Kif2 localizes to a subdomain of cortical endoplasmic reticulum that drives asymmetric spindle position

Actin Filament in the First Cell Cycle Contributes to the Determination of the Anteroposterior Axis in Ascidian Development

In many animal species, the body axis is determined by the relocalization of maternal determinants, organelles, or unique cell populations in a cytoskeleton-dependent manner. In the ascidian first cell cycle, the myoplasm, including mitochondria, endoplasmic reticulum (ER), and maternal mRNAs, move to the future posterior side concomitantly (called ooplasmic segregation or cytoplasmic and cortical reorganization). This translocation consists of first and second phases depending on the actin and microtubule, respectively. However, the transition from first to second phase, that is, translocation of myoplasmic components from microfilaments to microtubules, has been poorly investigated. In this study, we analyzed the relationship between these cytoskeletons and myoplasmic components during the first cell cycle and their role in morphogenesis by inhibitor experiments. Owing to our improved visualization techniques, there was unexpected F-actin accumulation at the vegetal pole during this transition period. When this F-actin was depolymerized, the microtubule structure was strongly affected, the myoplasmic components, including maternal mRNA, were mislocalized, and the anteroposterior axis formation was disordered. These results suggested the importance of F-actin during the first cell cycle and the existence of interactions between microfilaments and microtubules, implying the enigmatic mechanism of ooplasmic segregation. Solving this mystery leads us to an improved understanding of ascidian early development.

Developmental Table and Three-Dimensional Embryological Image Resource of the Ascidian Ascidiella aspersa

Ascidiella aspersa is an ascidian in the class of chordates—the closest relatives of vertebrates. A. aspersa is a potential model organism for bio-imaging studies due to its extremely transparent embryos as well as is a globally distributed cosmopolitan species. However, there is no standard developmental table for this organism. Here, as a first step to establish A. aspersa as a model organism, we report a standard developmental table as a web-based digital image resource. This resource used confocal laser scanning microscopy to scan more than 3,000 cross-sectional images and 3D-reconstructed images of A. aspersa embryos during embryogenesis. With reference to the standardized developmental table of Ciona intestinalis type A, 26 different developmental stages (Stages 1–26) from fertilized eggs to hatched larvae were redefined for A. aspersa. Cell lineages up to the cleavage period were annotated: The cleavage patterns, the embryonic morphology, and the developmental time were then compared with Ciona. We found that the cleavage patterns and developmental time up to the neurula period in A. aspersa were extremely conserved versus. Ciona. The ratio of the trunk and tail length in the tailbud period were smaller than Ciona indicating a relatively short tail. In addition, the timing of the bending of the tail is earlier than Ciona. This A. aspersa standard 3D digital resource is essential for connecting different omics data to different spatiotemporal hierarchies and is useful for a system-level understanding of chordate development and evolution.

Combined effect of cell geometry and polarity domains determines the orientation of unequal division

Cell division orientation is thought to result from a competition between cell geometry and polarity domains controlling the position of the mitotic spindle during mitosis. Depending on the level of cell shape anisotropy or the strength of the polarity domain, one dominates the other and determines the orientation of the spindle. Whether and how such competition is also at work to determine unequal cell division (UCD), producing daughter cells of different size, remains unclear. Here, we show that cell geometry and polarity domains cooperate, rather than compete, in positioning the cleavage plane during UCDs in early ascidian embryos. We found that the UCDs and their orientation at the ascidian third cleavage rely on the spindle tilting in an anisotropic cell shape, and cortical polarity domains exerting different effects on spindle astral microtubules. By systematically varying mitotic cell shape, we could modulate the effect of attractive and repulsive polarity domains and consequently generate predicted daughter cell size asymmetries and position. We therefore propose that the spindle position during UCD is set by the combined activities of cell geometry and polarity domains, where cell geometry modulates the effect of cortical polarity domain(s).

Cell division geometries as central organizers of early embryo development

Early cellular patterning is a critical step of embryonic development that determines the proper progression of morphogenesis in all metazoans. It relies on a series of rapid reductive divisions occurring simultaneously with the specification of the fate of different subsets of cells. Multiple species developmental strategies emerged in the form of a unique cleavage pattern with stereotyped division geometries. Cleavage geometries have long been associated to the emergence of canonical developmental features such as cell cycle asynchrony, zygotic genome activation and fate specification. Yet, the direct causal role of division positioning on blastomere cell behavior remain partially understood. Oriented and/or asymmetric divisions define blastomere cell sizes, contacts and positions, with potential immediate impact on cellular decisions, lineage specification and morphogenesis. Division positions also instruct daughter cells polarity, mechanics and geometries, thereby influencing subsequent division events, in an emergent interplay that may pattern early embryos independently of firm deterministic genetic programs. We here review the recent literature which helped to delineate mechanisms and functions of division positioning in early embryos.

