Mitotic spindle scaling during Xenopus development by kif2a and importin α

Jeremy D Wilbur,Rebecca Heald

doi:10.7554/elife.00290

Abstract

Early development of many animals is characterized by rapid cleavages that dramatically decrease cell size, but how the mitotic spindle adapts to changing cell dimensions is not understood. To identify mechanisms that scale the spindle during Xenopus laevis embryogenesis, we established an in vitro system using cytoplasmic extracts prepared from embryos that recapitulates in vivo spindle size differences between stage 3 (4 cells, 37 µm) and stage 8 (∼4000 cells, 18 µm). We identified the kinesin-13 kif2a as a driver of developmental spindle scaling whose microtubule-destabilizing activity is inhibited in stage 3 spindles by the transport receptor importin α, and activated in stage 8 when importin α partitions to a membrane pool. Altering spindle size in developing embryos impaired spindle orientation during metaphase, but chromosome segregation remained robust. Thus, spindle size in Xenopus development is coupled to cell size through a ratiometric mechanism controlling microtubule destabilization.DOI:http://dx.doi.org/10.7554/eLife.00290.001.

Highlights

Cell size varies widely among different organisms and cell types, and changes rapidly during early animal development when cell division occurs in the absence of growth
Reconstitution of spindle size differences in extracts of developing embryos To investigate how spindle size scales with cell size during development, we established a spindle reconstitution system utilizing cytoplasmic extracts from X. laevis embryos at different embryonic stages
This system is analogous to the cytostatic factor (CSF) arrested meiosis II extract derived from unfertilized X. laevis eggs, which has facilitated fundamental discoveries pertaining to the cell cycle and spindle assembly (Maresca and Heald, 2006)

Summary

Introduction

Cell size varies widely among different organisms and cell types, and changes rapidly during early animal development when cell division occurs in the absence of growth. There is an upper limit to spindle size (∼60 μm), and astral microtubules mediate movement of chromosomes long distances during anaphase (Wuhr et al, 2008). At these stages, spindle size is small compared to cell size, and cytoplasmic mechanisms likely operate, since in vivo spindle size is maintained in extracts prepared from eggs or two-cell stage embryos (Mitchison et al, 2005; Wuhr et al, 2008; Loughlin et al, 2011). To what extent cell size influences spindle size or if changes in cytoplasmic activities contribute to spindle scaling at these later stages is unknown

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Results

Conclusion