An endogenous calcium oscillator may control early embryonic division

An endogenous calcium oscillator may control early embryonic division

Department of Chemistry, Stanford University, Stanford, CA 94305

Abstract

Transient elevations in the concentration of free cytosolic calcium ion ([Ca²⁺]_i) promote cell phase transitions in early embryonic division and persist even if these transitions are blocked. These observations suggest that a [Ca²⁺]_i oscillator is an essential timing element of the early embryonic “master clock.” We explore this possibility by coupling a [Ca²⁺]_i oscillator model to an early embryonic cell cycle model based on the protein interactions that govern the activity of the M-phase-promoting factor (MPF). We hypothesize three dynamical states of the MPF system and choose parameter sets to represent each. We then investigate how [Ca²⁺]_i dynamics may control early embryonic division in both sea urchin and Xenopus embryos. To investigate both systems, distinct [Ca²⁺]_i profiles matching those observed in sea urchin embryos (in which [Ca²⁺]_i exhibits sharp transients) and Xenopus embryos (in which [Ca²⁺]_i is elevated and oscillates sinusoidally) are imposed on each of the hypothesized dynamical states of MPF. In the first hypothesis, [Ca²⁺]_i oscillations entrain the autonomous MPF oscillator. In the second and third hypotheses, where the MPF system rests in excitatory and bistable states, respectively, [Ca²⁺]_i oscillations drive MPF activation cycles. Simulation results show that hypotheses two and three, in which a [Ca²⁺]_i oscillator is a fundamental timing element of the master clock, best account for key experimental observations and the questions that they raise. Finally, we propose experiments to elucidate further [Ca²⁺]_i regulation and the fundamental components of the early embryonic master clock.

Footnotes

John Ross
Abbreviations: MPF, M-phase-promoting factor; [Ca²⁺]_i, concentration of free cytosolic calcium ion; BAPTA, 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetate; IP₃, inositol 1,4,5-trisphosphate; CamK II, Ca²⁺/calmodulin-dependent kinase; IE, intermediary enzyme; ICC, IP₃–Ca²⁺ cross-coupling.
↵ * The term transients is used to mean temporary increases in [Ca²⁺]_i. These increases may result from a [Ca²⁺]_i oscillator or other means. In this paper, the term calcium oscillations refers specifically to sustained periodic calcium transients.
↵ † MPF oscillations have been modeled with two-variable systems (17, 18) and, more recently, Novak and Tyson (19) have also published a comprehensive model that includes the assumed protein interactions that govern the activation state of MPF.
↵ ‡ The details of the [Ca²⁺]_i model are not fundamentally important. To verify this we have also studied the coupling of a two-variable calcium-induced calcium release model (23), and we found it made no significant difference. Other models could be considered (for example, see ref. 25). Most models, however, are similar to the one chosen here in that they consider agonist-induced oscillations in [Ca²⁺]_i. We have not seen a model that considers specifically the [Ca²⁺]_i transients associated with early embryonic development.
↵ § The parameter set to model extracts is used because we hope to study this system experimentally with extracts. In future theoretical work we hope to study intact systems; initial simulations of intact cells suggest that we can make conclusions similar to those posited in the present study.
↵ ¶ The shortest period to which [Ca²⁺]_i spikes can induce MPF activation peaks is longer than that of the autonomous MPF oscillator and than the periods reported in dividing embryos. We are not concerned with replicating period lengths exactly but only with the issue of [Ca²⁺]_i control. In simulations of this kind it is possible to generate any size period by scaling the kinetic constants, for example, we have used τ_scale to adjust the frequency of the ICC model.
↵ ‖ The deactivating transient may be generated through means distinct from the MPF system, or in response to active MPF. The latter possibility is suggested by the observation that the conserved PSTAIR peptide in p34^cdc2 induces a [Ca²⁺]_i transient (27).