Actomyosin Tube Formation in Polar Body Cytokinesis Requires Anillin in C. elegans

Abstract

Polar body extrusion (PBE) is the specialized asymmetric division by which oocytes accomplish reduction in ploidy and retention of cytoplasm. During maternal gametogenesis, as in male meiosis and mitosis, cytokinesis is accomplished by a ring rich in active Rho, myosin, and formin-nucleated F-actin [1-7]. However, unlike mitosis, wherein the contractile ring encircles the cell equator, the polar body ring assembles as a discoid cortical washer. Here we show that in Caenorhabditis elegans, the meiotic contractile ring transforms during closure from a disc above the spindle to a cylinder around the spindle midzone. The meiotic midbody tube comprises stacked cytoskeletal rings. This topological transition suggests a novel mechanism for constriction of an initially discoid cytokinetic ring. Analysis of mouse PBE indicates that midbody tube formation is a conserved process. Depletion of the scaffold protein anillin (ANI-1) from C. elegans results in large and unstable polar bodies that often fuse with the oocyte. Anillin is dispensable for contractile ring assembly, initiation, and closure but is required for the meiotic contractile ring to transform from a disc into a tube. We propose that cytoskeletal bundling by anillin promotes formation of the midbody tube, which ensures the fidelity of PBE.

Inhibition of ectopic microtubule assembly by the kinesin-13 KLP-7 prevents chromosome segregation and cytokinesis defects in oocytes

“…To test if the improper central spindle component localization in KLP-7-depleted oocytes correlated with defects in actomyosin organization, we analyzed oocytes expressing GFP-tagged myosin II (NMY-2) and mCherry-tagged H2B during anaphase I. Consistent with previous findings, in control oocytes NMY-2 formed a disc above the segregating chromosomes that progressively evolved into a cylinder, which ultimately formed the meiotic midbody between the segregated chromosomes (Dorn et al, 2010) (Fig. 3F, Movie 6, Table S1).…”

“…The precise mechanism of this atypical segregation is still unclear but involves microtubule-dependent forces exerted on chromosomes Muscat et al, 2015;McNally et al, 2016). The third specific adaptation of oocyte meiosis is polar body extrusion (PBE), which corresponds to an extremely asymmetric partitioning of the oocyte creating a tiny polar body with most of the cytoplasm maintained in the oocyte (Zhang et al, 2008;Dorn et al, 2010;Fabritius et al, 2011;Maddox et al, 2012).…”

F-actin binding protein, anillin, regulates integrity of intercellular junctions in human epithelial cells

“…Anillin plays a prominent role in cytokinesis by regulating cleavage furrow positioning and ingression during the separation of a dividing cell into daughter cells [27-29]. It is also involved in the transformation of the cellular cortex during early developmental events such as cellularization and polar body emission [30, 32, 36]. However, little is known about anillin’s function outside cell division.…”

Polar body emission

“…The contractile surface consists of thick F-actin with circular front, constricting as the anaphase spindle abuts against the protruding polar body, which itself has a much thinner cortical F-actin [Pielak et al, 2004]. Similar scheme appears to also apply to polar body emission in C. elegans embryos, except that an anillin-dependent actomyosin tube (a stack of rings) is proposed to ensure stability and strength to accomplish this task [Dorn et al, 2010]. A stronger contractile apparatus may be required to overcome the cytoplasmic pressure differential across the polar body neck due to the significant local membrane curvature differential of the smaller polar body and the large egg [Evans and Robinson, 2011].…”