The assembly of individual endothelial cells into multicellular tubes is a complex morphogenetic event in vascular development. Extracellular matrix cues and cell-cell junctional communication are fundamental to tube formation. Together they determine the shape of endothelial cells and the tubular structures that they ultimately form. Little is known regarding how mechanical signals are transmitted between cells to control cell shape changes during morphogenesis. Here we provide evidence that the scaffold protein amotL2 is needed for aortic vessel lumen expansion. Using gene inactivation strategies in zebrafish, mouse and endothelial cell culture systems, we show that amotL2 associates to the VE-cadherin adhesion complex where it couples adherens junctions to contractile actin fibres. Inactivation of amotL2 dissociates VE-cadherin from cytoskeletal tensile forces that affect endothelial cell shape. We propose that the VE-cadherin/amotL2 complex is responsible for transmitting mechanical force between endothelial cells for the coordination of cellular morphogenesis consistent with aortic lumen expansion and function.

The establishment and maintenance of apical-basal cell polarity is essential for the functionality of glandular epithelia. Cell polarity is often lost in advanced tumours correlating with acquisition of invasive and malignant properties. Despite extensive knowledge regarding the formation and maintenance of polarity, the mechanisms that deregulate polarity in metastasizing cells remain to be fully characterized. Here we show that AmotL2 expression correlates with loss of tissue architecture in tumours from human breast and colon cancer patients. We further show that hypoxic stress results in activation of c-Fos-dependent expression of AmotL2 leading to loss of polarity. c-Fos/hypoxia-induced p60 AmotL2 interacts with the Crb3 and Par3 polarity complexes retaining them in large vesicles and preventing them from reaching the apical membrane. The resulting loss of polarity potentiates the response to invasive cues in vitro and in vivo in mice. These data provide a molecular mechanism how hypoxic stress deregulates cell polarity during tumour progression.

Epithelial cells connect via cell-cell junctions to form sheets of cells with separate cellular compartments. These cellular connections are essential for the generation of cellular forms and shapes consistent with organ function. Tissue modulation is dependent on the fine-tuning of mechanical forces that are transmitted in part through the actin connection to E-cadherin as well as other components in the adherens junctions. In this report we show that p100 amotL2 forms a complex with E-cadherin that associates with radial actin filaments connecting cells over multiple layers. Genetic inactivation or depletion of amotL2 in epithelial cells in vitro or zebrafish and mouse in vivo, resulted in the loss of contractile actin filaments and perturbed epithelial packing geometry. We further showed that AMOTL2 mRNA and protein was expressed in the trophectoderm of human and mouse blastocysts. Genetic inactivation of amotL2 did not affect cellular differentiation but blocked hatching of the blastocysts from the zona pellucida. These results were mimicked by treatment with the myosin II inhibitor blebbistatin. We propose that the tension generated by the E-cadherin/AmotL2/actin filaments plays a crucial role in developmental processes such as epithelial geometrical packing as well as generation of forces required for blastocyst hatching.A central question during development is how single cells form functional multi-cellular organ structures. The high reproducibility indicates intricate synchronization of cellular processes such as migration, proliferation and cell shape changes. Much attention has been focused on how growth factors form biochemical gradients that govern some of these processes 1-3 . However, less is known regarding how mechanical signals or forces modulate cell shape and control cellular expansion 4, 5 . Cells perceive and respond to exogenous mechanical forces via different points of contact in the outer membrane. Forces exerted on the extra-cellular matrix are detected by epithelial cells via integrins in focal adhesions which transfer tension from the extracellular matrix to the cytoskeleton 6 . Low rigidity in the extra-cellular matrix transfers less extracellular force and thereby promotes the formation of organ-like epithelial structures in vitro whereas increased force or stiffness in the matrix causes loss of tissue architecture associated with tumor progression and promotes cell proliferation [7][8][9][10] . Recent evidence has also shown that actomyosin contractility is transmitted via the adherens junctions. External forces applied to cadherins have indicated a mechanical coupling between the cytoplasmic domain of cadherin and the actin cytoskeleton 11 . Cellular interactions and the force-mediated morphological changes are also important for the processes involved in organ development. One example is apical contraction where the

Endothelial cells respond to mechanical forces exerted by blood flow. Endothelial cell–cell junctions and the sites of endothelial adhesion to the matrix sense and transmit mechanical forces to the cellular cytoskeleton. Here we show that the scaffold protein AmotL2 connects junctional VE-cadherin and actin filaments to the nuclear lamina. AmotL2 is essential for the formation of radial actin filaments and the alignment of endothelial cells, and, in its absence, nuclear integrity and positioning are altered. Molecular analysis demonstrated that VE-cadherin binds to AmotL2 and actin, resulting in a cascade that transmits extracellular mechanical signals to the nuclear membrane. Furthermore, the endothelial deficit of AmotL2 in mice fed normal diet provoked a pro-inflammatory response and abdominal aortic aneurysms (AAAs). Transcriptome analysis of human AAA samples revealed a negative correlation between AmotL2 and inflammation of the aortic intima. These findings offer insight into the link between junctional mechanotransduction and vascular disease.

Arterial endothelial cells (ECs) have the ability to respond to mechanical forces exerted by fluid shear stress. This response is of importance, as it is protective against vascular diseases such as atherosclerosis and aortic aneurysms. Mechanical forces are transmitted at the sites of adhesion to the basal membrane as well as cell-cell junctions where protein complexes connect to the cellular cytoskeleton to relay force into the cell. Here we present a novel protein complex that connects junctional VE-cadherin and radial actin filaments to the LINC complex in the nuclear membrane. We show that the scaffold protein AmotL2 is essential for the formation of radial actin filaments and the flow-induced alignment of aortic and arterial ECs. The deletion of endothelial AmotL2 alters nuclear shape as well as subcellular positioning. Molecular analysis shows that VE-cadherin is mechanically associated with the nuclear membrane via binding to AmotL2 and Actin. Furthermore, the deletion of AmotL2 in ECs provokes a pro-inflammatory response and abdominal aortic aneurysms (AAA) in the aorta of mice on a normal diet. Remarkably, transcriptome analysis of AAA samples from human patients revealed a negative correlation between AmotL2 expression and aneurysm diameters, as well as a positive correlation between AmotL2 and YAP expression. These findings provide a conceptual framework regarding how mechanotransduction in the junctions is coupled with vascular disease.