Overview

Normal morphogenesis and physiology require that cellular signaling, gene expression and behavior be closely coordinated with tissue architecture and mechanical forces. Thus, the type and physical properties of the extracellular matrix, and the mechanical forces to which cells are exposed govern cell structure, gene expression and behavior. Nowhere is this more evident than in the cardiovascular system, where forces from blood flow (blood pressure and fluid shear stress, the frictional forces that flowing blood exerts on vessel walls) sculpt the developing heart and vasculature, and govern their remodeling throughout life. Conversely, the major diseases that afflict humankind, including atherosclerosis, heart failure and cancer, are due in essence to breakdowns in these processes.

The Schwartz lab combines basic cell biological studies of the fundamental basis for integrin signaling, signal integration and mechanotransduction with disease models, to elucidate the mechanistic basis of both normal physiology and disease. Our approaches integrate biophysical tools, high resolution imaging, protein engineering, cell culture and cell biological assays, with animal models of disease and analysis of human specimens. The goal of these studies is to deeply understand molecular and biophysical mechanisms, and to test and apply this knowledge to relevant biological systems, in particular to understanding and curing vascular diseases.

  

Fundamentals of Flow Sensing

Fluid shear stress is a very weak force, typically about 1/100th of the usual traction forces that endothelial cells exert on their extracellular matrix, yet it has very strong effects on vascular development, remodeling and function. Endothelial cells are specialized to sense and respond to these forces but the fundamental mechanisms of mechanotransduction are poorly understood. Our newest data shows that while the junctional mechanosensory complex is a true mechanotransducers, it is never the less downstream of other events [1]. This system therefore provides a starting point for elucidating the upstream pathway. We are currently doing siRNA and CRISPR-Cas9 based whole genome screening to identify all of the components involved in flow sensing. With the use of our other tools, such as in vitro assays and the molecular tensions sensor [2], we are working to delineate the complete upstream pathways by which cells sense fluid shear stress.
  1. Conway, D.E., et al., Fluid shear stress on endothelial cells modulates mechanical tension across VE-cadherin and PECAM-1. Curr Biol, 2013. 23(11): p. 1024-30.
  2. Grashoff, C., et al., Measuring mechanical tension across vinculin reveals regulation of focal adhesion dynamics. Nature, 2010. 466(7303): p. 263-6.

Mechanotransduction by Integrins

Tension on talin.  The talin tension sensor (TS) and the C-terminal, zero tension control (CS) were expressed in talin1-/- fibroblasts and imaged to determine FRET, which is displayed in pseudocolor.  Low FRET corresponds to high tension, which is evident in focal adhesions for cells expressing talinTS.

 

Integrins are the physical connectors that transmit mechanical forces between the extracellular matrix and the cytoskeleton; as such, they are well situated to sense the mechanical properties of the environment, both the stiffness of the matrix and externally applied forces, and transduce these forces into biochemical signals that govern cell behavior. While these sensing mechanisms have been found to be critically important for cell differentiation, growth control and other key functions, the molecular mechanisms are poorly understood. We are using a combination of our molecular tension sensor [1, 2] with actin speckle imaging and novel approaches to protein engineering to elucidate the fundamental mechanisms of stiffness sensing and mechanotransduction. Our current work is focused on understanding how integrin and talin conformations are affected by tension, and how these affects mediate cell responses.

1. Grashoff, C., et al., Measuring mechanical tension across vinculin reveals regulation of focal adhesion dynamics. Nature, 2010. 466(7303): p. 263-6.

2. Kumar, A., et al., Talin tension sensor measurements reveal novel features of focal adhesion force transmission and mechanosensitivity. J. Cell Biol., 2016. in press.

Fluid Shear Stress and Atherosclerosis

Cell polarity, flow sensing and atherosclerosis. Atherosclerotic plaque in mouse aortas visualized with Oil red O stain.  Endothelial cells normally align in the direction of flow, which is critical in activating anti-inflammatory pathways and suppressing atherosclerosis. In mice that are hyperlipidemic but WT for syndecan4, plaques form only at regions of disturbed shear stress where cells do not align, in this case, the arteries branching off the abdominal aorta. Syndecan4 is required for cell alignment, thus, syndecan4-/- mice lose the protective benefit of high laminar flow and in hyperlipidemia develop plaque throughout the aorta. From Baeyens, 2014.

