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

Abstract

During C. elegans oocyte meiosis I cytokinesis and polar body extrusion, cortical actomyosin is locally remodeled to assemble a contractile ring that forms within and remains part of a much larger and actively contractile cortical actomyosin network. This network both mediates contractile ring dynamics and generates shallow ingressions throughout the oocyte cortex during polar body extrusion. Based on our analysis of requirements for CLS-2, a member of the CLASP family of proteins that stabilize microtubules, we recently proposed that a balance of actomyosin-mediated tension and microtubule-mediated stiffness limits membrane ingression throughout the oocyte during meiosis I polar body extrusion. Here, using live cell imaging and fluorescent protein fusions, we show that CLS-2 is part of a group of kinetochore proteins, including the scaffold KNL-1 and the kinase BUB-1, that also co-localize during meiosis I to structures called linear elements, which are present within the assembling oocyte spindle and also are distributed throughout the oocyte in proximity to, but appearing to underlie, the actomyosin cortex. We further show that KNL-1 and BUB-1, like CLS-2, promote the proper organization of sub-cortical microtubules and also limit membrane ingression throughout the oocyte. Moreover, nocodazole or taxol treatment to destabilize or stabilize oocyte microtubules leads to, respectively, excess or decreased membrane ingression throughout the oocyte. Furthermore, taxol treatment, and genetic backgrounds that elevate the levels of cortically associated microtubules, both suppress excess membrane ingression in cls-2 mutant oocytes. We propose that linear elements influence the organization of sub-cortical microtubules to generate a stiffness that limits cortical actomyosin-driven membrane ingression throughout the oocyte during meiosis I polar body extrusion. We discuss the possibility that this regulation of sub-cortical microtubule dynamics facilitates actomyosin contractile ring dynamics during C. elegans oocyte meiosis I cell division.

Securin Regulates the Spatiotemporal Dynamics of Separase

“…Many of the outer kinetochore proteins in the fibrous corona are also found in the cytoplasmic linear elements. Although a definitive function of linear elements remains elusive, they have recently been shown to regulate cortical microtubules to control plasma membrane dynamics during polar body extrusion (Quiogue et al, 2023). One interesting possibility is that the microtubule regulatory function of these cortical filaments could contribute to separase relocalization.…”

“…The kinetochore localization of separase and securin may reflect the local recruitment of separase to the vicinity of anaphase target sites while preventing it from directly interacting with substrates. The linear elements were first observed by staining for HIM-10 (Howe et al, 2001), and subsequently numerous outer kinetochore proteins were also found to localize to them (Hattersley et al, 2016; Monen et al, 2015; Pereira et al, 2018; Quiogue et al, 2023). Similar kinetochore linear element structures were observed in fly and bovine oocytes, suggesting these are widely conserved structures (Gilliland et al, 2007; Wu et al, 2023).…”