The interaction of cells with the surrounding extracellular matrix (ECM) affects many aspects of cell behavior, including the migration of cells, their growth and differentiation. In this lab, we have for many years studied the interactions of cells with ECM, using the focal adhesion as a model system. Focal adhesions are sites of tightest adhesion made to the underlying ECM by cells in culture. They serve a structural role, linking the ECM on the outside to the actin cytoskeleton on the inside. In addition, they are sites of signal transduction, initiating signaling pathways in response to adhesion. The major transmembrane proteins in focal adhesions are integrins, receptors for many ECM components. We have identified many of the proteins at the cytoplasmic face of focal adhesions. We are interested in how they link to integrins and the cytoskeleton, and in the roles of particular proteins in signaling at these sites.

The assembly of focal adhesions is regulated by the low molecular weight GTPase RhoA. We demonstrated several years ago that RhoA promotes assembly of focal adhesions by stimulating contractility of the actin filaments associated with integrins. This leads to clustering of integrins into focal adhesions. We are also interested in other members of the Rho family of GTPases, such as Rac and Cdc42. These proteins affect the organization of the actin cytoskeleton and regulate cell migration. The activity of these proteins is controlled by guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs), that are governed by signaling pathways downstream from various cell surface receptors. In a complex feedback loop, the activities of RhoA, Rac and Cdc42 are themselves regulated by integrin-mediated adhesion, and we are studying the mechanism by which this occurs.

Many of the focal adhesion proteins are also found in cell-cell junctions of the adherens type that are prominent in epithelia. However, the major transmembrane proteins in cell-cell junctions are cadherins rather than integrins. Cadherins interact with a set of cytoskeletal proteins known as catenins. Just as with focal adhesions, cadherin-mediated adhesions serve both structural and signaling functions. The assembly and maintenance of these adhesions is also regulated by Rho family proteins. Adherens junctions are disrupted in many cancers and this disruption contributes to the increased invasiveness and migration of tumor cells. In recent work, we have identified a catenin, p120catenin, which regulates the activity of Rho, Rac and Cdc42, when it is dissociated from cadherins. We have found that this catenin interacts with the Rho family GEF, Vav2. We have also found that cadherin-mediated adhesion depresses the activity of RhoA, but elevates Rac and Cdc42 activities.

One clinically important set of adhesive interactions that we are investigating is the trans-endothelial migration of leukocytes. This migration is a critical aspect of the inflammatory response. It involves a complex series of interactions between leukocytes and endothelial cells, and requires modulation of the cell-cell junctions made between adjacent endothelial cells. Multiple signaling pathways appear to be involved in this process, as do several members of the Rho family of regulatory proteins. In recent work, we have found that inhibiting RhoA blocks the ability of monocytes to migrate across an endothelial monolayer. We have shown that this is caused by an inability of the monocytes to detach their tails while migrating. As a consequence, the cells remained tethered in their rears, although the leading edge of the cell often continues to migrate for some distance, giving rise to cells with long drawn out tails.

Worthylake, RA, Lemoine, S, Watson, JM, and Burridge, K. RhoA is required for monocyte tail retraction during transendothelial migration. J. Cell Biol. 154: 147-160 (2001).

Arthur, WT, and Burridge, K. RhoA inactivation by p190RhoGAP regulates cell spreading and migration by promoting membrane protrusion and polarity. Mol. Biol. Cell 12: 2711-20 (2001).

Worthylake, RA, and Burridge,K. Leukocyte transendothelial migration: Orchestrating the underlying molecular machinery. Curr. Op. Cell Biol. . 13: 569-577 (2001).

Noren, NK, Niessen, CM, Gumbiner, BM, and Burridge, K. Cadherin engagement regulates Rho family GTPases. J. Biol. Chem.276: 33305-33308 (2001).

Sastry, SK, Lyons, PD, Schaller, MD, and Burridge, K. PTP-PEST controls motility through regulation of Rac1. J. Cell Sci. 115: 4305-16. (2002).

Arthur, WT, Ellerbroek, SM, Der, CJ, Burridge, K, and Wennerberg, K. XPLN, a guanine nucleotide exchange factor for RhoA and RhoB, but not RhoC. J. Biol. Chem. 277: 42964-72 (2002).

Wennerberg, K, Ellerbroek, SM, Liu, RY, Karnoub, AE, Burridge, K, and Der, CJ. RhoG signals in parallel with Rac1 and Cdc42. J Biol Chem. 277: 47810-7. (2002).

