Exosome cargoes are highly varied and include proteins, small RNAs, and genomic DNA (gDNA). The presence of gDNA suggests that different intracellular compartments contribute to exosome loading, resulting in distinct exosome subpopulations. However, the loading of gDNA and other nuclear contents into exosomes (nExo) remains poorly understood. Here, we identify the relationship between cancer cell micronuclei (MN), which are markers of genomic instability, and nExo formation. Imaging flow cytometry analyses reveal that 10% of exosomes derived from cancer cells and <1% of exosomes derived from blood and ascites from patients with ovarian cancer carry nuclear contents. Treatment with genotoxic drugs resulted in increased MN and nExos both in vitro and in vivo. We observed that multivesicular body precursors and exosomal markers, such as the tetraspanins, directly interact with MN. Collectively, this work provides new insights related to nExos, which have implications for cancer biomarker development.

Abstract. To investigate the intracellular role of the clathrin heavy chain in living cells, we have used "antisense" RNA to engineer mutant Dictyostelium discoideum cells that are severely deficient in clathrin heavy chain expression. Immunoblots stained with an anti-clathrin heavy chain antiserum revealed that mutant cells contained undetectable amounts of clathrin heavy chain protein. Similarly, Northern blots showed an absence of clathrin heavy chain mRNA. Clathrin heavy chain-deficient Dictyostelium cells were viable, but exhibited growth rates twofold slower than parental cells. Whereas many morphological features of the mutant cells were normal, mutant cells lacked coated pits and coated vesicles. Clathrin-deficient cells were also missing large translucent vacuoles that serve as endosomes and contractile vacuoles. In the absence of clathrin heavy chain, mutant cells displayed three distinct functional defects: (a) impairment in endocytosis of fluid phase markers, but competence in another endocytic pathway, the phagocytosis of solid particles; (b) defects in osmoregulation; and (c) inability to complete the starvation-induced development cycle.

Using clathrin-minus Dictyostelium cells, we identified a novel requirement for clathrin during cytokinesis. In suspension culture, clathrin-minus cells failed to divide and became large and multinucleate. This cytokinesis deficiency was not attributable to a pleiotropic effect on the actomyosin cytoskeleton, since other cellular events driven by myosin II (e.g., cortical contraction and capping of concanavalin A receptors) remained intact in clathrin-minus cells. Examination of cells expressing myosin II tagged with green f luorescent protein showed that clathrin-minus cells failed to assemble myosin II into a functional contractile ring. This inability to localize myosin II to a particular location was specific for cytokinesis, since clathrin-minus cells moving across a substrate localized myosin II properly to their posterior cortexes. These results demonstrate that clathrin is essential for construction of a functional contractile ring during cell division.Cytokinesis, which follows nuclear division in a coordinated fashion, is the process by which one cell divides into two (1). During late anaphase/early telophase, as condensed chromosomes move toward opposite poles of a cell, a contractile ring assembles in the center of the cell. As the contractile ring pinches the cell in half, two cells are formed. While many of the cellular properties involved in cytokinesis have been studied extensively, central questions remain. Especially lacking is information about how the plasma membrane and associated proteins integrate with the contractile ring to form a cleavage furrow during cytokinesis.The best characterized proteins of the contractile ring are actin and myosin II. Actin filaments form the scaffold for the contractile ring, whereas myosin II is the motor that drives constriction of the ring. Additional proteins, both membranebound and cytosolic, are certainly involved in contractile ring formation and function. Some proteins identified recently include racE, a small GTPase required for cytokinesis in Dictyostelium (2), and septins, identified first in Saccharomyces cerevisiae (3) and then in Drosophila (4). In addition, the actin-binding protein anillin (5, 6) and the formin-like protein diaphanous (7) are required for cytokinesis in Drosophila. In Dictyostelium, the actin-binding protein cortexillin (8) and the cytoskeletal protein coronin (9) both function in cytokinesis.