Probabilistic modeling and analysis of the effects of extra-cellular matrix density on the sizes, shapes, and locations of integrin clusters in adherent cells

To quantify how the concentration of ECM affects integrin clustering, we seeded cells expressing the integrin αIIbβ3 on different concentrations of the complementary ECM protein fibrinogen (Fg) and measured the resulting integrin cluster properties. We observed heterogeneity in the properties of integrin clusters, and to characterize this population heterogeneity we use a probabilistic modeling approach to quantify changes to the distributions of integrin cluster size, shape, and location.Our results indicate that in response to increasing ECM density cells form smaller integrin clusters that are less elongated and closer to the cell periphery. These results suggest that cells can sense the availability of ECM binding sites and consequently regulate integrin clustering as a function of ECM density.Integrins are transmembrane adhesion receptors that facilitate cell adhesion by binding extra-cellular ligands to provide a mechanical linkage between a cell and the extracellular matrix (ECM). Many types of integrins form micron-sized clusters, which create the foundation for various cell-matrix adhesion complexes including focal adhesions. These complexes are populated by a diverse group of membrane, structural, adaptor, and enzymatic proteins [1], and signaling via these complexes affects many important cellular processes [2]. Integrin clusters thus provide the platform for signal propagation as well as force transduction through focal adhesions; as a result cell signaling and adhesion depend directly on the spatial and temporal characteristics of integrin cluster formation and dispersion [3-6].Because integrin binding, clustering, and signaling depend on the availability of insoluble extracellular ligands [7-9], the availability of integrin binding sites is a critical property of the ECM proteins to which cells adhere. There is also a growing body of experimental evidence indicating that cells sense and respond to the concentration of ECM ligands available to them. Cel