DeMali, KA, Barlow, CA, and Burridge, K. Recruitment of the Arp2/3 complex to vinculin: coupling membrane protrusion to matrix adhesion. J. Cell Biol. 159: 881-891 (2002).

Maddox, AS, and Burridge, K. RhoA is required for cortical retraction and rigidity during mitotic cell rounding. J Cell Biol. 160:255-65 (2003).

Worthylake, RA and Burridge, K. RhoA and ROCK Promote Migration by Limiting Membrane Protrusions. J Biol Chem. 278:13578-84 (2003).

Noren, NK, Arthur, WT, and Burridge, K. Cadherin Engagement Inhibits RhoA via p190RhoGAP. J Biol Chem. 278:13615-8 (2003).

Ellerbroek, SM, Wennerberg, K, and Burridge, K. Serine Phosphorylation Negatively Regulates RhoA in Vivo. J Biol Chem. 278:19023-31 (2003).

DeMali, KA, and Burridge, K. Coupling membrane protrusion and cell adhesion. J. Cell Sci. 116:2389-97 (2003).

Wennerberg, K, Forget, M-A, Ellerbroek, SM, Arthur WT, Burridge, K, Settleman J, Der CJ, Hansen, SH. Rnd proteins function as RhoA antagonists by activating p190 RhoGAP. Current Biology 13:1106-15 DeMali, KA, Wennerberg, K, and Burridge, K. Integrin signaling to the actin cytoskeleton Curr. Op. Cell Biol. 15: 582-592 (2003).

Ridley, AJ, Schwartz, MA, Burridge, K, Firtel, RA, Ginsberg, MH, Borisy, G, Parsons, JT and Horwitz, AR. Cell migration: Integrating signals from front to back. Science 302: 1704-1709 (2003).

Burridge, K and Wennerberg, K. Rho and Rac take center stage. Cell 116: 167-179 (2004).

Peterson, LJ, Rajfur, Z, Maddox, AS, Freel, CD, Chen, Y, Edlund, M, Otey, C, Burridge, K. Simultaneous stretching and contraction of stress fibers in vivo. Mol. Biol. Cell 15:3497-508 (2004).

Ellerbroek, S, Wennerberg, K, Arthur, W, Dunty, J, Bowman, D, DeMali, K, Der, C, Burridge, K. SGEF, a RhoG guanine nucleotide exchange factor that stimulates macropinocytosis. Mol.Biol. Cell 15:3309-19.

Avalos, AM, Arthur, WT, Schneider, P, Quest, AFG, Burridge, K, Leyton, L. Aggregation of integrins and RhoA activation are required for Thy-1-induced morphological changes in astrocytes. J. Biol. Chem. 279:39139-39145 (2004).

Wittchen, ES, van Buul, JD, Burridge, K, Worthylake, RA. Trading spaces: Rap, Rac, and Rho as architects of transendothelial migration. Curr. Op. Hematol. 12: 14-21 (2005).

Wittchen, ES, Worthylake, RA, Kelly, P, Casey, PJ, Quilliam, LA, and Burridge, K. Rap1 GTPase inhibits leukocyte transmigration by promoting endothelial barrier function. J. Biol. Chem. 11675-82 (2005).

van Buul JD, Anthony EC, Fernandez-Borja M, Burridge K, Hordijk PL. Proline-rich tyrosine kinase 2 (PYK2) mediates VE-cadherin-based cell-cell adhesion by regulating beta -catenin tyrosine phosphorylation. J. Biol. Chem. 280:21129-36. (2005).

Burridge, K. Foot in mouth: Do focal adhesions disassemble by endocytosis? Nature Cell Biol. 7: 545-7 (2005).

Garcia-Mata, R, Wennerberg, K, Arthur, WT, Noren, NK, Ellerbroek, SM and Burridge, K. Analysis of activated GAPs and GEFs in cell lysates. Methods Enzymol. 406:425-437 (2006).

Burridge, K. A break in the chain? Nature 440: 38-39 (2006).

Burridge, K, Sastry, SK, Sallee, JL. Regulation of cell adhesion by protein tyrosine phosphatases 1. Cell-matrix adhesion. J Biol Chem. In press (2006).

Sallee, JL, Wittchen, ES, Burridge, K. Regulation of cell adhesion by protein tyrosine phosphatases 2. Cell-cell adhesion. J. Biol. Chem. In press (2006).