C. elegans XMAP215/ZYG-9 and TACC/TAC-1 act at multiple times during oocyte meiotic spindle assembly and promote both spindle pole coalescence and stability

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. In C. elegans, XMAP215/zyg-9 and TACC/tac-1 mutant oocytes exhibit multiple and indistinguishable oocyte spindle assembly 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 their requirements at high temporal resolution. 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, stabilizing pole foci as they grow and limiting their growth rate, with these requirements being independent of an earlier defect in microtubule organization that occurs upon nuclear envelope breakdown. Second, during metaphase, ZYG-9 and TAC-1 maintain spindle bipolarity by suppressing ectopic pole formation. Third, we show that ZYG-9 and TAC-1 also are required for spindle assembly during meiosis II, independently of their meiosis I requirements. The metaphase pole stability requirement appears to be important for maintaining chromosome congression, 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.

Cortical microtubules oppose actomyosin-driven membrane ingression duringC. elegansmeiosis I polar body extrusion

“…The results we report here, and other investigations of C. elegans meiotic cell division, clearly indicate that microtubules are an integral part of the oocyte cortex (18,19,28,29).…”

“…In addition to CLS-2, two other widely conserved regulators of microtubule stability are known to influence oocyte cortical microtubule levels: the kinesin 13/MCAK microtubule depolymerase family member called KLP-7, and another TOG domain protein, ZYG-9/chTOG. Both KLP-7 and ZYG-9 are required for oocyte meiotic spindle assembly, and reducing the function of either results in elevated levels of both spindle and cortical microtubules (27)(28)(29). While the consequences of increased cortical microtubules in klp-7 and zyg-9 mutant oocytes have not been investigated, microtubules clearly are a regulated component of the oocyte cortex and therefore might influence cortical elasticity and stiffness, and contractile ring dynamics, during meiotic cell division.…”

Microtubules oppose cortical actomyosin-driven membrane ingression during C. elegans meiosis I polar body extrusion

“…Instead, it was equally or more defective in the double mutants compared to cls-2 , klp-7 and zyg-9 single mutants. Indeed, polar body extrusion only rarely succeeded even in all single mutant oocytes ( S4B Fig ), which all have extensive oocyte spindle assembly defects [ 24 – 26 ]. Furthermore, although membrane ingression in both klp-7 and zyg-9 mutant oocytes generally appeared reduced, we nevertheless observed deep and prominent ingressions near the oocyte chromosomes in some cls-2 , klp-7 and zyg-9 single mutants (Figs 10C, 10E and S3 and S3 Movie ).…”

“…In addition to CLS-2, two other widely conserved proteins are known to regulate microtubules both within the meiotic spindle and throughout the oocyte: the kinesin 13/ MCAK microtubule depolymerase family member called KLP-7, and another TOG domain protein, ZYG-9/chTOG. Both KLP-7 and ZYG-9 are required for oocyte meiotic spindle assembly, and reducing the function of either results in elevated levels of both spindle microtubules and microtubules enriched near the cortex throughout the oocyte [24][25][26]. While KLP-7 and ZYG-9 clearly regulate their levels, the functional significance of these cortically associated microtubules and their regulation by KLP-7 and ZYG-9 have received little attention.…”

Structural characterization and inhibition of the interaction between ch-TOG and TACC3

“…First in Drosophila spindles, the PCM is templated by a cnn-containing structure with a more liquid-like D-TACC region associated with it (Wong et al, 2024). Second in C. elegans oocyte meiotic spindles, XMAP215/ZYG-9 and TACC/TAC-1 act at multiple times during assembly to promote spindle pole integrity and stability (Harvey et al, 2023). Our data suggest that the TACC3–ch-TOG interaction is important for maintaining the PCM around the centrosomes and that fragmentation results in a mitotic delay.…”