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Focal Adhesion Strength and Lifetime Depend on Substrate Stiffness and
Directionality of Detachment Forces
Sangjin Ryu
Brown University
182 Hope Street, Box D, Providence, 02912, US
Phone: 617-861-7625, Email: sangjin_ryu@brown.edu
Christian Franck
Brown University, Providence RI,
Abstract:
Motile cells interact with their extracellular matrix (ECM) through focal adhesions (FAs). A cell crawls
by anchoring its protruding membrane to the ECM or planar substrata through FAs, pulling its body
and then detaching FAs behind. During this process, the cell exerts a force on the substrate via FAs in
the lateral direction (the direction of movement) as well as in the normal direction. On the other hand,
cells sense mechanical changes in ECM through FAs: they show different morphology and motility
depending on the elastic properties of the substrate. Therefore, investigating the dynamics of FAs
promotes a better understanding of cell-ECM interactions.
In this study, we study FA contacts through the interaction between cellular integrins and extracellular
ligands (fibronectin) and measure their rupture force with the atomic force microscopy (AFM) as a
function of substrate elastic modulus and detachment force orientation. Integrins are one of most
abundant transmembrane receptors of FAs, and they specifically bind to fibronectin ligands of the ECM.
In our model system FAs are represented by colloidal AFM probes functionalized with human α5β1-
integrin, and the ECM by planar substrata patterned with human plasma fibronectin. To simulate
changes in the elastic property of the substrate, we employ materials of various elastic moduli including
glass, polydimethylsiloxane (PDMS) and polyacrylamide (PAAM). To mimic the lateral component of
the force that crawling cells exert on the substrate, we disengage the probe from the substrate in the
lateral direction as well as in the normal direction.
Our measurements show that bond strength between the integrin and fibronectin clusters are
dependent on both changes in the elastic modulus of the substrates and the direction of pulling.
Furthermore, rupture force curves between the ligand and receptor clusters show a similar behavior that
is influenced by the direction of the AFM probe disengagement as well as the substrate elastic modulus.
This seems to be due to differences in the association/dissociation behavior between integrin and
fibronectin molecules. In the normal disengagement, chance of rebinding decreases as the probe
recedes from the substrate because the distance between the receptor and ligand increases. By contrast,
the receptor can find new ligands to bind in the lateral disengagement because the distance between the
bead and substrate is kept constant. Understanding the observed behavior of our model system will
provide us with a better understanding how cells might modulate their internal forces during crawling
and how force directionality and the elastic properties of the underlying matrix influence the cell’s
ligand-receptor interactions during cell locomotion and adhesion.
Society of Engineering Science ▪ 47 th Annual Technical Meeting 132