, how are these forces transduce to the extracellular environment and how are the cells guided within complex tissues?We could show for the first time in tissues how cells are guided along a chemokine gradient within tissues. In this study we used a quantitative imaging approach, which revealed that the instructive gradients were not soluble but bound to connective tissue (Weber et al, Science 2013). We also uncovered the molecular details of a new way how a guidance cue receptor can distinguish between a soluble and an immobilized chemokine (Kiermaier et al, Science 2016). Together with our previous finding that leukocytes do not rely on adhesive interactions to migrate through three dimensional tissues (Lämmermann et al, Nature 2008), this turned around the traditional view of guided cell migration, where the cells are guided by soluble factors but pull themselves along immobilized connective tissue scaffolds. We investigated the cytoskeletal mechanics underlying such interstitial locomotion and found that the leading front of the cell is not, as previously assumed, the structure that pulls the cell forward, but rather a sensing organelle that makes the directional choices. Without this leading “lamellipodium” cells migrated even faster but were not able to turn towards the gradient of guidance cue (Leithner et al. Nat Cell Biol 2016). A quantitative study of cytoskeletal dynamics further revealed a fundamental feature of directed cell motility: the faster a cell is the more straight it migrates. We showed that this was due to signaling molecules, which are flushed to the rear end of the cell by the flow of the cytoskeleton (Maiuri et al, Cell 2015).Our findings are based on a combination of top down investigations of (genetically manipulated) cells migrating in physiological tissues and a bottom up reconstitution approach, where we re-engineered isolated aspects of the environment. Here, we established new approaches in microfabrication, surface chemistry and cell culture (Schwarz et al, Curr Biol, 2017; Schwarz et al, Sci Rep, 2016; Vaahtomeri et al, Cell Reports 2017). A major objective of the project was to find out how leukocytes can transduce intracellular forces, without using transmembrane receptors to couple the cytoskeletal drag across the plasma membrane. We could solve this question by re-engineering topographical aspects of the microenvironment, while controlling the chemical composition. We were able to show that leukocytes can locomote by the sheer deformation of the cell body (Reversat et al. in preparation). This finding completes the picture of “amoeboid” migration, which essentially describes the fact that some cells constantly change their shape while migrating. We show that these shape changes are also causative for migration.
