β1-Integrin-Mediated Adhesion Is Lipid-Bilayer Dependent

Seoyoung Son, George J. Moroney, Peter J.Butler

Integrin-mediated adhesion is a central feature of cellular adhesion, locomotion, and endothelial cell mechanobiology. Although integrins are known to be transmembrane proteins, little is known about the role of membrane biophysics and dynamics in integrin adhesion. We treated human aortic endothelial cells with exogenous amphiphiles, shown previously in model membranes, and computationally, to affect bilayer thickness and lipid phase separation, and subsequently measured single-integrin-molecule adhesion kinetics using an optical trap, and diffusion using fluorescence correlation spectroscopy. Benzyl alcohol (BA) partitions to liquid-disordered (Ld) domains, thins them, and causes the greatest increase in hydrophobic mismatch between liquid-ordered (Lo) and Ld domains among the three amphiphiles, leading to domain separation. In human aortic endothelial cells, BA increased β1-integrin-Arg-Gly-Asp-peptide affinity by 18% with a transition from single to double valency, consistent with a doubling of the molecular brightness of mCherry-tagged β1-integrins measured using fluorescence correlation spectroscopy. Accordingly, BA caused an increase in the size of focal-adhesion-kinase/paxillin-positive peripheral adhesions and reduced migration speeds as measured using wound-healing assays. Vitamin E, which thickens Lo domains and disperses them by lowering edge energy on domain boundaries, left integrin affinity unchanged but reduced binding probability, leading to smaller focal adhesions and equivalent migration speed relative to untreated cells. Vitamin E reversed the BA-induced decrease in migration speed. Triton X-100 also thickens Lo domains, but partitions to both lipid phases and left unchanged binding kinetics, focal adhesion sizes, and migration speed. These results demonstrate that only the amphiphile that thinned Ld lipid domains increased β1-integrin-Arg-Gly-Asp-peptide affinity and valency, thus implicating Ld domains in modulation of integrin adhesion, nascent adhesion formation, and cell migration.

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