Probing the Force-Balancing Mechanism of the Meiotic Spindle in Xenopus Egg Extracts

Abstract

During cell division, the meiotic spindle equally segregates replicated genomes into two daughter cells. Errors in this process cause birth defect and cancer.Spindles are mainly composed of microtubules (MTs) and molecular motors. Various studies have revealed key regulators, such as Kinesin-5 (plus-end directed kinesin tetramer), depolymerizing kinesins and microtubule-associated proteins (MAPs). These exert forces for sliding MTs or regulating MT dynamics in the spindle, so that the spindle maintains a rugby ball-like structure at metaphase. These forces are also known to generate a poleward flux of spindle MTs. At metaphase, size and shape of the spindle are maintained constant in spite of the dynamic nature of spindle MTs. This indicates forces exerted by molecular motors and MTs are well balanced in the spindle.In this study, we developed micromanipulation techniques for changing spindle shape to disrupt steady state force balance of the spindle without any changes in molecular components of the spindle. Spindles spontaneously assembled in Xenopus egg extracts were stretched along their pole-to-pole axis using two glass micro-needles. When the spindles were briefly stretched, they recovered their original size and shape after a while. In contrast, when spindles were kept stretched, they gradually recovered their original shape with the increase in the spindle width, resulting in the enlargement in size. This result indicates that the meiotic spindle has an ability to adjust its size and shape to the externally applied force. Our findings provide new insights into the force-balancing mechanism of the spindle.