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.

Burridge, K, Sastry, SK, Sallee, JL. Regulation of cell adhesion by protein tyrosine phosphatases 1. Cell-matrix adhesion. J Biol Chem. 281:15593-96 (2006)

Sallee, JL, Wittchen, ES, Burridge, K. Regulation of cell adhesion by protein tyrosine phosphatases 2. Cell-cell adhesion. J. Biol. Chem. 281:16189-92 (2006)

Sastry, SK, Rajfur, Z, Liu, BP, Cote, JF, Tremblay, M, Burridge, K. PTP-PEST couples membrane protrusion and tail retraction via VAV2 and p190RhoGAP. J Biol Chem. 281:11627-36 (2006)

Burridge, K, Doughman, R. Front and Back by Rho and Rac. Nat. Cell Biol. 8: 781-782 (2006)

Sandquist, JC, Swenson, KI, DeMali, KA, Burridge, K, Means AR. Rho kinase differentially regulates phosphorylation of nonmuscle myosin II isoforms A and B during cell rounding and migration. J. Biol. Chem. 281:35873-83 (2006)

DeMali, KA, Jue, AL, Burridge, K. IpaA targets beta 1 integrins and Rho to promote actin cytoskeleton rearrangements necessary for Shigella entry. J. Biol. Chem. 281:39534-41 (2006)

Garcia-Mata, R, Burridge, K. Catching a GEF by its tail. Trends Cell Biology 17: 36-43 (2007)

Peterson, LJ, Wittchen, ES, Geisen, P, Burridge, K, Hartnett, ME. Heterotypic RPE-cell-cell contact increases choroidal endothelial cell transmigration through PI 3-kinase and Rac1. Exp. Eye Res. 84: 737-744 (2007) PMCID: PMC2270476

Samson, T, Will, C, Knoblauch, A, Sharek, C, von der Mark, K, Burridge, K, Wixler, V. Def-6, a guanine-nucleotide exchange factor for Rac1, interacts with the skeletal muscle integrin chain alpha 7A and influences myoblast differentiation. J. Biol. Chem. 282: 15730-42 (2007)

Garrett, TA, van Buul, JD, Burridge, K. VEGF-induced Rac1 activation in endothelial cells is regulated by the guanine nucleotide exchange factor Vav2. Exp. Cell Res. 313: 3285-97 (2007) PMCID: MC2034315

Allingham, MJ, van Buul, JD, Burridge, K. ICAM1-mediated, Src and Pyk2 dependent VE-cadherin tyrosine phosphorylation is required for leukocyte transendothelial migration. J. Immunol. 179: 4053-64 (2007)

van Buul, JD, Allingham, MJ, Samson, T, Meller, J, Boulter, E, Garcia-Mata, R, Burridge, K. RhoG regulates endothelial apical cup assembly downstream from ICAM1 engagement and is involved in leukocyte transendothelial migration. J. Cell Biol. 178: 1279-93 (2007) PMCID: PMC2064659

Dubash, A, Wennerberg, K, Garcia-Mata, R, Menold, MM, Arthur, WT, Burridge, K. A novel role for Lsc/p115 RhoGEF and LARG in regulating RhoA activity downstream of adhesion to fibronectin. J. Cell Sci. 120: 3989-98 (2007)

Garcia-Mata, R, Dubash, AD, Sharek, L, Carr, HS, Frost, JA, Burridge, K. The nuclear RhoA-exchange factor Net1 interacts with proteins of the Dlg family, affects their localization and influences their tumor suppressor activity. Mol. Cell. Biol. 7: 8683-97 (2007) PMCID: PMC2169424

Hermosilla, T, Muñoz, D, Herrera-Molina R, Valdivia A, Nicolás Muñoz N, Sang-Uk Nham S-U, Schneider P, Burridge K, Quest AFG, Leyton L. Direct Thy-1/aVß3 integrin interaction mediates neuron to astrocyte communication. Biochem. Biophys. Acta 1783:1111-1120 (2008)

Aghajanian, A, Wittchen, ES, Allingham, MJ, Garrett, TA, Burridge, K. Endothelial cell junctions and the regulation of vascular permeability and leukocyte transmigration. J. Thromb. Haem. In press (2008)

Wittchen, ES, Burridge, K. Analysis of Low Molecular Weight GTPase Activity in Endothelial Cell Cultures. Methods in Enzymol. 443: In press (2008)