Spatial Coupling of Biological Oscillators: Towards a Quantitative Understanding of Mitotic Waves

Abstract

Over the years, various models were developed to capture the mechanism behind the mitotic clock. In short, the clock network centers on the cyclin-dependent kinase (Cdk1), such that its oscillatory rising and falling activity directs the cell through a series of steps which define one mitotic cycle. When a collection of these oscillators couple, they synchronize. In particular, early embryogenesis is marked by a series of synchronous cell divisions across the length of the embryo in various systems, e.g. Drosophila (approximately 0.5mm in length) and Xenopus (approximately 1.2mm in diameter). This synchrony allows embryos to simply grow in viability prior to differentiation, at which point, cell types diverge, and the physical form of the organism begins to take shape. However, the large size of these embryos implies a faster coordinating effect than diffusion alone. Work in the field proposes a mechanism for such spatial coordination in the form of mitotic waves. However, this literature describes waves which fall into two categories: canonical trigger, or bistable waves, and recently proposed sweep waves. These two types of waves are separated by both the speeds at which they propagate and the biochemical mechanisms behind their formation. Nevertheless, recent work proposes a model by which sweep waves may transition to trigger waves if cycles slow heterogeneously over time. To this point, little-to-no work exists studying the time dependence of mitotic waves in either context. Using Xenopus extracts and a Cdk1 FRET sensor, I exploit the slowing of oscillations in extracts to demonstrate such a transition for the first time. Moreover, I show how the addition of nuclei entrains the system to the trigger wave regime. Finally, using a novel approach utilizing metaphase-arrested extracts, I produce one-dimensional directed mitotic waves without reconstituted nuclei. With this setup, I reinforce the notion of entrainment explicitly, as well as probe the possible differences between mitotic waves in systems with and without nuclei, finding the speed scaling of the former to be significantly slower. Additionally, my work lays the groundwork for this system to be used in the future to systematically study perturbations to these waves. In total, the presented work offers the first direct observation of mitotic waves in Xenopus, explains their time-dependent behavior, and displays a unique method for exploring biochemical waves experimentally.