Mitosis is a dramatic process that affects all parts of the cell. It is driven by an oscillator whose various components are localized in the nucleus, centrosome, and cytoplasm. In principle, the cellular location with the fastest intrinsic rhythm should act as a pacemaker for the process. Here we traced the waves of tubulin polymerization and depolymerization that occur at mitotic entry and exit in Xenopus egg extracts back to their origins. We found that mitosis was commonly initiated at sperm-derived nuclei and their accompanying centrosomes. The cell cycle was ~20% faster at these initiation points than in the slowest regions of the extract. Nuclei produced from phage DNA, which did not possess centrosomes, also acted as trigger wave sources, but purified centrosomes in the absence of nuclei did not. We conclude that the nucleus accelerates mitotic entry and propose that it acts as a pacemaker for cell cycle.

The results confirm previous in vivo and in vitro observations obtained from animal models that epinastine exerts antiallergic and antiinflammatory effects. Whether the observed effects are due to non specific membrane interactions or by influencing intracellular signal transduction elements has to be further elucidated.

Mitosis is a dramatic cellular process that affects all parts of the cell. In Xenopus embryos and extracts it is driven by the activation of a bistable trigger circuit, whose various components are localized in the nucleus, centrosome, and cytoplasm. In principle, whichever cellular location has the fastest intrinsic rhythm should act as a pacemaker for the process. Here 15we followed tubulin polymerization and depolymerization in Xenopus egg extracts supplemented with demembranated sperm, and thereby identified locations where mitosis first occurred. We found that mitosis was commonly first initiated at sperm-derived nuclei and their accompanying centrosomes, and then spread outward in circular trigger waves. The cell cycle was ~20% more rapid at the nucleus/centrosome-associated trigger wave sources than in the regions of the extract 20 that appeared not to be entrained by trigger waves. Nuclei produced from phage DNA, which did not possess centrosomes, also acted as trigger wave sources, but purified centrosomes in the absence of nuclei did not. We conclude that the nucleus accelerates mitotic entry and propose that it acts as a pacemaker for cell cycle.[170 words] 25One Sentence Summary: Studies in cycling Xenopus egg extracts show that mitosis first occurs in the nucleus and then spreads outward through the cytoplasm in circular trigger waves. Main Text: 30Mitotic entry is driven by a circuit of proteins that regulates cyclin-dependent protein kinase-1 (Cdk1) and its opposing phosphatases. The circuit includes at least 5 interlinked positive and double-negative feedback loops and functions as a bistable toggle switch, which transitions from a stable interphase state with low Cdk1 activity and high PP2A-B55 activity, to a stable M-phase state with high Cdk1 activity and low PP2A-B55 activity (1-5). In some contexts, like the 35Xenopus laevis embryonic cell cycle, the cell cycle operates as a relaxation oscillator (6), with the bistable triggering firing, and the cell cycle repeating, with a precise (± a few percent) (7-9) fixed period.

No abstract

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]