Cortical and cytoplasmic flows driven by actin microfilaments polarize the cortical ER-mRNA domain along the a–v axis in ascidian oocytes

Aberrant cortex contractions impact mammalian oocyte quality

The cortex controls cell shape. In mouse oocytes, the cortex thickens in an Arp2/3-complex-dependent manner, ensuring chromosome positioning and segregation. Surprisingly, we identify that mouse oocytes lacking the Arp2/3 complex undergo cortical actin remodeling upon division, followed by cortical contractions that are unprecedented in mammalian oocytes. Using genetics, imaging, and machine learning, we show that these contractions stir the cytoplasm, resulting in impaired organelle organization and activity. Oocyte capacity to avoid polyspermy is impacted, leading to a reduced female fertility. We could diminish contractions and rescue cytoplasmic anomalies. Similar contractions were observed in human oocytes collected as byproducts during IVF (in vitro fertilization) procedures. These contractions correlate with increased cytoplasmic motion, but not with defects in spindle assembly or aneuploidy in mice or humans. Our study highlights a multiscale effect connecting cortical F-actin, contractions, and cytoplasmic organization and affecting oocyte quality, with implications for female fertility.

Friction forces determine cytoplasmic reorganization and shape changes of ascidian oocytes upon fertilization

Contraction and flow of the actin cell cortex have emerged as a common principle by which cells reorganize their cytoplasm and take shape. However, how these cortical flows interact with adjacent cytoplasmic components, changing their form and localization, and how this affects cytoplasmic organization and cell shape remains unclear. Here we show that in ascidian oocytes, the cooperative activities of cortical actomyosin flows and deformation of the adjacent mitochondria-rich myoplasm drive oocyte cytoplasmic reorganization and shape changes following fertilization. We show that vegetal-directed cortical actomyosin flows, established upon oocyte fertilization, lead to both the accumulation of cortical actin at the vegetal pole of the zygote and compression and local buckling of the adjacent elastic solid-like myoplasm layer due to friction forces generated at their interface. Once cortical flows have ceased, the multiple myoplasm buckles resolve into one larger buckle, which again drives the formation of the contraction pole-a protuberance of the zygote's vegetal pole where maternal mRNAs accumulate. Thus, our findings reveal a mechanism where cortical actomyosin network flows determine cytoplasmic reorganization and cell shape by deforming adjacent cytoplasmic components through friction forces.

Yolk granule fusion and microtubule aster formation regulate cortical granule translocation and exocytosis in zebrafish oocytes

Dynamic reorganization of the cytoplasm is key to many core cellular processes, such as cell division, cell migration, and cell polarization. Cytoskeletal rearrangements are thought to constitute the main drivers of cytoplasmic flows and reorganization. In contrast, remarkably little is known about how dynamic changes in size and shape of cell organelles affect cytoplasmic organization. Here, we show that within the maturing zebrafish oocyte, the surface localization of exocytosis-competent cortical granules (Cgs) upon germinal vesicle breakdown (GVBD) is achieved by the combined activities of yolk granule (Yg) fusion and microtubule aster formation and translocation. We find that Cgs are moved towards the oocyte surface through radially outward cytoplasmic flows induced by Ygs fusing and compacting towards the oocyte center in response to GVBD. We further show that vesicles decorated with the small Rab GTPase Rab11, a master regulator of vesicular trafficking and exocytosis, accumulate together with Cgs at the oocyte surface. This accumulation is achieved by Rab11-positive vesicles being transported by acentrosomal microtubule asters, the formation of which is induced by the release of CyclinB/Cdk1 upon GVBD, and which display a net movement towards the oocyte surface by preferentially binding to the oocyte actin cortex. We finally demonstrate that the decoration of Cgs by Rab11 at the oocyte surface is needed for Cg exocytosis and subsequent chorion elevation, a process central in egg activation. Collectively, these findings unravel a yet unrecognized role of organelle fusion, functioning together with cytoskeletal rearrangements, in orchestrating cytoplasmic organization during oocyte maturation.

A novel automated image analysis pipeline for quantifying morphological changes to the endoplasmic reticulum in cultured human cells

Background

In mammalian cells the endoplasmic reticulum (ER) comprises a highly complex reticular morphology that is spread throughout the cytoplasm. This organelle is of particular interest to biologists, as its dysfunction is associated with numerous diseases, which often manifest themselves as changes to the structure and organisation of the reticular network. Due to its complex morphology, image analysis methods to quantitatively describe this organelle, and importantly any changes to it, are lacking.

