Changes in oscillatory dynamics in the cell cycle of early Xenopus laevis embryos

Timing by rhythms: Daily clocks and developmental rulers

Biological rhythms are widespread, allowing organisms to temporally organize their behavior and metabolism in advantageous ways. Such proper timing of molecular and cellular events is critical to their development and health. This is best understood in the case of the circadian clock that orchestrates the daily sleep/wake cycle of organisms. Temporal rhythms can also be used for spatial organization, if information from an oscillating system can be recorded within the tissue in a manner that leaves a permanent periodic pattern. One example of this is the "segmentation clock" used by the vertebrate embryo to rhythmically and sequentially subdivide its elongating body axis. The segmentation clock moves with the elongation of the embryo, such that its period sets the segment length as the tissue grows outward. Although the study of this system is still relatively young compared to the circadian clock, outlines of molecular, cellular, and tissue-level regulatory mechanisms of timing have emerged. The question remains, however, is it truly a clock? Here we seek to introduce the segmentation clock to a wider audience of chronobiologists, focusing on the role and control of timing in the system. We compare and contrast the segmentation clock with the circadian clock, and propose that the segmentation clock is actually an oscillatory ruler, with a primary function to measure embryonic space.

Size Scaling of Microtubule Assemblies in Early Xenopus Embryos

The first 12 cleavage divisions in Xenopus embryos provide a natural experiment in size scaling, as cell radius decreases ∼16-fold with little change in biochemistry. Analyzing both natural cleavage and egg extract partitioned into droplets revealed that mitotic spindle size scales with cell size, with an upper limit in very large cells. We discuss spindle-size scaling in the small- and large-cell regimes with a focus on the "limiting-component" hypotheses. Zygotes and early blastomeres show a scaling mismatch between spindle and cell size. This problem is solved, we argue, by interphase asters that act to position the spindle and transport chromosomes to the center of daughter cells. These tasks are executed by the spindle in smaller cells. We end by discussing possible mechanisms that limit mitotic aster size and promote interphase aster growth to cell-spanning dimensions.

How Does the Xenopus laevis Embryonic Cell Cycle Avoid Spatial Chaos?

Theoretical studies have shown that a deterministic biochemical oscillator can become chaotic when operating over a sufficiently large volume and have suggested that the Xenopus laevis cell cycle oscillator operates close to such a chaotic regime. To experimentally test this hypothesis, we decreased the speed of the post-fertilization calcium wave, which had been predicted to generate chaos. However, cell divisions were found to develop normally, and eggs developed into normal tadpoles. Motivated by these experiments, we carried out modeling studies to understand the prerequisites for the predicted spatial chaos. We showed that this type of spatial chaos requires oscillatory reaction dynamics with short pulse duration and postulated that the mitotic exit in Xenopus laevis is likely slow enough to avoid chaos. In systems with shorter pulses, chaos may be an important hazard, as in cardiac arrhythmias, or a useful feature, as in the pigmentation of certain mollusk shells.

Ultrasensitivity part III: cascades, bistable switches, and oscillators

Switch-like, ultrasensitive responses - responses that resemble those of cooperative enzymes but are not necessarily generated by cooperativity - are widespread in signal transduction. In the previous installments in this series, we reviewed several mechanisms for generating ultrasensitivity: zero-order ultrasensitivity; multistep ultrasensitivity; inhibitor ultrasensitivity; and positive feedback (or double negative feedback) loops. In this review, we focus on how ultrasensitive components can be important for the functioning of more complex signaling circuits. Ultrasensitivity can allow the effective transmission of signals down a signaling cascade, can contribute to the generation of bistability by positive feedback, and can promote the production of biochemical oscillations in negative feedback loops. This makes ultrasensitivity a key building block in systems biology and synthetic biology.

Ultrasensitivity part II: multisite phosphorylation, stoichiometric inhibitors, and positive feedback

In this series of reviews, we are examining ultrasensitive responses, the switch-like input-output relationships that contribute to signal processing in a wide variety of signaling contexts. In the first part of this series, we explored one mechanism for generating ultrasensitivity, zero-order ultrasensitivity, where the saturation of two converting enzymes allows the output to switch from low to high over a tight range of input levels. In this second installment, we focus on three conceptually distinct mechanisms for ultrasensitivity: multisite phosphorylation, stoichiometric inhibitors, and positive feedback. We also examine several related mechanisms and concepts, including cooperativity, reciprocal regulation, coherent feed-forward regulation, and substrate competition, and provide several examples of signaling processes where these mechanisms are known or are suspected to be applicable.

Ultrasensitivity part I: Michaelian responses and zero-order ultrasensitivity

Quantitative studies of signal transduction systems have shown that ultrasensitive responses - switch-like, sigmoidal input/output relationships - are commonplace in cell signaling. Ultrasensitivity is important for various complex signaling systems, including signaling cascades, bistable switches, and oscillators. In this first installment of a series on ultrasensitivity we survey the occurrence of ultrasensitive responses in signaling systems. We review why the simplest mass action systems exhibit Michaelian responses, and then move on to zero-order ultrasensitivity, a phenomenon that occurs when signaling proteins are operating near saturation. We also discuss the physiological relevance of zero-order ultrasensitivity to cellular regulation.

Cdc25 and the importance of G2 control: insights from developmental biology

While cell proliferation is an essential part of embryonic development, cells within an embryo cannot proliferate freely. Instead, they must balance proliferation and other cellular events such as differentiation and morphogenesis throughout embryonic growth. Although the G1 phase has been a major focus of study in cell cycle control, it is becoming increasingly clear that G2 regulation also plays an essential role during embryonic development. Here we discuss the role of Cdc25, a key regulator of mitotic entry, with a focus on several recent examples that show how the precise control of Cdc25 activity and the G2/M transition are critical for different aspects of embryogenesis. We finish by discussing a promising technology that allows easy visualization of embryonic and adult cells potentially regulated at mitotic entry, permitting the rapid identification of other instances where the exit from G2 plays an essential role in development and tissue homeostasis.