Anillin localization suggests distinct mechanisms of division plane specification in mouse oogenic meiosis I and II

MRCK activates mouse oocyte myosin II for spindle rotation and male pronucleus centration

Asymmetric meiotic divisions in oocytes rely on spindle positioning in close vicinity to the cortex. In metaphase II mouse oocytes, eccentric spindle positioning triggers cortical polarization, including the build-up of an actin cap surrounded by a ring of activated myosin II. While the role of the actin cap in promoting polar body formation is established, ring myosin II activation mechanisms and functions have remained elusive. Here, we show that ring myosin II activation requires myotonic dystrophy kinase-related Cdc42-binding kinase (MRCK), downstream of polarized Cdc42. MRCK inhibition resulted in spindle rotation defects during anaphase II, precluding polar body extrusion. Remarkably, disengagement of segregated chromatids from the anaphase spindle could rescue rotation. We further show that the MRCK/myosin II pathway is activated in the fertilization cone and is required for male pronucleus migration toward the center of the zygote. These findings provide novel insights into the mechanism of myosin II activation in oocytes and its role in orchestrating asymmetric division and pronucleus centration.

Identification of a new p53 responsive element in the promoter region of anillin

The expression of anillin mRNA and protein is regulated in a cell cycle‑dependent manner. However, the mechanism underlying this process is unclear. Previous studies analyzing the sequence of the 5'‑untranslated region of anillin have unveiled several putative p53 binding sites. Therefore, the present study hypothesized that the anillin gene may be repressed by p53 and that the commonly observed mutation (or loss of function) of p53 may serve a role in this phenotype. Bioinformatic analysis of the anillin promoter region revealed potential p53 responsive elements. Of those identified, 2 were able to bind p53 protein, as determined via a chromatin immunoprecipitation assay. Although it was hypothesized that DNA damage and resultant p53 expression would repress anillin expression, the results revealed that anillin mRNA and protein expression levels were negatively regulated by DNA damage in the wild‑type p53 cells, but not in the isogenic p53 null cells. Furthermore, DNA sequences encompassing the p53 binding site downregulated luciferase transgenes in a p53 dependent manner. Taken together, these data indicated that anillin was negatively regulated by p53 and that anillin overexpression observed in cancer may be a p53‑mediated phenomenon. The data from the present study provided further evidence for the role of p53 in the biologically crucial process of cytokinesis.

Production of mouse androgenetic embryos using spindle perturbation

To study the functional differences between maternal and paternal genomes in mammalian development, embryos with only one parental genome are often used. Androgenetic embryos are produced by the removal of maternal chromosomes before or after fertilization by techniques that require specialized skills and are associated with high risk of cellular damage. Here, we developed a novel method for producing androgenetic mouse embryos without the invasive enucleation process. We found that during in vitro fertilization in the presence of low-dose nocodazole, a microtubule destabilizing drug, whole oocyte chromosomes were extruded into the second polar body resulting in the production of androgenetic embryos. We further demonstrated that low-dose nocodazole decreased the spindle size and prevented chromosome segregation but did not compromise oocyte meiotic resumption. This led to the formation of a protrusion around the chromosomes, accumulation of protein regulator of cytokinesis 1 (PRC1) to the microtubules around the chromosomes, and assembly of a contractile ring at the neck region of the protrusion. Our method uses the intrinsic cytokinetic mechanism to exclude maternal chromatin from zygotes and may be applicable to other mammals.

Cdk1 inactivation induces post-anaphase-onset spindle migration and membrane protrusion required for extreme asymmetry in mouse oocytes

Female meiotic divisions are extremely asymmetric, producing large oocytes and small polar bodies (PBs). In mouse oocytes, the spindle relocates to the cortex before anaphase of meiosis I (MI). It is presumed that by displacing the future midzone, pre-anaphase spindle repositioning alone ensures asymmetry. But how subsequent anaphase events might contribute to asymmetric PB extrusion (PBE) is unknown. Here, we find that inactivation of cyclin-dependent kinase 1 (Cdk1) induces anaphase and simultaneously triggers cytoplasmic formin-mediated F-actin polymerisation that propels the spindle into the cortex causing it to protrude while anaphase progresses. Significantly, if post-anaphase-onset spindle migration fails, protrusion and asymmetry are severely threatened even with intact pre-anaphase migration. Conversely, post-anaphase migration can completely compensate for failed pre-anaphase migration. These data identify a cell-cycle-triggered phase of spindle displacement occurring after anaphase-onset, which, by inducing protrusion, is necessary for extreme asymmetry in mouse oocytes and uncover a pathway for maximising unequal division.

