Aurora B kinase promotes cytokinesis by inducing centralspindlin oligomers that associate with the plasma membrane

Abstract

In metazoans, cytokinesis is triggered by activation of the GTPase RhoA at the equatorial plasma membrane. ECT-2, the guanine nucleotide exchange factor (GEF) required for RhoA activation, is activated by the centralspindlin complex that concentrates on spindle midzone microtubules. However, these microtubules and the plasma membrane are not generally in apposition, and thus the mechanism by which RhoA is activated at the cell equator remains unknown. Here we report that a regulated pool of membrane-bound, oligomeric centralspindlin stimulates RhoA activation. The membrane-binding C1 domain of CYK-4, a centralspindlin component, promotes furrow initiation in C. elegans embryos and human cells. Membrane localization of centralspindlin oligomers is globally inhibited by PAR-5/14-3-3. This activity is antagonized by the chromosome passenger complex (CPC), resulting in RhoA activation at the nascent cleavage site. Therefore, CPC-directed centralspindlin oligomerization during anaphase induces contractile ring assembly at the membrane.

Figure 1. PAR-5/14-3-3 Depletion Leads to Enhanced Centralspindlin-Dependent Cortical Contractility

(A) Representative maximum intensity projections of time-lapse movies of one-cell C. elegans embryos expressing the membrane marker PH∷GFP during polarity establishment. (B) 3D maximum intensity projections of representative PH∷GFP-expressing embryos after completion of cytokinesis. (C) One-cell C. elegans embryos were filmed by DIC microscopy shortly after fertilization until completion of the first division. Shown are representative images of embryos of the indicated genotypes at pseudocleavage and late anaphase. ect-2(ax751ts) is a hypomorphic allele (Zonies et al., 2010). Arrows indicate ectopic membrane furrows. (D) Embryos of the genotypes shown in (C). were scored for the extent of contractility observed during pseudocleavage and cytokinesis (see Supplemental Experimental Procedures and Figure S1C for definition of the contractility index). Error bars indicate SEM. n.s. indicates that the populations are not significantly different at p = 0.05. ***p ≤ 0.01 and *p ≤ 0.05 (Mann-Whitney U test). a.u., arbitrary unit. (E) Schematic illustrating the genetic pathway leading to RhoA activation and cortical contractility in C. elegans embryos. n ≥ 5. Scale bars, 10 μm.

Figure 2. Cortical Levels of Active RhoA and Centralspindlin Increase upon PAR-5/14-3-3 Depletion

(A) One-cell C. elegans embryos expressing the chromatin marker mCherry∷HIS and a GFP-tagged RhoA biosensor were filmed starting at metaphase. Representative images of wild-type and par-5(RNAi) embryos at the indicated times after anaphase onset are shown. (B) Wild-type or PAR-5-depleted embryos expressing either CYK-4∷GFP or ZEN-4∷GFP were analyzed by time-lapse microscopy. Representative images of embryos 50 s after anaphase onset are shown. (C) PAR-5 depleted embryos expressing labeled centralspindlin subunits were analyzed by time-lapse microscopy. Bottom, PAR-5 was depleted in combination with CYK-4 or ZEN-4 as indicated. (D) Cortical localization of centralspindlin does not require microtubules. Wild-type and par-5(RNAi) embryos were treated with 50 μg/ml nocodazole. The pronuclei do not meet under these conditions, and the embryo sets up a small posterior spindle during anaphase. Arrows indicate cortical localization of the GFP-tagged protein. n ≥ 5. Scale bars, 10 μm. Quantitation of these data is shown in Figure S2.

Figure 3. The Aurora B Kinase AIR-2, a CPC Component, Antagonizes PAR-5 Inhibition of Centralspindlin

(A) One-cell C. elegans embryos were analyzed by time-lapse DIC microscopy shortly after fertilization until completion of the first division. Shown are representative images of embryos of the indicated genotypes at pseudocleavage and late anaphase. The temperature-sensitive allele air-2(or207ts) abolishes Aurora B kinase activity (Severson et al., 2000). AIR-2 depletion/inactivation results in defects in chromosome segregation that are independent of its role in cytokinesis. Changes in contractility are quantified in Figure S3. n ≥ 7. Scale bar, 10 μm. (B) Schematic for the genetic pathway activating RhoA and generating cortical contractility in C. elegans embryos.

Figure 4. Astral Microtubules Pattern RhoA Activity and Furrow Position

One-cell stage C. elegans embryos expressing TUB∷GFP and mCherry∷HIS were depleted of the microtubule-associated protein ZYG-9, resulting in the formation of a small posterior spindle. With the exception of the posterior, midzone-directed furrow, cortical contractility is inversely correlated with the position of the asters. Representative images of cells of the indicated genotypes at anaphase are shown. A schematic of the observed cortical contractility is depicted for each condition. The yellow circles represent centrosomes in the posterior spindle. Scale bar, 10 μm. Changes in contractility are quantified in Figure S4.

