C. elegans XMAP215/ZYG-9 and TACC/TAC-1 act at multiple times throughout oocyte meiotic spindle assembly to promote both the coalescence of pole foci into a bipolar structure, and the stability of coalesced poles, during oocyte meiotic cell division

Abstract

The conserved two-component XMAP215/TACC modulator of microtubule stability is required in multiple animal phyla for acentrosomal spindle assembly during oocyte meiotic cell division, with C. elegans XMAP215/zyg-9 and TACC/tac-1 mutant oocytes exhibiting multiple and indistinguishable defects beginning early in meiosis I. To determine if these defects represent one or more early requirements with additional later and indirect consequences, or multiple temporally distinct and more direct requirements, we have used live cell imaging and fast-acting temperature-sensitive zyg-9 and tac-1 alleles to dissect at high temporal resolution their oocyte meiotic spindle assembly requirements. Our results from temperature upshift and downshift experiments indicate that the ZYG-9/TAC-1 complex has multiple temporally distinct and separable requirements throughout oocyte meiotic cell division. First, we show that during prometaphase ZYG-9 and TAC-1 promote the coalescence of early pole foci into a bipolar structure both by stabilizing pole foci as they grow and by limiting their growth rate, with these requirements being independent of an earlier defect in microtubule organization. Second, during metaphase, ZYG-9 and TAC-1 maintain spindle bipolarity by suppressing ectopic pole formation, and this pole stability is important for maintaining chromosome congression. Finally, we show that ZYG-9 and TAC-1 also are required for the proper coalescence of pole foci during meiosis II, independently of their requirements during meiosis I. Our findings highlight the value of fast-acting temperature-sensitive alleles for high resolution temporal dissection of gene requirements, and we 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.

ZYG-9ch-TOG promotes the stability of acentrosomal poles via regulation of spindle microtubules in C. elegans oocyte meiosis

“…We found that when ZYG-9 was removed from a pre-formed spindle, acentrosomal poles destabilize and the overlap zone of the spindle is disrupted, causing the spindle to become unstable and ultimately lose bipolarity. Similar results were recently reported in a study using temperature-sensitive mutants, confirming an essential role for ZYG-9 in stabilizing the acentrosomal spindle [36].…”

“…We speculate that this interaction could spatially regulate microtubule polymerase activity, promoting microtubule growth locally but minimizing potential ectopic growth throughout the cytoplasm. When disrupting ZYG-9/TAC-1 function via fast-acting ts mutants, a recent study reported an overall increase in microtubules within Meiosis I oocytes in tac-1 mutants, but not zyg-9 mutants [36]. This observation is consistent with the hypothesis that TAC-1 interacts with ZYG-9 to constrict where microtubule polymerization can occur during the meiotic divisions.…”

“…RNAi led to a notable increase in ectopic microtubule asters throughout oocytes [6]. In addition, rapid upshifts and downshifts to/from restrictive temperatures with zyg-9 temperature sensitive mutants revealed that ASPM-1 foci grew larger in volume and intensity when ZYG-9 was inactivated, and reverted to control levels upon downshifting [36]; these results raise the possibility that ZYG-9 activity suppresses microtubule growth and pole formation instead of promoting it.…”