Parameters that Regulate Size and Shape of the Vertebrate Meiotic Spindle

Abstract

During meiosis, the spindle equally segregates replicated genomes into two daughter cells. Spindles are mainly composed of microtubules (MTs) and molecular motors. Various studies have revealed key regulators of the spindle structure, such as kinesin-5 and depolymerizing kinesins, which produce force for the sliding of MTs or regulating the polymerization/depolymerization dynamics of MTs. Forces generated by the spindle are known to play an essential role in the checkpoint of the metaphase-anaphase transition. At metaphase, the size of the spindle is maintained almost constant despite the dynamic nature of the spindle MTs. This indicates that the forces generated by molecular motors and MTs are well balanced along the pole-to-pole axis; however, how this is achieved remains enigmatic. In this study, using a pair of glass microneedles, we developed quantitative micromanipulation techniques to deform the spindle directly, such as pushing, stretching, cutting, and fusing spindles, and 3D analysis of the spindle. We found that the meiotic spindles assembled in the Xenopus egg extracts have the ability to spontaneously reorganize, acquiring stable size and shape, after physical perturbations. Surprisingly, the density and the amount of MTs within the spindle were also regulated during the spontaneous reorganization. These results indicate that force generators, such as the molecular motors and MTs, dynamically regulate the spindle structure. We also found that the stable spindle length is fully determined by 4 three-dimensional parameters, indicating that the fluctuations of the length of the stable spindle are characterized by the variability of these 4 parameters.