Because adhesion receptors connect ECM to intracellular filaments, they transmit forces between these structures. These may be intracellular forces from myosin that act on the surrounding ECM, or external forces acting on the cell. Integrins are involved in “measuring” the mechanical properties of the ECM, which critically govern cell behavior. Both integrins and cell-cell adhesion receptors including cadherins and the Ig family protein PECAM-1 (CD31) mediate responses of cells to applied forces such as stretch and fluid shear stress. Integrins and other cell adhesion receptors therefore stand at the intersection of a 3-way link between cell adhesion, mechanical forces and signal transduction pathways. As such understanding their behavior offers the key to understanding major biological and medical problems including anchorage dependence of growth, regulation of gene expression and differentiation by ECM, and how cells respond to mechanical stimuli (mechanotransduction).
My laboratory has therefore developed an integrated program addressing interesting problems in cell adhesion, signaling and mechanotransduction. We are currently working in 4 major areas.
Regulation of Lipid Raft Trafficking
Integrin-mediated adhesion not only directly regulates signaling pathways but also controls how other receptors signal. We first identified synergistic effects of integrins and growth factor receptors in the early 1990’s [1, 2] and subsequent work has revealed multiple layers of mechanism. It now appears that regulation of lipid raft trafficking mediates many of these effects [3-5]. In brief, detaching anchorage-dependent cells from the ECM stimulates caveolin1-dependent endocytosis of a large fraction of the cholesterol, GPI-linked proteins and sphingolipids in the plasma membrane, resulting in a highly fluid membrane that does not support signaling through Rho GTPases, Erk MAP kinase or PI 3-kinase. This process is reversed upon replating cells on ECM, with Arf6 and RalA as the major regulators of exocytosis [6, 7], which is required for cell spreading and restores signaling through these pathways.
There is a strong correlation between the proteins that regulate this pathway and control of anchorage dependence of growth and metastasis by cancer cells, which supports the notion that anchorage dependence of growth is a key checkpoint against tumor metastasis. We are currently working to understand how integrins regulate these events and elucidating the role this pathway plays in cell motility.
Mechanotransduction by Integrins
Cells sense the mechanical properties of their ECM and respond accordingly . They also respond to external forces applied through the ECM . Exhaustive evidence has shown that integrins mediate these responses but the molecular mechanisms are not well understood.
We recently developed a fluorescence-based method to measure forces across specific proteins in live cells . We are currently combining this with high resolution microscopy with tension sensors to elucidate how integrin-mediated adhesions respond to force at the near-single molecule level, in order to elucidate these molecular mechanisms.
Fluid Shear Stress Mechanotransduction in Blood Vessels
Flowing blood exerts a frictional force called fluid shear stress on the endothelial cells that line the vessels; this force is a major determinant of vascular development, physiology and disease . Atherosclerosis arises in regions of arteries subject to disturbances in fluid flow patterns, while high fluid shear stress suppresses inflammatory and atherosclerotic pathways. We have identified complex between VE-cadherin, PECAM-1 and VEGFR2 as a critical mechanotransducer that mediates a subset of these effects .
One major pathway downstream of the junctional complex involves activation of integrins, binding to the subendothelial extracellular matrix and subsequent signaling. An important consequence of this pathway is that cell responses to flow are modulated by the ECM. We have found that basement membrane proteins promote flow-dependent activation of anti-inflammatory pathways; by contrast, endothelial cells on provisional ECM proteins such as fibronectin activate multiple inflammatory pathways [13-15]. Both functional and genetic evidence provide further links between ECM proteins and atherosclerosis.
We are currently using tension sensors combined with other approaches to elucidate the molecular mechanisms by which fluid flow acts on the junctional complex proteins. We are also investigating the mechanisms of matrix-dependent flow signaling and the role of these pathways in atherosclerosis.
Collateral Artery Formation
Blockage of a coronary artery after myocardial infarction leads to downstream ischemia and myocardial cell death. Blockage of a major artery also triggers increased flow through parallel vessels, which then remodel to accommodate the higher flow. In human patients, this ability to form collateral arteries that perfuse the affected region is a major determinant of recovery after MI. However, the key steps by which high flow stimulates arterialization of small vessels are not well understood, nor are the reasons why some patients are unable to do so.
We are currently applying our expertise in flow signaling to address this medical problem. Our goal is to first elucidate the basic pathway of flow-dependent vessel remodeling, then identify the steps that are inhibited in poor responders in order to devise therapies to improve outcomes.