Atherosclerosis has many well publicized, systemic risk factors such as hyperlipidemia, hypertension, and diabetes; however, atherosclerotic plaques show a strong preference for regions of arteries where flow patterns are disturbed, with lower average flow magnitude and complex changes in direction. Worse still, low, disturbed shear stress is strongly correlated with the small subset of vulnerable plaques that rupture to cause myocardial infarction and stroke [1]. These results can be traced to the pro-inflammatory effects of low/disturbed flow on the endothelium, in contrast to the strong anti-inflammatory effect of normal arterial blood flow. A major goal in the lab is to understand the molecular mechanisms by which disturbed flow induces endothelial inflammatory activation and identify drug targets that can reduce inflammation without harmful side effects. One direction for our current work is to test mutations that specifically affect flow sensing through the junctional complex without affecting other functions [2]. Another major direction concerns the role of cell polarity in determining whether flow activates pro- vs. anti-inflammatory pathways [3]. Lastly, we have found that the endothelial extracellular matrix and cognate integrins are potent modulators of disturbed flow-induced endothelial inflammatory activation [4-6]. We are therefore investigating the mechanisms by which integrins modulate inflammatory pathways and how they govern plaque development and vulnerability.

1. Morbiducci, U., et al., Atherosclerosis at arterial bifurcations: evidence for the role of haemodynamics and geometry. Thromb Haemost, 2016. 115(3): p. 484-92.

2. Coon, B.G., et al., Intramembrane binding of VE-cadherin to VEGFR2 and VEGFR3 assembles the endothelial mechanosensory complex. J Cell Biol, 2015.

3. Baeyens, N., et al., Syndecan 4 is required for endothelial alignment in flow and atheroprotective signaling. Proc Natl Acad Sci U S A, 2014. 111(48): p. 17308-13.

4. Orr, A.W., et al., The subendothelial extracellular matrix modulates NF-kappaB activation by flow: a potential role in atherosclerosis. J Cell Biol, 2005. 169(1): p. 191-202.

5. Orr, A.W., et al., Matrix-specific p21-activated kinase activation regulates vascular permeability in atherogenesis. J Cell Biol, 2007. 176(5): p. 719-27.

6. Orr, A.W., et al., p21-activated kinase signaling regulates oxidant-dependent NF-kappa B activation by flow. Circ Res, 2008. 103(6): p. 671-9.

Collateral Artery Formation

Control of artery diameter by the junctional mechanosensory complex. The endothelial fluid shear stress set point is determined by components of the junctional mechanosensory complex.  Deletion of VEGF receptor 3 in mice reduces endothelial sensitivity to flow, shifting the set point to higher levels.  This triggers striking inward remodeling of the aorta, to increase shear stress to the new, higher set point.  See Baeyens et al 2015.

Collateral blood vessels provide “natural bypasses” to perfuse tissues downstream of a blocked artery. Indeed, the “coronary collateral flow index”, a measure of patients’ abilities to form new circulation after a blockage, is a major prognostic indicator of favorable clinical outcome after myocardial infarction [1]. Collateral growth is stimulated by the increased fluid shear stress after blockage of the major vessel. Our exploration of how flow determines vessel remodeling led to the discovery that endothelial cells encode a fluid shear stress “set point”, such that flow within this narrow range stabilizes blood vessels, whereas flow either above or below activates pathways that lead to vessel remodeling, inward in the case of low flow and outward in the case of high flow [2]. We also found that this set point was determined in part by expression levels of the junctional mechanosensory complex. Current work is directed at understanding the sensing and signaling pathways that determine set point behavior and subsequent vessel remodeling, using this information to improve outcomes in patients with artery disease.

1. Zimarino, M., et al., The dynamics of the coronary collateral circulation. Nat Rev Cardiol, 2014. 11(4): p. 191-7.

2. Baeyens, N., et al., Vascular remodeling is governed by a VEGFR3-dependent fluid shear stress set point. Elife, 2015. 4.