Cytoskeletal force generation and transduction of leukocytes

Cell migration is a universal feature of all metazoan life and crucially involved in most developmental, homeostatic and pathological processes. In order to move, cells transduce intracellular forces to the extracellular environment. Hence, cell migration is essentially a mechanical phenomenon and knowledge about force coupling at the interface between the intracellular cytoskeleton and the extracellular substrate is rapidly increasing. However, almost all efforts to understand cellular locomotion focused on the classical textbook paradigm that has been established by Abercrombie in the 1970s: here cells migrate over two dimensional surfaces by assembling substrate adhesions at the leading edge and disassembling them at the trailing edge in a cyclic process termed haptokinesis. Although there is no doubt that this working model led to fundamental insights it also turned out that haptokinesis represents just one out of many different locomotion strategies. Our recent work demonstrated that especially leukocytes, the class of animal cells that migrates with highest speed and efficiency, shift within an enormous space of biophysical modes to generate and transduce force. Although all these strategies depend on the actomyosin cytoskeleton, the common mechanical principle that allows such plasticity has hardly been explored. Goal of this proposal is to establish a combined molecular and mechanical model of how leukocytes migrate in a physiological interstitium. Key questions to be addressed are: 1) Which molecular mechanisms drive the nucleation and elongation of actin filaments and thereby force generation at the leading edge? How are these pathways modulated when the cell shifts between extracellular environments of varying geometry and adhesive properties? 2) Apart from polymerization cytoskeletal mechanics are largely determined by depolymerising factors, myosins and actin-crosslinkers. How do these affect the morphodynamic plasticity of leukocytes? 3) Force transduction to the extracellular environment can occur via transmembrane receptors or by other means like deformations of the cell body. How is deformation mechanically translated into locomotion? Technically, these questions will be addressed by employing advanced live cell imaging in combination with artificial environments, which are engineered using microfluidic and micropatterning approaches. Cellular manipulation will include genetic as well as pharmacological tools and experiments will mainly be performed with primary cells. The principle findings made in artificial environments will ultimately be challenged in living tissues to guarantee that the environmental parameters stay in a physiological range. This integrated and multidisciplinary approach to study the plasticity programs of a "professionally motile" class of cells will not only provide fundamental information regarding immune cell motility but is aimed to aid the understanding locomotion principles of other cell types including metastatic cancers.

Locomotion is one of the most evident manifestations of life and the development and maintenance of most multicellular organisms crucially depends on the ability of single cells to actively migrate. Depending on the cell type locomotion can come in many different forms: some cells are tightly incorporated into the tissue context while they migrate, while others are not. We investigate primarily cells of the immune system. Immune cells are extremely motile because it is part of their surveillance function that they have to quickly access affected tissues. In fact, the cumulative distance all immune cells contained within a human body crawl is at least 120 000 kilometres per hour. We found that immune cells achieve this goal by using a very promiscuous type of crawling, where they hardly interact mechanically with the connective tissue or other cells. They rather use a very unspecific form of interaction with their environment and never change the tissue architecture by digestion or remodelling, as do other migrating cells. This mechanically autonomous all-terrain locomotion allows the immune cells to move through every type of organ. We uncovered the mechanical principles behind the different types of migration and found that they are all variations of one unifying mechanical scheme of the actin cytoskeleton generating and transducing forces. Mechanical autonomy leaves all the orchestrating functions to extracellular guidance cues, which direct the cellular traffic by forming chemoattractive gradients. We were the first to quantitatively study the build-up, distribution and function of such gradients in tissues and investigated fundamental aspects, how these gradients are interpreted by the migrating cells. While our work is driven by the curiosity to understand how a cell achieves the immensely complex task to crawl, our findings are usually tightly linked to the understanding of human disease. Notably, the autonomous type of immune cell movement we discovered is also adopted by several types of extremely aggressive metastatic cancer cells.