Anillin controls cleavage furrow formation in the course of asymmetric division during mouse oocyte maturation

Anillin is a scaffold protein that recruits several proteins involved in cleavage furrow formation during cytokinesis. The role of anilllin in symmetric cell divisions in somatic cells has been intensively studied, yet its involvement in cleavage furrow formation is still elusive. In this study, we investigated the role of anillin in mammalian oocyte maturation and cytokinesis. We found that anillin is localized around the nucleus during the oocyte germinal-vesicle stage, and spreads to the cytoplasm after germinal vesicle breakdown. Thereafter, anillin concentrates at the site of the cleavage furrow from anaphase I to metaphase II. Disruption of anillin activity by microinjecting oocytes with specific siRNAs resulted in a failure of polar body extrusion and asymmetric division, and caused abnormal chromosome segregation during anaphase I. Furthermore, pharmacological inhibition of myosin light chain using Y-27632 or ML-7 resulted in decreased anillin expression. Collectively, our data suggest that anillin is an essential intracellular component that maintains the integrity of asymmetric division in mouse oocytes. Mol. Reprod. Dev. 83: 792-801, 2016 © 2016 Wiley Periodicals, Inc.

The Zebrafish Anillin-eGFP Reporter Marks Late Dividing Retinal Precursors and Stem Cells Entering Neuronal Lineages

Monitoring cycling behaviours of stem and somatic cells in the living animal is a powerful tool to better understand tissue development and homeostasis. The tg(anillin:anillin-eGFP) transgenic line carries the full-length zebrafish F-actin binding protein Anillin fused to eGFP from a bacterial artificial chromosome (BAC) containing Anillin cis-regulatory sequences. Here we report the suitability of the Anillin-eGFP reporter as a direct indicator of cycling cells in the late embryonic and post-embryonic retina. We show that combining the anillin:anillin-eGFP with other transgenes such as ptf1a:dsRed and atoh7:gap-RFP allows obtaining spatial and temporal resolution of the mitotic potentials of specific retinal cell populations. This is exemplified by the analysis of the origin of the previously reported apically and non-apically dividing late committed precursors of the photoreceptor and horizontal cell layers.

Prediction of blastocyst development and implantation potential in utero based on the third cleavage and compaction times in mouse pre-implantation embryos

Cytokinesis and cell division during pre-implantation embryonic development occur as an orchestrated spatiotemporal program. Cleavage, compaction, and blastulation in pre-implantation embryos are essential for successful implantation and pregnancy. Their alteration is associated with chromosomal imbalance and loss of developmental competence. In this study, we evaluated the time of cleavage and compaction as predictors for in vitro pre- and peri-implantation development and in utero implantation potential by time-lapse monitoring. Mouse 2-cell embryos were collected on 1.5 days post coitum (dpc) and were individually cultured to the outgrowth (OG) stage (7.5 dpc). Developmental stages were classified as 3-cell, 4-cell, 8-cell, morula, blastocyst, and OG. Cut-off times for successful blastocyst development were determined by receiver operating characteristic curve analysis. When cut-off times were set as 9 h for the third cleavage from the 2- to 4-cell stage, and 40 h for compaction from the 2-cell to morula stage, blastocyst and OG development rates, respectively, were significantly higher (P < 0.0001). Embryos were grouped according to the above cut-off time and transferred to the contralateral uterine horn on 3.5 dpc. Implantation rates in utero on 5.5 dpc were significantly higher in early third cleaved (≤ 9 h from 2- to 4-cell) and early compacted embryos (≤ 40 h from 2-cell to morula) than those in delayed embryos (P < 0.05). Therefore, the time of the third cleavage from 2- to the 4-cell stage and compaction from 2-cell to morula stage may be a useful morphokinetic parameter for predicting developmental potential, including successful implantation and pregnancy in human in vitro fertilization-embryo transfer programs.