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Although polyploidization is a hallmark of adult mammalian cardiomyocytes and may constrain their proliferation, the mechanisms leading to ploidy increase in cardiomyocytes remain elusive.1 Our laboratory and others have reported the formation of DNA bridges between daughter nuclei as a potential route to cardiomyocyte polyploidization.2,3 These earlier in vivo studies found DNA bridges on thin tissue sections, but this approach does not cover all events because not all cardiomyocytes were oriented in the same 2-dimensional plane. To detect all DNA bridges, we dispersed cardiomyocytes obtained from Myh6-H2B-mCh mice4 at postnatal day 4 on a culture plate and labeled them with the mitosis marker phospho-histone H3 (pH3) and DAPI (4’6-diamidino-2-phenylindole). No apparent mitotic errors were identified in prophase or metaphase, but DNA bridges were apparent in 54±7.9% of cardiomyocytes in early anaphase. The percentage of cardiomyocytes with DNA bridges decreased rapidly to 17.8±9.5% in late anaphase. On the contrary, the percentage of cardiomyocytes with resolved or broken DNA bridges (cardiomyocytes with discontinuous DNA fibers between sister chromatids) increased from 11.7±6.4% in early anaphase to 28.9±16.3% in late anaphase. DNA bridge remnants were present in 8±3.7% of telophase cardiomyocytes. This analysis indicates that DNA bridges form in early anaphase and are resolved in the subsequent mitotic stages. Time-lapse videomicroscopy of postnatal day 4 cardiomyocytes reinforces the finding that DNA bridges between sister chromatids resolve as mitosis advances.

Caveolin-1 (Cav1)/caveolae biology is intimately linked to actin dynamics and adhesion receptors. Caveolar domains are organized in hierarchical levels of complexity from curved or flatten caveolae to large, higher-order caveolar rosettes. We report that stress fibers controlled by Abl kinases and mDia1 determine the level of caveolar domain organization, which conditions the subsequent inward trafficking of caveolar domains induced upon loss of cell adhesion from the extracellular matrix. Abl-deficient cells show decreased content of stress fibers, a smaller stress-fiber co-aligned Cav1 pool and increased clustering of Cav1/caveolae at the cell surface. Defective caveolar linkage to stress fibers prevents the formation of big caveolar rosettes upon loss of cell adhesion, correlating with a lack of inward trafficking. Live imaging of stress fibers and Cav1 showed that the actin-linked Cav1 pool loses its spatial organization in the absence of actin polymerization and is dragged and clustered by depolymerizing filaments. We identify mDia1 as the actin polymerization regulator downstream of Abl kinases that controls the stress fiber-linked Cav1 pool. mDia1 knockdown results in Cav1/caveolae clustering and defective inward trafficking upon loss of cell adhesion. In contrast, cell elongation imposed by the excess of stress fibers induced by active mDia1 flattens caveolae. Furthermore, active mDia1 rescues the actin co-aligned Cav1 pool and Cav1 inward trafficking upon loss of adhesion in Abl-deficient cells. Thus, caveolar domain organization and trafficking are tightly coupled to adhesive and stress fiber regulatory pathways

The adhesion molecule CD44 and the membrane-type matrix metalloproteinase MT1-MMP act coordinately in tumor cells to promote cell invasion through a yet unclear mechanism. We are interested in studying the interplay between CD44 and MT1-MMP in carcinoma cells embedded in HA containing three-dimensional collagen I matrices (3D HA-Col I) by time-lapse confocal microscopy imaging. Here we report the in vivo interaction between CD44 and MT1-MMP, revealed by fluorescence resonance energy transfer (FRET) microscopy. MT1-MMP interacts with CD44 preferentially at the trailing edge of the invading tumor cells during rear retraction and on membrane fragments released during the invasion process. A fluorescent biosensor designed to monitor the proteolytic processing of CD44 by live cell imaging demonstrates that cleavage of the CD44 extracellular domain is enriched in the retracting rear ends of invasive tumor cells. Invasion assays showed that MT1-MMP mediates CD44-dependent tumor-cell invasion, whereas CD44 is not essential for MT1-MMP-mediated invasion of 3D HA-Col I matrices. Together, our results support a role for MT1-MMP in cell retraction during CD44-mediated cell invasion

Polarized migrating T cells possess 2 poles, the uropod protrusion at the rear and the leading edge at the front, with specific protein composition and function. The influenza virus hemagglutinin (HA) is a prototypical molecule that uses lipid rafts for biosynthetic transport to the apical surface in polarized epithelial Madin-Darby canine kidney (MDCK) cells. In this study, HA was used as a tool to investigate the role of lipid rafts in vectorial protein traffic in polarized T lymphocytes. Results show that newly synthesizedHAbecomes selectively targeted to the uropod subdomain in polarized T lymphoblasts. HA incorporates into rafts soon after biosynthesis, suggesting that delivery of HA to the uropod occurs through a pathway of transport reminiscent of that used for its specific targeting to the apical surface. HA and the adhesion molecules, intercellular adhesion molecule 3 (ICAM-3), CD44, and CD43, 3 endogenous uropod markers, were detected in surface rafts of T lymphoblasts. Cholesterol, a major component of lipid rafts, was predominantly located in the uropod. Disruption of lipid raft integrity by cholesterol sequestration produced unclustering of ICAM-3 and the loss of uropodia and severely impaired processes that require a polarized phenotype such as intercellular aggregation and cell migration. Collectively, these results indicate that lipid rafts constitute a route for selective targeting of proteins to the uropod and that the rafts are essential for the generation, maintenance, and functionality of T-cell anteroposterior polarity.