Collaborative Research: Mechanisms of Gradient Sensing by 'Feel' in Cell Migration Directed by Extracellular Matrix

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Cell migration is an important step in many processes in the body, such as the healing of wounds and the response to infection. The cells are in contact with a matrix that allows them to crawl and explore. To do so, cells must actively sense matrix properties by touch and feel. An overarching goal of this collaborative project is to unify or distinguish cell sensing of the quantity (stickiness by touch) versus quality (stiffness by feel) of contacts with the matrix. The multidisciplinary research will be integrated with graduate student training, teaching of engineering courses, and outreach that will foster science communication.Cell migration is a fundamental process in tissue development and homeostasis. The molecular determinants of chemotaxis, cell migration biased by a gradient of a soluble attractant, are reasonably well understood, and a paradigm has emerged characterizing certain intracellular signaling pathways as a molecular "compass". In contrast, haptotaxis and durotaxis, migration directed by gradients of immobilized ligand density and of mechanical stiffness, respectively, are poorly understood and require a new paradigm, considering that cells must actively encounter and respond to physical cues (i.e., by "feel") rather than by passive sensing of diffusible ligands. Under Objective 1, the aim is to define feedback mechanisms that control coupled cytoskeletal dynamics in migrating cells. It is hypothesized that adhesion-mediated signaling pathways control the duration of lamellipodial protrusion and are spatially coordinated by F-actin bundles. High-resolution, live-cell microscopy and molecular perturbations of the putative signaling and cytoskeletal processes will be applied to systematically relate perturbations of putative mechanisms regulating the actin cytoskeleton to changes in lamellipodial protrusion dynamics. These studies are expected to show how cell motility is biased by physical cues. Under Objective 2, it is proposed to elucidate the nature of haptotactic bias at the level of adhesion, signaling, and cytoskeletal dynamics. It is hypothesized that F-actin bundles/filopodia direct, and lamellipodia propagate, haptotactic exploration. A related hypothesis is that the haptotactic bias (comparing up- vs. down-gradient) manifests as differences in signaling and/or cytoskeletal dynamics, integrated by adhesions. It is proposed to analyze the dynamics of adhesion, signaling, and cytoskeletal structures during migration on haptotactic gradients generated using microfluidic devices. These studies will characterize leading-edge motility dynamics during haptotaxis, a poorly understood mode of directed migration that is distinct from chemotaxis. Under Objective 3, the proposed work will elucidate the molecular and biophysical determinants of durotaxis and define features that are common or distinct between haptotaxis and durotaxis. Hydrogels with gradients of crosslinking will be generated and integrated with traction force microscopy, a method for measuring local stress in an elastic gel. These studies will test the role of fascin-rich filopodia as sensory organelles guiding durotactic migration. The results will further elucidate the relationship between cell protrusion, focal adhesion dynamics, and traction force exerted by durotaxing cells. The interdisciplinary nature of the project offers a unique environment for the training of two graduate students, who will be engaged in research and an outreach activity fostering science communication.