I have been fascinated by the molecular mechanisms of cell migration ever since I made my first time-lapse movie. Although many of the molecules involved in this phenomenon have been identified, the integration and regulation of these components are incompletely understood. Cell migration is essential for many physiological processes, such as wound healing, immune response, and morphogenesis, and plays a significant role in pathological processes, such as cancer metastasis. Although metastasis is by far the most clinically significant step in cancer progression, no therapeutic agents that directly target this step are currently available to oncologists. This largely reflects the fact that we simply do not understand cell migration sufficiently to target this process.
All cells depend on the dynamic reorganization of the actin cytoskeleton in order to migrate. Actin exists in two states: monomeric (G-actin) and polymeric (F-actin). In purified preparations, under the appropriate conditions, G-actin can polymerize spontaneously into filaments. Cells have a remarkable array of proteins that tightly regulate actin polymerization in vivo so that it occurs only at appropriate places and times. Once filaments have formed, they can be cross-linked or bundled into a variety of forms for use in specific cellular structures. Polymerization, filament reorganization, and depolymerization are all regulated by signal transduction pathways that allow cells to respond to extracellular cues by remodeling their actin cytoskeleton appropriately.
Migrating cells execute a four-step cycle to move forward. The first step is to extend a new protrusion, called a lamellipodia, in the direction of movement. A large increase in actin polymerization is seen at this leading edge of the cell and arises from the creation of a complex, branched network of filaments. New actin polymerization is thought to provide the driving force to push the membrane forward. The second step in the translocation process is to anchor the new process to the substratum through structures called focal adhesions. Cells have multiple cell surface proteins that can engage the extracellular matrix in a reversible manner to serve as anchor points for the next step. Once the cell has extended and anchored a new process, the cell body must slide forward over the new process by traction. This step is thought to involve a contraction at the rear of the cell that squeezes the cell body forward. In the final step, the focal adhesions at the rear of the cell have to be dissolved to allow the cell to continue forward. In order for cells to migrate, they must actively and coordinately remodel their actin cytoskeleton and adhesion systems on multiple time scales, ranging from milliseconds to minutes.
My lab focuses on coronins, a family of highly conserved WD-repeat proteins that regulate cell migration both at the leading edge and at adhesive structures called focal adhesions. Type I coronins (such as coronin 1B) are enriched in the rear of branched actin networks found at the leading edge and other cellular locations. Our lab recently showed that coronin 1B remodels branched actin networks at the leading edge of motile cells into a new network containing more variable and possibly more flexible branches. This remodeled network is destined either for disassembly or incorporation into other actin-containing structures in the cell. Type II coronins (such as coronin 2A) are excluded from the branched network and accumulate instead along actin stress fibers and at focal adhesions. At focal adhesions, coronin 2A controls the disassembly process through local regulation of the actin filament severing protein cofilin.
Our current research program involves studying cell migration and coronins with three distinct approaches: First, we are using a combination of biochemical, biophysical, and single-molecule imaging approaches to elucidate the molecular mechanism of coronin function. Not only are these studies uncovering new insights into the regulation of coronins and actin, they are essential if we are to interpret the results of coronin manipulation in vivo. Second, we are using gene manipulation strategies (e.g., RNAi) and live-cell imaging to study the function of coronin proteins at the cellular level. We are particularly interested in how coronins interact with other components in cell migration pathways. In addition, we are generating correspondence maps to compare the location of coronins and other components in cells migrating on two-dimensional (2D) surfaces and in 3D matrices. Our third approach is to use intravital multiphoton microscopy to understand the migration of cells inside of animals. Specifically, we are developing a model of melanoma invasion in the ear skin of live mice to better understand cell migration in a physiologically relevant milieu.