Role of PB1 Midbody Remnant Creating Tethered Polar Bodies during Meiosis II

Polar body (PB) formation is an extreme form of unequal cell division that occurs in oocytes due to the eccentric position of the small meiotic spindle near the oocyte cortex. Prior to PB formation, a chromatin-centered process causes the cortex overlying the meiotic chromosomes to become polarized. This polarized cortical subdomain marks the site where a cortical protrusion or outpocket forms at the oocyte surface creating the future PBs. Using ascidians, we observed that PB1 becomes tethered to the fertilized egg via PB2, indicating that the site of PB1 cytokinesis directed the precise site for PB2 emission. We therefore studied whether the midbody remnant left behind following PB1 emission was involved, together with the egg chromatin, in defining the precise cortical site for PB2 emission. During outpocketing of PB2 in ascidians, we discovered that a small structure around 1 µm in diameter protruded from the cortical outpocket that will form the future PB2, which we define as the “polar corps”. As emission of PB2 progressed, this small polar corps became localized between PB2 and PB1 and appeared to link PB2 to PB1. We tested the hypothesis that this small polar corps on the surface of the forming PB2 outpocket was the midbody remnant from the previous round of PB1 cytokinesis. We had previously discovered that Plk1::Ven labeled midbody remnants in ascidian embryos. We therefore used Plk1::Ven to follow the dynamics of the PB1 midbody remnant during meiosis II. Plk1::Ven strongly labeled the small polar corps that formed on the surface of the cortical outpocket that created PB2. Following emission of PB2, this polar corps was rich in Plk1::Ven and linked PB2 to PB1. By labelling actin (with TRITC-Phalloidin) we also demonstrated that actin accumulates at the midbody remnant and also forms a cortical cap around the midbody remnant in meiosis II that prefigured the precise site of cortical outpocketing during PB2 emission. Phalloidin staining of actin and immunolabelling of anti-phospho aPKC during meiosis II in fertilized eggs that had PB1 removed suggested that the midbody remnant remained within the fertilized egg following emission of PB1. Dynamic imaging of microtubules labelled with Ens::3GFP, MAP7::GFP or EB3::3GFP showed that one pole of the second meiotic spindle was located near the midbody remnant while the other pole rotated away from the cortex during outpocketing. Finally, we report that failure of the second meiotic spindle to rotate can lead to the formation of two cortical outpockets at anaphase II, one above each set of chromatids. It is not known whether the midbody remnant of PB1 is involved in directing the precise location of PB2 since our data are correlative in ascidians. However, a review of the literature indicates that PB1 is tethered to the egg surface via PB2 in several species including members of the cnidarians, lophotrochozoa and echinoids, suggesting that the midbody remnant formed during PB1 emission may be involved in directing the precise site of PB2 emission throughout the invertebrates.

Diversity of activator of G-protein signaling (AGS)-family proteins and their impact on asymmetric cell division across taxa

Asymmetric cell division (ACD) is a cellular process that forms two different cell types through a cell division and is thus critical for the development of all multicellular organisms. Not all but many of the ACD processes are mediated by proper orientation of the mitotic spindle, which segregates the fate determinants asymmetrically into daughter cells. In many cell types, the evolutionarily conserved protein complex of Gαi/AGS-family protein/NuMA-like protein appears to play critical roles in orienting the spindle and/or generating the polarized cortical forces to regulate ACD. Studies in various organisms reveal that this conserved protein complex is slightly modified in each phylum or even within species. In particular, AGS-family proteins appear to be modified with a variable number of motifs in their functional domains across taxa. This apparently creates different molecular interactions and mechanisms of ACD in each developmental program, ultimately contributing to developmental diversity across species. In this review, we discuss how a conserved ACD machinery has been modified in each phylum over the course of evolution with a major focus on the molecular evolution of AGS-family proteins and its impact on ACD regulation.

Emergence of Embryo Shape During Cleavage Divisions

Cells are arranged into species-specific patterns during early embryogenesis. Such cell division patterns are important since they often reflect the distribution of localized cortical factors from eggs/fertilized eggs to specific cells as well as the emergence of organismal form. However, it has proven difficult to reveal the mechanisms that underlie the emergence of cell positioning patterns that underlie embryonic shape, likely because a systems-level approach is required that integrates cell biological, genetic, developmental, and mechanical parameters. The choice of organism to address such questions is also important. Because ascidians display the most extreme form of invariant cleavage pattern among the metazoans, we have been analyzing the cell biological mechanisms that underpin three aspects of cell division (unequal cell division (UCD), oriented cell division (OCD), and asynchronous cell cycles) which affect the overall shape of the blastula-stage ascidian embryo composed of 64 cells. In ascidians, UCD creates two small cells at the 16-cell stage that in turn undergo two further successive rounds of UCD. Starting at the 16-cell stage, the cell cycle becomes asynchronous, whereby the vegetal half divides before the animal half, thus creating 24-, 32-, 44-, and then 64-cell stages. Perturbing either UCD or the alternate cell division rhythm perturbs cell position. We propose that dynamic cell shape changes propagate throughout the embryo via cell-cell contacts to create the ascidian-specific invariant cleavage pattern.

