actin filament organization in a migrating fibroblast
The Cramer laboratory studies cytoskeleton mechanisms in cell polarity and cell migration. Cell migration is essential for life; required throughout embryo development, and for tissue repair and immunity in both the embryo and the adult. It also contributes to several important diseases, including inflammatory diseases, mental retardation and the spread of cancer. Determining molecular mechanisms controlling cell migration thus promises to provide effective therapeutic strategies for treating disease. We are interested in how migrating cells reach new territory and how they polarize to activate motility. We focus on determining actin cytoskeleton-based mechanical forces that drive these processes analysed at high spatiotemporal resolution, mainly using primary fibroblasts as model systems.
Model for how cells initiate cell polarization from the cell front and cell rear.
To initiate directed migration a cell must break cell symmetry and form a cell front and cell back, at opposing ends of the cell along an axis approximately aligned with the direction of locomotion. In the main, actin-based mechanical forces first shape - and then propel - the cell front and cell rear forwards in the direction of migration. We recently discovered that constitutive cell migration is initiated from the cell rear, not the cell front, the common view. Our findings provide a new perspective on how cells polarize. To determine precise mechanism, we are currently investigating the precise temporal order and spatial organization of actin-based mechanical forces likely to be important for triggering initial breaking of cell symmetry from the cell rear.
Ratio of actin monomer and actin filaments in a migrating fibroblast.
In collaboration with Prof. Jim Bamburg’s lab at Colorado State University we were the first to identify that actin depolymerization and the ADF/cofilin family of proteins that regulate actin depolymerization are necessary for cell polarization in migrating cells. This was subsequently also discovered for several other migrating cell types by other labs, thus increasingly revealing the importance of these proteins for triggering migration. Our current endeavours focus on our finding that ADF/cofilin controls the formation of oriented actomyosin II bundles in the cell body that in turn specifies the formation of the front and back of the cell around the ends of the bundles during cell polarization. We are investigating the mechanism of formation of oriented actin bundles and the spatiotemporal regulation of ADF/cofilin in triggering polarization and maintaining migration.
We found that the ratio of monomer:polymer is low (blue) within the cell front. Consistent with this, we also discovered that actin filaments are immediately recycled (depolymerised) to provide monomer fuel to sustain actin polymerization-driven cell protrusion in migrating cells.