Cycling cells maintain centriole number at precisely two per cell in part by limiting their duplication to S phase under the control of the cell cycle machinery. In contrast, postmitotic multiciliated cells (MCCs) uncouple centriole assembly from cell cycle progression and produce hundreds of centrioles in the absence of DNA replication to serve as basal bodies for motile cilia. Although some cell cycle regulators have previously been implicated in motile ciliogenesis, how the cell cycle machinery is employed to amplify centrioles is unclear. We use transgenic mice and primary airway epithelial cell culture to show that Cdk2, the kinase responsible for the G1 to S phase transition, is also required in MCCs to initiate motile ciliogenesis. While Cdk2 is coupled with cyclins E and A2 during cell division, cyclin A1 is required during ciliogenesis, contributing to an alternative regulatory landscape that facilitates centriole amplification without DNA replication.
New adsorbent materials, many of which rely on supported Ag nanoclusters or exchanged Ag + ions, have recently been employed in petroleum processing to further reduce sulfur content in fuels following catalytic hydrodesulfurization (HDS). HDS refractory species include aromatic heterocycles, such as dibenzothiophene (DBT) and its methylated derivatives. Herein, we report a fundamental study of the adsorption of two structurally analogous petroleum-relevant molecules, DBT and fluorene, on a clean TiO 2 (110) surface and one with supported Ag nanoclusters. Using thermal desorption spectroscopy, we determined the desorption activation energies to be 106 ± 2 and 103 ± 2 kJ/mol for DBT and fluorene, respectively. The similar desorption activation energies imply that the interaction of DBT on TiO 2 is not strongly dependent upon the S atom (which fluorene lacks). When adsorbed on supported Ag nanoparticles, both desorption activation energies shifted to 111 ± 2 kJ/mol, suggesting a nonselective binding enhancement, which likely involves the π-electron systems. After heating the Ag/TiO 2 (110) surface to 650 K to force agglomeration of the particles, no enhancement in binding was observed for either molecule, suggesting that the cluster size is critical for the observed enhancement. These results point to the importance of the metal particle size in addition to the oxidation state in commercially employed sorbents.
DNA replication is normally coupled with centriole duplication in the cell cycle. Trophoblast giant cells (TGCs) of the placenta undergo endocycles resulting in polyploidy but their centriole state is not known. We used a cell culture model for TGC differentiation to examine centriole and centrosome number and properties. Prior to differentiation, trophoblast stem cells (TSCs) have either two centrioles before duplication, or four centrioles after. We find that the average nuclear area increases approximately 8-fold over differentiation, but most TGCs do not have more than four centrioles. However, these centrioles become disengaged, acquire centrosome proteins, and can nucleate microtubules. In addition, some TGCs undergo further duplication and disengagement of centrioles, resulting in substantially higher numbers. Live imaging revealed that disengagement and separation are centriole autonomous and can occur asynchronously. Centriole amplification, when present, occurs by the standard mechanism of one centriole generating one procentriole. PLK4 inhibition blocks centriole formation in differentiating TGCs but does not affect endocycle progression. In summary, centrioles in TGC endocycles undergo disengagement and conversion to centrosomes. This increases centrosome number, but to a limited extent compared with DNA reduplication. [Media: see text] [Media: see text] [Media: see text] [Media: see text]
DNA replication is normally coupled with centriole duplication in the cell cycle. Trophoblast giant cells (TGCs) of the placenta undergo endocycles resulting in polyploidy but their centriole state is not known. We used a cell culture model for TGC differentiation to examine centriole and centrosome number and properties. Prior to differentiation, trophoblast stem cells (TSCs) have either two centrioles before duplication, or four centrioles after. We find that average nuclear area increases approximately 8-fold over differentiation, but most TGCs do not have more than four centrioles. However, these centrioles become disengaged, acquire centrosome proteins, and can nucleate microtubules. In addition, some TGCs undergo further duplication and disengagement of centrioles, resulting in substantially higher numbers. Live imaging revealed that disengagement and separation are centriole autonomous and can occur asynchronously. Centriole amplification, when present, occurs by the standard mechanism of one centriole generating one procentriole. PLK4 inihibition blocks centriole formation in differentiating TGCs but does not affect endocycle progression. In summary, centrioles in TGC endocycles undergo disengagement and conversion to centrosomes. This increases centrosome number, but to a limited extent compared with DNA reduplication.
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