Recently we demonstrated that the bistability of the Cdk1/Cdc25/Wee1 system allows Cdk1 activation to propagate rapidly through Xenopus cytoplasm by means of what are termed trigger waves, waves of activation and inactivation that spread the way action potentials spread down an axon. This work was made possible through the development of a Teflon tube system that is compatible with fluorescence microscopy and allows cycling extracts to carry out a dozen or more complete cell cycles without dying. The speed of the Cdk1 trigger wave (~60 ?m/min) is sufficient to account for the dynamics of mitosis and of the surface contraction waves (SCWs) that occur prior to cleavage in intact embryos. This work was published last year in Nature. We propose to build upon this work through studies divided into three Specific Aims:
Aim 1. Mitotic and meiotic trigger waves in eggs, oocytes, and extracts. We now plan to examine mitotic waves in greater spatial detail and examine the roles of nuclei and centrosomes in the generation and propagation of these waves. We also plan to examine the interplay between mitotic waves, which we suspect helps keep ectopic foci of Cdk1 activation from disorganizing the first cell cycle. Finally, we plan to characterize the mechanism and significance of a newly discovered meiotic trigger wave phenomenon in oocytes, which we suspect may be involved in the expulsion of the first polar body and the completion of meiosis 1.
Aim 2. Intercellular coupling and the synchronization of multicellular embryos. Once the fertilized egg begins to divide, the issue of keeping mitosis spatially coordinated within individual blastomeres becomes less problematic, but the issue of keeping mitosis coordinated between separate cells becomes more problematic. In preliminary studies we have shown that when an embryo is desynchronized with a transitory temperature gradient, the cells subsequently return toward synchrony. Several mechanisms, singly or together, may explain this synchronization, including communication through cytoplasmic bridges, gap junctions, and cytoskeletal elements. We plan to test these ideas through experiments in intact embryos and egg extracts.
Aim 3. Spatial coordination of apoptosis. The caspase system includes multiple positive feedback loops that could generate bistability and help ensure that apoptosis is all-or-none and irreversible in character. We plan to test whether caspase activation is, in fact, bistable, using Xenopus egg extracts as a model system. If it is, then it is possible that this bistability allows the apoptotic state to propagate rapidly through te egg via trigger waves. Preliminary studies indicate that this is the case: the apoptotic state apparently propagates through Xenopus cytoplasm at a constant speed of ~15 ?m/min. We plan to characterize these waves in the Teflon tube extract system and to dissect the feedback loops that generate them.
One common view is that intracellular communication occurs largely through random walk diffusion, and long range communication occurs largely through flow (e.g. the mammalian circulatory system). Recently we discovered that in Xenopus eggs, the process of mitosis is coordinated through what are termed trigger waves of Cdk1 activation and inactivation. Other examples of trigger waves include action potentials in neurons, calcium waves in tissues, and cAMP waves in aggregating slime molds. We suspect that evolution has repeatedly converged upon trigger waves as a means of coordinating events over large distances. Here we are studying trigger waves in the coordination of cell cycles and cell death.
| Cheng, Xianrui; Ferrell Jr, James E (2018) Apoptosis propagates through the cytoplasm as trigger waves. Science 361:607-612 |
| Anderson, Graham A; Gelens, Lendert; Baker, Julie C et al. (2017) Desynchronizing Embryonic Cell Division Waves Reveals the Robustness of Xenopus laevis Development. Cell Rep 21:37-46 |
| Ferrell Jr, James E (2016) Perfect and Near-Perfect Adaptation in Cell Signaling. Cell Syst 2:62-7 |
| Gelens, Lendert; Huang, Kerwyn Casey; Ferrell Jr, James E (2015) How Does the Xenopus laevis Embryonic Cell Cycle Avoid Spatial Chaos? Cell Rep 12:892-900 |
| Gelens, Lendert; Anderson, Graham A; Ferrell Jr, James E (2014) Spatial trigger waves: positive feedback gets you a long way. Mol Biol Cell 25:3486-93 |
