A giant disruption of the heart: Certain forms of heart failure originate from genetic mutations. Understanding how the culprit mutant proteins alter normal heart function could lead to more effective treatments. One candidate is the giant protein tintin, which is mutated in a subset of patients with dilated cardiomyopathy. Through a combination of patient-derived stem cells, tissue engineering, and gene editing, Hinson et al. found that disease-associated titin mutations disrupt the function of the contractile unit in heart muscle. As a result, the heart does not respond properly to mechanical and other forms of stress. Science , this issue p. 982

Interstitial flow is the convective transport of fluid through tissue extracellular matrix. This creeping fluid flow has been shown to affect the morphology and migration of cells such as fibroblasts, cancer cells, endothelial cells, and mesenchymal stem cells. A microfluidic cell culture system was designed to apply stable pressure gradients and fluid flow and allow direct visualization of transient responses of cells seeded in a 3D collagen type I scaffold. We used this system to examine the effects of interstitial flow on cancer cell morphology and migration and to extend previous studies showing that interstitial flow increases the metastatic potential of MDA-MB-435S melanoma cells [Shields J, et al. (2007) Cancer Cell 11:526–538]. Using a breast carcinoma line (MDA-MB-231) we also observed cell migration along streamlines in the presence of flow; however, we further demonstrated that the strength of the flow as well as the cell density determined directional bias of migration along the streamline. In particular, we found that cells either at high seeding density or with the CCR-7 receptor inhibited migration against, rather than with the flow. We provide further evidence that CCR7-dependent autologous chemotaxis is the mechanism that leads to migration with the flow, but also demonstrate a competing CCR7-independent mechanism that causes migration against the flow. Data from experiments investigating the effects of cell concentration, interstitial flow rate, receptor activity, and focal adhesion kinase phosphorylation support our hypothesis that the competing stimulus is integrin mediated. This mechanism may play an important role in development of metastatic disease.

Brillouin microscopy can be used to analyze the mechanical properties of cells in a contact-free fashion. Cells in 2D and 3D environments are accessible to this technology, which provides measurements of longitudinal moduli at optical resolution. Current measurements of the biomechanical properties of cells require physical contact with cells or lack subcellular resolution. Here we developed a label-free microscopy technique based on Brillouin light scattering that is capable of measuring an intracellular longitudinal modulus with optical resolution. The 3D Brillouin maps we obtained of cells in 2D and 3D microenvironments revealed mechanical changes due to cytoskeletal modulation and cell-volume regulation.

The transmembrane domain of NOTCH1 plays a key role in the assembly of adherens junctions and the non-transcriptional regulation of vascular permeability that links transcriptional programs with adhesive and cytoskeletal remodelling. The interaction between the Notch receptor and its ligands causes proteolysis of the receptor's intracellular domain, which then relocates to the nucleus and regulates transcription of a battery of genes. William Polacheck et al. show that ligand binding to the Notch receptor in the endothelium—the layer of cells that line the interior of blood and lymphatic vessels—triggers a non-canonical signalling cascade that is mediated by the receptor's transmembrane domain. This domain catalyses the formation of a complex composed of vascular endothelial cadherin, the transmembrane protein tyrosine phosphatase LAR, and the RAC1 guanidine-exchange factor TRIO. This complex drives the assembly of adherens junctions and is crucial for the establishment and maintenance of the endothelial barrier, which is essential to vascular health. The vascular barrier that separates blood from tissues is actively regulated by the endothelium and is essential for transport, inflammation, and haemostasis1. Haemodynamic shear stress plays a critical role in maintaining endothelial barrier function2, but how this occurs remains unknown. Here we use an engineered organotypic model of perfused microvessels to show that activation of the transmembrane receptor NOTCH1 directly regulates vascular barrier function through a non-canonical, transcription-independent signalling mechanism that drives assembly of adherens junctions, and confirm these findings in mouse models. Shear stress triggers DLL4-dependent proteolytic activation of NOTCH1 to expose the transmembrane domain of NOTCH1. This domain mediates establishment of the endothelial barrier; expression of the transmembrane domain of NOTCH1 is sufficient to rescue defects in barrier function induced by knockout of NOTCH1. The transmembrane domain restores barrier function by catalysing the formation of a receptor complex in the plasma membrane consisting of vascular endothelial cadherin, the transmembrane protein tyrosine phosphatase LAR, and the RAC1 guanidine-exchange factor TRIO. This complex activates RAC1 to drive assembly of adherens junctions and establish barrier function. Canonical transcriptional signalling via Notch is highly conserved in metazoans and is required for many processes in vascular development, including arterial–venous differentiation3, angiogenesis4 and remodelling5. We establish the existence of a non-canonical cortical NOTCH1 signalling pathway that regulates vascular barrier function, and thus provide a mechanism by which a single receptor might link transcriptional programs with adhesive and cytoskeletal remodelling.

Significance: Interstitial flow (IF) is elevated in solid tumors and imparts fluid stresses on tumor cells within the extracellular matrix (ECM), and these fluid stresses must be balanced by stress in matrix adhesions to maintain static equilibrium. This force balance results in greater matrix adhesion tension on the upstream side of the cell, and we demonstrate that this tension activates β1-integrin adhesion complexes, resulting in localization and activation of focal adhesion (FA) proteins near the upstream membrane. Importantly, we demonstrate that this asymmetric FA activation governs the direction of cell migration in 3D, and therefore our data show that mechanical stress acting on cells within a 3D ECM is a fundamental directional migratory stimulus.