Dynamic ordering of nuclei in syncytial embryos: a quantitative analysis of the role of cytoskeletal networks

Takuma Kanesaki,Jörg Grosshans,Ulrich S Schwarz,Carina M Edwards

doi:10.1039/c1ib00059d

Abstract

In syncytial embryos nuclei undergo cycles of division and rearrangement within a common cytoplasm. It is presently unclear to what degree and how the nuclear array maintains positional order in the face of rapid cell divisions. Here we establish a quantitative assay, based on image processing, for analysing the dynamics of the nuclear array. By tracking nuclear trajectories in Drosophila melanogaster embryos, we are able to define and evaluate local and time-dependent measures for the level of geometrical order in the array. We find that after division, order is re-established in a biphasic manner, indicating the competition of different ordering processes. Using mutants and drug injections, we show that the order of the nuclear array depends on cytoskeletal networks organised by centrosomes. While both f-actin and microtubules are required for re-establishing order after mitosis, only f-actin is required to maintain the stability of this arrangement. Furthermore, f-actin function relies on myosin-independent non-contractile filaments that suppress individual nuclear mobility, whereas microtubules promote mobility and attract adjacent nuclei. Actin caps are shown to act to prevent nuclear incorporation into adjacent microtubule baskets. Our data demonstrate that two principal ordering mechanisms thus simultaneously contribute: (1) a passive crowding mechanism in which nuclei and actin caps act as spacers and (2) an active self-organisation mechanism based on a microtubule network.

Highlights

In syncytial embryos nuclei undergo cycles of division and rearrangement within a common cytoplasm
In the absence of separating cell membranes the prime candidates for generating and transmitting forces in the embryo are f-actin, which builds up a cortical layer and forms the actin caps, and the microtubule network, which forms a basket around each nucleus (Fig. 1A).[10,15]
In which cells interact by a combination of extra- and intracellular forces,[4,26,40,41,42,43,44,45] interactions of syncytial nuclei involve only cytoplasmic components and intracellular forces

Summary

Introduction

In syncytial embryos nuclei undergo cycles of division and rearrangement within a common cytoplasm. We use automatic image processing to follow changes in the spatial organisation of nuclear positioning in the early (syncytial) insect embryo throughout the cell cycle. We combine this analysis with a range of drug-injection experiments and mutations. The main forces driving this process are assumed to be cytoskeletal in origin, with each nucleus in the array being associated with an actin cap, a pair of centrosomes and an enclosing microtubule basket.[9,10] These elements change dynamically, altering their arrangement, size and function as the nuclei pass through the various stages of the cell cycle. In contrast to the case of epithelial tissues, our setup enables direct observation of the underlying cellular mechanisms

Methods

Results

Conclusion