The nucleus serves as the pacemaker for the cell cycle

Abstract

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.

Synchronizing an oscillatory medium: The speed of pacemaker-generated waves

“…The biochemical underpinnings of these cell cycle oscillations are well characterized. Moreover, in extracts made of these frog eggs, waves of mitosis have been observed [12], and recently the importance of pacemakers in these systems has been shown as well [26,27].…”

“…In such chemical systems, pacemakers can appear through impurities such as dust particles, or they can be induced, for example, to suppress chaotic behavior [22]. In biological systems, target patterns occur in cyclic adenosine monophosphate (cAMP) signaling in Dictyostelium discoideum [23], cardiac tissue [18], yeast glycolysis [24], neural tissue [25], and cell-free extracts of Xenopus laevis frog eggs [12], where nuclei act as pacemakers to organize the dynamics [26,27]. Pacemaker-generated waves have a clear function in these biological systems: they synchronize the system over long distances and transmit information inside the cell or between cells [1,11,16].…”

Dynamics of diffusive cell signaling relays

“…While diffusive waves have been observed in a variety of biological processes, they have also been experimentally probed in a variety of spatial contexts – in quasi-1D tubes ( Cheng and Ferrell, 2018 ; Chang and Ferrell, 2013 ; Nolet et al, 2020 ), in quasi-2D droplets and chambers ( Nolet et al, 2020 ; Afanzar et al, 2020 ), on 2D surfaces in fly eggs ( Vergassola et al, 2018 ), and on substrates of finite thickness ( Parkin and Murray, 2018 ; Pálsson and Cox, 1996 ). And while the phenomenology of diffusive waves has been studied for years, in the context of cell signaling it is less well-understood how the propagation and initiation of such waves are affected by the dimensionalities of the cellular distribution and the diffusive environment – or even how to identify the system dimensionality – as previous modeling work has largely assumed quasi-1D dynamics ( Kessler and Levine, 1993 ; Meyer, 1991 ; Gelens et al, 2014 ; Vergassola et al, 2018 ).…”

Dynamic bistable switches enhance robustness and accuracy of cell cycle transitions

“…Waves of Cdk1 activity spread throughout the cell at mitotic entry, as has been observed in Xenopus cell-free extracts [71][72][73] and in the early Drosophila embryo [74,75]. In the cell, and in extracts, spatial heterogeneities are present through nuclei, which concentrate certain proteins [72].…”

“…Models that do include a spatial component often focus on traveling waves which can play a role in synchronizing large cells, such as Xenopus [71] or Drosophila embryos [74]. In Xenopus cell-free extracts, nuclei play an essential role as a pacemaker [72,73], possibly due to the fact that nuclei locally increase concentrations of key PLOS COMPUTATIONAL BIOLOGY regulatory proteins [72]. This can possibly be interpreted as changing bistable response curves in nucleus and cytoplasm.…”