XMAP215/ZYG-9 and TACC/TAC-1 Promote Bipolar Spindle Assembly and Stability during C. elegans Oocyte Meiotic Cell Division
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Date
2022-10-26
Authors
Harvey, Austin
Journal Title
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Publisher
University of Oregon
Abstract
Despite lacking canonical, centriole-containing microtubule organizing centers, oocytes can still organize microtubules into a bipolar spindle and accurately separate chromosomes. How oocyte acentrosomal spindles achieve bipolarity and separate chromosomes remains unclear, yet accuracy is critical as defects can lead to severe health consequences such as aneuploidy. The conserved two-component modulator of microtubule stability, comprised of XMAP215/ZYG-9 and TACC/TAC-1 in Caenorhabditis elegans, is required in multiple animal phyla for acentrosomal spindle assembly during oocyte meiotic cell division, with C. elegans zyg-9 and tac-1 mutant oocytes exhibiting multiple and indistinguishable defects beginning early in meiosis I. To better understand the role of XMAP215/ZYG-9 and TACC/TAC-1 during acentrosomal spindle assembly, we aimed to determine if these defects represent one early requirement, with additional later and indirect consequences, or multiple temporally distinct and more direct requirements. To accomplish this, we used live cell imaging and fast-acting temperature-sensitive zyg-9 and tac-1 alleles to dissect at high resolution their meiotic spindle assembly requirements in C. elegans oocytes.
Our results from temperature upshift and downshift experiments indicate that the XMAP215/ZYG-9 and TAC-1/TACC complex has multiple and temporally distinct requirements throughout oocyte meiotic cell division. First, we show that ZYG-9 and TAC-1 appear to promote the coalescence of early pole foci into a bipolar structure both by promoting pole stability and by limiting pole growth during meiosis I, with these requirements being independent of earlier defects in both microtubule organization and levels. Second, during metaphase I, ZYG-9 and TAC-1 maintain spindle bipolarity by suppressing ectopic pole formation, and this pole stability is important for maintaining chromosome congression at the metaphase plate. Finally, we show that ZYG-9/TAC-1 also are required for the proper coalescence of pole foci during meiosis II, independently of their requirements during meiosis I. Together, these analyses reveal that the ZYG-9 and TAC-1 complex has separable, stage-specific requirements throughout meiosis I and II, and we also discuss how negative regulation of microtubule stability by ZYG-9/TAC-1 during oocyte meiotic cell division might account for the observed defects in spindle pole coalescence and stability.
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Keywords
Acentrosomal spindle pole assembly, conditional gene knockdown, Microtubule stability, Oocyte meiotic cell division