Eukaryotic cells crawl over two-dimensional surfaces by coordinating the extension of their front, and the retraction of their rear, all while adhering the surface below them. In most animal cells the protrusion mechanism of the front is provided by the growth of rigid polymers that push the cell membrane forward in a region called the lamellipodium. The polymers, made of the protein actin, are a key component of the eukaryotic cytoskeleton. The actin cytoskeleton is dynamic and changes constantly. Actin polymerization is regulated by a host of other proteins that can enhance or cap growth, cause polymer branching, sever polymers and nucleate new filaments. In Xenopus(frog) tissue cells the amount of actin polymer near the cell front is anti-correlated with the cell protrusion rate1. In collaboration with the Vavylonis and Watanabe groups, we investigate the complicated relationship between actin polymer growth rate and the lamellipodial protrusion rate.
1 G. L. Ryan, H. Petroccia, N. Watanabe and D. Vavylonis, 2012. “Excitable actin dynamics in lamellipodial protrusion and retraction,” Biophys. J. 102:1493-1502 (2012).
Symmetry Breaking in Cells
Before cells can crawl over surfaces they must first establish a ‘front’ and a ‘rear.’ Unlike animals, individual eukaryotic cells do not have a fixed front and back, rather they dynamically reorganize themselves in response to their environment. In order to move, many cells must first undergo a symmetry-breaking event, or become polarized. In Dictyostelium discoideum (slime mold) this polarization is well characterized by the separation of a lipid, PIP3 (associated with the cell front), and a protein, PTEN (associated with the cell rear). In the absence of external cues, or pre-existing cytoskeletal markers the direction of the new front/back axis is random, and dynamic2,3. Under certain conditions, the cells can even switch from ‘all front’ to ‘all back’ states3. We are currently investigating the interactions required to generate this dynamic phase switching and separation of the front and back signals.
2Arai Y, Shibata T, Matsuoka S, Sato MJ, Yanagida T, Ueda M. Self-organization of the phosphatidylinositol lipids signaling system for random cell migration. Proc Natl Acad Sci. 2010. 107:12399–404.
3Gerisch G, Schroth-Diez B, Muller-Taubenberger A, Ecke M. PIP3 Waves and PTEN Dynamics in the Emergence of Cell Polarity. Biophys J. 2012. 103:1170–8.
Nucleation During Endocytosis
Cells take in large molecules from their surrounding environment by engulfing them via a process called endocytosis. Although there are multiple possible pathways for endocytosis, one common mechanism depends on the protein clathrin. Clathrin coordinates with other proteins to first form rafts or patches on the cell membrane, which then deforms and pinches off to engulf the cargo. Clathrin patches have been imaged in live cells experimentally, and it has been observed that not all patches result in a successful intake of cargo. Some clathrin patches form and disperse quickly, while still others persist for longer times before dispersing without cargo delivery4. We are investigating the nucleation events leading to patch formation to better understand the distributions of short-lived and long-lived unsuccessful patches.
4Loerke D, Mettlen M, Yarar D, Jaqaman K, Jaqaman H, Danuser G, et al. Cargo and Dynamin Regulate Clathrin-Coated Pit Maturation. 2009. PLoS Biol. 7.