Viscotaxis: Novel cell migration mechanisms regulated by microenvironmental viscosity

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Cell motility is a key step in the metastatic cascade of events, as it enables cancerous cells dissociating from a primary tumor to navigate through interstitial tissues and ultimately colonize distant organs. Cell locomotion is governed by cell-matrix interactions, the actomyosin cytoskeleton, and cell volume regulation via the involvement of ion transporters, such as the Na+/H+ exchanger 1 (NHE1). To date, most cell motility assays are performed in medium with a viscosity close to that of water (0.77 cP). However, the viscosity of the interstitial fluid varies up to 2-3 cP, and can be further augmented by the presence of macromolecules secreted not only by resident epithelial cells in various tissues but also by tumor cells. Cancer cell plasticity is a key feature in metastasis, as tumor cells need to adapt to and navigate through diverse tissue microenvironments presenting different stiffness, degrees of confinement, viscosity and extracellular matrix (ECM) composition. It is currently unknown how tumor cells sense and respond to (patho)physiologically relevant levels of viscosity. The overarching goal of this project is to employ a multidisciplinary approach involving state-of-the-art bioengineering and imaging tools, quantitative analysis and in vivo models to elucidate the effects of extracellular viscosity on breast cancer cell migration, invasion and metastasis. This application will test the hypothesis, supported by intriguing preliminary data, that elevated extracellular viscosity (≥3cP) promotes NHE1-dependent cell swelling, which triggers the activation of the mechanosensitive ion channel TRPV4, thereby initiating downstream signaling. In Aim 1a, we will establish that TRPV4 is the key mechanosensor of elevated viscosity, which initiates RhoA activation, and delineate the presence of a potential feedback loop between NHE1-dependent TRPV4 activation and RhoA. In Aim 1b, we will demonstrate that the coordinated action of local isosmotic swelling at the leading edge and shrinkage at the trailing edge mediated by NHE1 and potassium-chloride cotransporter 4, KCC4, respectively, supports confined migration at elevated viscosities. Cells, as active mechanical objects upon sensing elevated extracellular viscosity, respond by balancing forces in the cell cytoplasm with those in the extracellular microenvironment, thus resulting in increased cytoskeletal tension, higher RhoA-dependent cell contractility and actin reorganization, which ultimately precipitate nuclear translocation of YAP (Aim 1c). We will characterize the roles of viscosity-sensing mechanisms in discrete steps of metastatic dissemination in a live zebrafish model that affords the unique advantages of optical transparency and exceptionally high-resolution along with high-speed imaging of transplanted tumor cells (Aim 2a). We will complement these studies with mouse models to characterize the localization patterns and functional roles of TRPV4, NHE1, KCC4 and YAP in cell migration in natural mammary tissue tracks in vivo (Aim 2b) and in breast cancer growth and metastasis (Aim 2c), using triple-negative breast cancer cell lines and patient-derived xenografts (PDXs). In sum, this project will define how cells sense and respond to extracellular viscosity and identify novel targets to reduce metastasis.