SUMMARYThe endocycle is a variant cell cycle consisting of successive DNA synthesis and Gap phases that yield highly polyploid cells. Although essential for metazoan development, relatively little is known about its control or physiologic role in mammals. Using novel lineage-specific cre mice we identified two opposing arms of the E2F program, one driven by canonical transcription activation (E2F1, E2F2 and E2F3) and the other by atypical repression (E2F7 and E2F8), that converge on the regulation of endocycles in vivo. Ablation of canonical activators in the two endocycling tissues of mammals, trophoblast giant cells in the placenta and hepatocytes in the liver, augmented genome ploidy, whereas ablation of atypical repressors diminished ploidy. These two antagonistic arms coordinate the expression of a unique G2/M transcriptional program that is critical for mitosis, karyokinesis and cytokinesis. These results provide in vivo evidence for a direct role of E2F family members in regulating non-traditional cell cycles in mammals.

Notch signaling is a key regulator of angiogenesis, in which sprouting is regulated by an equilibrium between inhibitory Dll4-Notch signaling and promoting Jagged-Notch signaling. Whereas Fringe proteins modify Notch receptors and strengthen their activation by Dll4 ligands, other mechanisms balancing Jagged and Dll4 signaling are yet to be described. The intermediate filament protein vimentin, which has been previously shown to affect vascular integrity and regenerative signaling, is here shown to regulate ligand-specific Notch signaling. Vimentin interacts with Jagged, impedes basal recycling endocytosis of ligands, but is required for efficient receptor ligand transendocytosis and Notch activation upon receptor binding. Analyses of Notch signal activation by using chimeric ligands with swapped intracellular domains (ICDs), demonstrated that the Jagged ICD binds to vimentin and contributes to signaling strength. Vimentin also suppresses expression of Fringe proteins, whereas depletion of vimentin enhances Fringe levels to promote Dll4 signaling. In line with these data, the vasculature in vimentin knockout (VimKO) embryos and placental tissue is underdeveloped with reduced branching. Disrupted angiogenesis in aortic rings from VimKO mice and in endothelial 3D sprouting assays can be rescued by reactivating Notch signaling by recombinant Jagged ligands. Taken together, we reveal a function of vimentin and demonstrate that vimentin regulates Notch ligand signaling activities during angiogenesis.

The intermediate filament (IF) cytoskeleton has been proposed to regulate morphogenic processes by integrating the cell fate signaling machinery with mechanical cues. Signaling between endothelial cells (ECs) and vascular smooth muscle cells (VSMCs) through the Notch pathway regulates arterial remodeling in response to changes in blood flow. Here we show that the IF-protein vimentin regulates Notch signaling strength and arterial remodeling in response to hemodynamic forces. Vimentin is important for Notch transactivation by ECs and vimentin knockout mice (VimKO) display disrupted VSMC differentiation and adverse remodeling in aortic explants and in vivo . Shear stress increases Jagged1 levels and Notch activation in a vimentin-dependent manner. Shear stress induces phosphorylation of vimentin at serine 38 and phosphorylated vimentin interacts with Jagged1 and increases Notch activation potential. Reduced Jagged1-Notch transactivation strength disrupts lateral signal induction through the arterial wall leading to adverse remodeling. Taken together we demonstrate that vimentin forms a central part of a mechanochemical transduction pathway that regulates multilayer communication and structural homeostasis of the arterial wall.

Metastases are initiated by disseminated tumor cells (DTCs) that colonize distant organs. Growing evidence suggests that the microenvironment of the primary tumor primes DTCs for dormant or proliferative fates. However, the manner in which this occurs remains poorly understood. Here, using the Window for High-Resolution Intravital Imaging of the Lung (WHRIL), we study the live lung longitudinally and follow the fate of individual DTCs that spontaneously disseminate from orthotopic breast tumors. We find that spontaneously DTCs have increased levels of retention, increased speed of extravasation, and greater survival after extravasation, compared to experimentally metastasized tumor cells. Detailed analysis reveals that a subset of macrophages within the primary tumor induces a pro-dissemination and pro-dormancy DTC phenotype. Our work provides insight into how specific primary tumor microenvironments prime a subpopulation of cells for expression of proteins associated with dissemination and dormancy.