Figure 5. The CYK-4 C1 Domain Promotes RhoA Activation and Cortical Localization of Centralspindlin in C. elegans Embryos

(A) nop-1(it142) embryos expressing RNAi-resistant wild-type CYK-4∷GFP or CYK-4ΔC1∷GFP transgenes were depleted of endogenous CYK-4 by RNAi and filmed during the first division. Representative images of embryos 200 s after anaphase onset are shown. n ≥ 5. (B) C. elegans embryos expressing RNAi-resistant wild-type CYK-4∷GFP or CYK-4ΔC1∷GFP were simultaneously depleted of endogenous CYK-4 and PAR-5. The arrow indicates cortical CYK-4∷GFP. Cortical recruitment of CYK-4∷GFP is quantified in Figure S5A. n ≥ 10. (C) C. elegans embryos expressing a mCherry∷PH membrane marker with an RNAi-resistant wild-type CYK-4∷GFP or CYK-4ΔC1∷GFP transgene were depleted of endogenous CYK-4 by RNAi. These embryos were filmed starting at metaphase in the first division cycle. Shown are montages of the equatorial region as the cell divides (n ≥ 5). The arrow indicates the appearance of cortical CYK-4∷GFP under wild-type conditions. Scale bar, 10 μm. Recruitment of the GFP-tagged protein to the furrow tip is quantified in Figure S5B.

Figure 6. A zen-4 Variant Predicted to be Defective in PAR-5 Binding Phenocopies PAR-5 Depletion

(A) Domain structure of the ZEN-4 protein showing its oligomerization domain and conserved sites in the globular tail that are predicted to regulate centralspindlin clustering. NL, neck linker; CC, coiled coil. (B) One-cell C. elegans embryos homozygous for zen-4(S682A) at the endogenous locus were analyzed by time-lapse DIC microscopy shortly after fertilization until completion of the first division. Shown are representative images at pseudocleavage and cytokinesis. Arrows indicate membrane furrows. n = 13. See also Movie S1. (C) Contractility observed in zen-4(S682A) was quantified (see Supplemental Experimental Procedures) and plotted to compare with wild-type and par-5(RNAi) embryos (n ≥ 7). Error bars represent ± SEM. n.s. indicates that the populations are not significantly different at p = 0.05, ***p ≤ 0.01, Mann-Whitney U test. (D) Wild-type and zen-4(S682A) embryos expressing CYK-4∷GFP during pseudocleavage (at maximal ingression) and at 40 s after anaphase onset. Arrows indicate ectopic membrane recruitment of CYK-4∷GFP. Pseudocleavage furrows in zen-4(S682A) embryos often ingress completely, and CYK-4∷GFP can be detected on the furrow tips. n ≥ 5. Scale bar, 10 μm. Membrane recruitment of CYK-4∷GFP is quantified in Figure S6A.

Figure 7. Membrane Association of Centralspindlin Promotes RhoA Activation in Human Cells

(A) Schematic depicting conditions used for mitotic arrest and release of HeLa cells. Shown is the domain structure of the full-length HsCyk4 construct, truncations of which are used in (B). N, N-terminal domain. (B) HeLa cells transfected with siRNA targeting endogenous HsCyk4 and the indicated siRNA-resistant construct were imaged by DIC microscopy at different time intervals following nocodazole (Noc)/flavopiridol (Flavo) treatment. These data are quantified in Figure S7. FLAc, N-terminally tagged with a single FLAG epitope and AcGFP (A. coerulescens GFP). (C) HeLa cells treated as described in (A) were stained with anti-RhoA (white) and anti-anillin (white) antibody to visualize recruitment to the membrane. Percentages indicate the fraction of cells exhibiting cortical localization of each marker. Chromatin is marked in red. (D and E) Working model for membrane activation of RhoA in wild-type cells. (D) Our data indicate that PAR-5/14-3-3 proteins (red) act globally to prevent centralspindlin oligomerization and maintain the complex in its soluble, inactive form (pale yellow). During anaphase, Aurora B kinase (CPC) activity antagonizes PAR-5 activity, allowing centralspindlin (dark yellow) to form clusters that can bind microtubules and the plasma membrane with high avidity. (E) Schematic to illustrate spatial regulation of RhoA activation during anaphase. The CPC (gray) is known to be active in the equatorial region of a dividing cell, specifically at the central spindle and the plasma membrane during anaphase. This could result in zones of centralspindlin activity where centralspindlin (dark yellow) locally forms clusters, allowing ECT-2 (orange) and RhoA (blue) activation at the equatorial membrane and subsequent recruitment of actomyosin (green) to generate the contractile ring. Centralspindlin is inactive (pale yellow) in other regions of the cell because of PAR-5/14-3-3 (red) inhibition.