Spindle assembly and chromosome dynamics during oocyte meiosis

Bowerman

2020

PLoS Genet

The requirements for oocyte meiotic cytokinesis during polar body extrusion are not well understood. In particular, the relationship between the oocyte meiotic spindle and polar body contractile ring dynamics remains largely unknown. We have used live cell imaging and spindle assembly defective mutants lacking the function of CLASP/CLS-2, kinesin-12/ KLP-18, or katanin/MEI-1 to investigate the relationship between meiotic spindle structure and polar body extrusion in C. elegans oocytes. We show that spindle bipolarity and chromosome segregation are not required for polar body contractile ring formation and chromosome extrusion in klp-18 mutants. In contrast, oocytes with similarly severe spindle assembly defects due to loss of CLS-2 or MEI-1 have penetrant and distinct polar body extrusion defects: CLS-2 is required early for contractile ring assembly or stability, while MEI-1 is required later for contractile ring constriction. We also show that CLS-2 both negatively regulates membrane ingression throughout the oocyte cortex during meiosis I, and influences the dynamics of the central spindle-associated proteins Aurora B/AIR-2 and MgcRacGAP/CYK-4. We suggest that proper regulation by CLS-2 of both oocyte cortical stiffness and central spindle protein dynamics may influence contractile ring assembly during polar body extrusion in C. elegans oocytes.

Section: Using Spindle Assembly Defective Mutants To Explore Contractmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

C. elegans CLASP/CLS-2 negatively regulates membrane ingression throughout the oocyte cortex and is required for polar body extrusion

Bowerman

2020

PLoS Genet

“…While the requirements for several factors that control oocyte meiotic spindle assembly in C. elegans have been described (1–3), the impact of the resulting spindle assembly defects on polar body extrusion has remained largely unexplored. Indeed, anecdotal observations have suggested that polar body extrusion can occur even in mutants with severe oocyte spindle assembly defects.…”

Section: Discussionmentioning

confidence: 99%

C. elegansCLASP/CLS-2 negatively regulates membrane ingression throughout the oocyte cortex and is required for polar body extrusion

Bowerman

2020

Preprint

1 2 3 C. elegans CLASP/CLS-2 negatively regulates membrane ingression 4 throughout the oocyte cortex and is required for polar body extrusion 5 6 Aleesa 11 12 13 Short Title: Spindle structure and polar body extrusion 14 15 2 16 Abstract 17The requirements for oocyte meiotic cytokinesis during polar body extrusion are 18 not well understood. In particular, the relationship between the oocyte meiotic spindle 19 and polar body contractile ring dynamics remains largely unknown. We have used live 20 cell imaging and spindle assembly defective mutants lacking the function of 21 CLASP/CLS-2, kinesin-12/KLP-18, or katanin/MEI-1 to investigate the relationship 22 between meiotic spindle structure and polar body extrusion in C. elegans oocytes. We 23 show that spindle bipolarity and chromosome segregation are not required for polar 24 body contractile ring formation and chromosome extrusion in klp-18 mutants, but 25 oocytes with severe spindle assembly defects due to loss of CLS-2 or MEI-1 have 26 penetrant and distinct polar body extrusion defects: CLS-2 is required early for 27 contractile ring assembly or stability, while MEI-1 is required later for contractile ring 28 constriction. We also show that CLS-2 negatively regulates membrane ingression 29 throughout the oocyte cortex during meiosis I, and we explore the relationship between 30 global cortical dynamics and oocyte meiotic cytokinesis. 313 32 Author Summary 33The precursor cells that produce gametes-sperm and eggs in animals-have 34 two copies of each chromosome, one from each parent. These precursors undergo 35 specialized cell divisions that leave each gamete with only one copy of each 36 chromosome; defects that produce incorrect chromosome number cause severe 37 developmental abnormalities. In oocytes, these cell divisions are highly asymmetric, 38 with extra chromosomes discarded into small membrane bound polar bodies, leaving 39 one chromosome set within the much larger oocyte. How oocytes assemble the 40 contractile apparatus that pinches off polar bodies remains poorly understood. To better 41 understand this process, we have used the nematode Caenorhabditis elegans to 42 investigate the relationship between the bipolar structure that separates oocyte 43 chromosomes, called the spindle, and assembly of the contractile apparatus that 44 pinches off polar bodies. We used a comparative approach, examining this relationship 45 in three spindle assembly defective mutants. Bipolar spindle assembly and 46 chromosome separation were not required for polar body extrusion, as it occurred 47 normally in mutants lacking a protein called KLP-18. However, mutants lacking the 48 protein CLS-2 failed to assemble the contractile apparatus, while mutants lacking the 49 protein MEI-1 assembled a contractile apparatus that failed to fully constrict. We also 50 found that CLS-2 down-regulates membrane ingression throughout the oocyte surface, 51 and we explored the relationship between oocyte membrane dynamics and polar body 52 extrusion. 53 Introduction 54Oocyte meiosis co...

“…C. elegans oocyte meiotic spindles form in the absence of centrosomes and reduce the duplicated diploid genome to a haploid content through two rounds of cell division called meiosis I and II (Dumont and Desai, 2012; Mullen et al, 2019; Ohkura, 2015; Severson et al, 2016). While acentrosomal microtubule nucleation pathways have been shown to mediate meiotic and mitotic spindle assembly in some settings, how C. elegans oocytes nucleate microtubules and assemble bipolar spindles remains poorly understood.…”

Section: Introductionmentioning

confidence: 99%

Microtubule Assembly and Pole Coalescence: Early Steps inC. elegansOocyte Meiosis I Spindle Assembly

Chuang

Yang

et al. 2020

Preprint

How oocytes assemble bipolar meiotic spindles in the absence of centrosomes as microtubule organizing centers remains poorly understood. We have used live cell imaging in C. elegans to investigate requirements for the nuclear lamina and for conserved regulators of microtubule dynamics during oocyte meiosis I spindle assembly, assessing these requirements with respect to recently identified spindle assembly steps. We show that the nuclear lamina is required for microtubule bundles to form a cage-like structure that appears shortly after oocyte nuclear envelope breakdown and surrounds the oocyte chromosomes, although bipolar spindles still assembled in its absence. Although two conserved regulators of microtubule nucleation, RAN-1 and γ-tubulin, are not required for bipolar spindle assembly, both contribute to normal levels of spindle-associated microtubules and spindle assembly dynamics. Finally, the XMAP215 ortholog ZYG-9 and the nearly identical minus-end directed kinesins KLP-15/16 are required for proper assembly of the early cage-like structure of microtubule bundles, and for early spindle pole foci to coalesce into a bipolar structure. Our results provide a framework for assigning molecular mechanisms to recently described steps in C. elegans oocyte meiosis I spindle assembly.