KIF2 mediates the neuroprotection in cerebral ischaemia injury by affecting NF-κB pathway

Stroke is the most common cerebrovascular disease with high morbidity and mortality around the world. However, the underlying mechanisms involved in nerve injury and cerebral ischaemia/reperfusion (I/R) during cerebrovascular disease are still not completely clear. In the present study, we investigate the role of kinesin family member 2 (KIF2) in the neuroprotection after cerebral I/R injury. KIF2 was aberrantly expressed in the cerebral tissues from middle cerebral artery occlusion (MCAO) rat model in a time dependent manner. A similar changing pattern was found in the cultured hypoxic neurons as well as SK-N-SH cells in vitro. Compared to the control, KIF2 inhibition significantly increased the level of malonic dialdehyde (MDA), and reduced the level of superoxide dismutase (SOD) as well as glutathione peroxidase (GSH-px) activity in cerebral tissues of MCAO rat model. The reactive oxygen species (ROS) level was also up-regulated after KIF2 siRNA knockdown in cultured hypoxic SK-N-SH cells. The apoptosis rates of hypoxic neurons and SK-N-SH cells as well as activated-caspase-3 level were obviously increased after KIF2 silencing. Furthermore, we found that the nuclear factor-kappa B (NF-κB) pathway was involved in KIF2-mediated neuroprotection after cerebral I/R injury, and induced apoptosis of hypoxic SK-N-SH cells by KIF2 silencing could be attenuated by the specific inhibitor BAY11-7082 of NF-κB. In conclusion, we demonstrate that KIF2 could mediate the neuroprotection in cerebral I/R injury by inhibiting activation of NF-κB pathway. This might provide a novel therapeutic target for cerebral I/R injury.

Spatiotemporally Controlled Myosin Relocalization and Internal Pressure Generate Sibling Cell Size Asymmetry

Metazoan cells can generate unequal-sized sibling cells during cell division. This form of asymmetric cell division depends on spindle geometry and Myosin distribution, but the underlying mechanics are unclear. Here, we use atomic force microscopy and live cell imaging to elucidate the biophysical forces involved in the establishment of physical asymmetry in Drosophila neural stem cells. We show that initial apical cortical expansion is driven by hydrostatic pressure, peaking shortly after anaphase onset, and enabled by a relief of actomyosin contractile tension on the apical cell cortex. An increase in contractile tension at the cleavage furrow combined with the relocalization of basally located Myosin initiates basal and sustains apical extension. We propose that spatiotemporally controlled actomyosin contractile tension and hydrostatic pressure enable biased cortical expansion to generate sibling cell size asymmetry. However, dynamic cleavage furrow repositioning can compensate for the lack of biased expansion to establish physical asymmetry.

Massive cytoplasmic transport and microtubule organization in fertilized chordate eggs

Eggs have developed their own strategies for early development. Amphibian, teleost fish, and ascidian eggs show cortical rotation and an accompanying structure, a cortical parallel microtubule (MT) array, during the one-cell embryonic stage. Cortical rotation is thought to relocate maternal deposits to a certain compartment of the egg and to polarize the embryo. The common features and differences among chordate eggs as well as localized maternal proteins and mRNAs that are related to the organization of MT structures are described in this review. Furthermore, recent studies report progress in elucidating the molecular nature and functions of the noncentrosomal MT organizing center (ncMTOC). The parallel array of MT bundles is presumably organized by ncMTOCs; therefore, the mechanism of ncMTOC control is likely inevitable for these species. Thus, the molecules related to the ncMTOC provide clues for understanding the mechanisms of early developmental systems, which ultimately determine the embryonic axis.

Dynamic interplay of cell fate, polarity and force generation in ascidian embryos

A fundamental challenge in developmental biology is to understand how forces produced by individual cells are patterned in space and time and then integrated to produce stereotyped changes in tissue-level or embryo-level morphology. Ascidians offer a unique opportunity to address this challenge by studying how small groups of cells collectively execute complex, but highly stereotyped morphogenetic movements. Here we highlight recent progress and open questions in the study of ascidian morphogenesis, emphasizing the dynamic interplay of cell fate determination, cellular force generation and tissue-level mechanics.