Results

In this work we detail a methodological approach that utilises automated high-content screening microscopy to capture images of cells fluorescently-labelled for various ER markers, followed by their quantitative analysis. We propose that two key metrics, namely the area of dense ER and the area of polygonal regions in between the reticular elements, together provide a basis for measuring the quantities of rough and smooth ER, respectively. We demonstrate that a number of different pharmacological perturbations to the ER can be quantitatively measured and compared in our automated image analysis pipeline. Furthermore, we show that this method can be implemented in both commercial and open-access image analysis software with comparable results.

Conclusions

We propose that this method has the potential to be applied in the context of large-scale genetic and chemical perturbations to assess the organisation of the ER in adherent cell cultures.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12859-021-04334-x.

Cell cycle control during early embryogenesis

Understanding the mechanisms of embryonic cell cycles is a central goal of developmental biology, as the regulation of the cell cycle must be closely coordinated with other events during early embryogenesis. Quantitative imaging approaches have recently begun to reveal how the cell cycle oscillator is controlled in space and time, and how it is integrated with mechanical signals to drive morphogenesis. Here, we discuss how the Drosophila embryo has served as an excellent model for addressing the molecular and physical mechanisms of embryonic cell cycles, with comparisons to other model systems to highlight conserved and species-specific mechanisms. We describe how the rapid cleavage divisions characteristic of most metazoan embryos require chemical waves and cytoplasmic flows to coordinate morphogenesis across the large expanse of the embryo. We also outline how, in the late cleavage divisions, the cell cycle is inter-regulated with the activation of gene expression to ensure a reliable maternal-to-zygotic transition. Finally, we discuss how precise transcriptional regulation of the timing of mitosis ensures that tissue morphogenesis and cell proliferation are tightly controlled during gastrulation.

Cytoplasm's Got Moves

Cytoplasm is a gel-like crowded environment composed of various macromolecules, organelles, cytoskeletal networks, and cytosol. The structure of the cytoplasm is highly organized and heterogeneous due to the crowding of its constituents and their effective compartmentalization. In such an environment, the diffusive dynamics of the molecules are restricted, an effect that is further amplified by clustering and anchoring of molecules. Despite the crowded nature of the cytoplasm at the microscopic scale, large-scale reorganization of the cytoplasm is essential for important cellular functions, such as cell division and polarization. How such mesoscale reorganization of the cytoplasm is achieved, especially for large cells such as oocytes or syncytial tissues that can span hundreds of micrometers in size, is only beginning to be understood. In this review, we will discuss recent advances in elucidating the molecular, cellular, and biophysical mechanisms by which the cytoskeleton drives cytoplasmic reorganization across different scales, structures, and species.

Cytoplasmic streaming drifts the polarity cue and enables posteriorization of the Caenorhabditis elegans zygote at the side opposite of sperm entry

Cell polarization is required to define body axes during development. The position of spatial cues for polarization is critical to direct the body axes. In Caenorhabditis elegans zygotes, the sperm-derived pronucleus/centrosome complex (SPCC) serves as the spatial cue to specify the anterior-posterior axis. Approximately 30 min after fertilization, the contractility of the cell cortex is relaxed near the SPCC, which is the earliest sign of polarization and called symmetry breaking (SB). It is unclear how the position of SPCC at SB is determined after fertilization. Here, we show that SPCC drifts dynamically through the cell-wide flow of the cytoplasm, called meiotic cytoplasmic streaming. This flow occasionally brings SPCC to the opposite side of the sperm entry site before SB. Our results demonstrate that cytoplasmic flow determines stochastically the position of the spatial cue of the body axis, even in an organism like C. elegans for which development is stereotyped.

Bulk Actin Dynamics Drive Phase Segregation in Zebrafish Oocytes

Segregation of maternal determinants within the oocyte constitutes the first step in embryo patterning. In zebrafish oocytes, extensive ooplasmic streaming leads to the segregation of ooplasm from yolk granules along the animal-vegetal axis of the oocyte. Here, we show that this process does not rely on cortical actin reorganization, as previously thought, but instead on a cell-cycle-dependent bulk actin polymerization wave traveling from the animal to the vegetal pole of the oocyte. This wave functions in segregation by both pulling ooplasm animally and pushing yolk granules vegetally. Using biophysical experimentation and theory, we show that ooplasm pulling is mediated by bulk actin network flows exerting friction forces on the ooplasm, while yolk granule pushing is achieved by a mechanism closely resembling actin comet formation on yolk granules. Our study defines a novel role of cell-cycle-controlled bulk actin polymerization waves in oocyte polarization via ooplasmic segregation.

All Oocytes Attach to the Dorsal Ovarian Epithelium in the Ovary of Medaka, Oryzias latipes

In the teleost fish medaka, an adult ovary simultaneously contains developing oocytes at all phases of oogenesis during the breeding season. However, it remains unclear where oocytes at each developmental stage are located in the ovary by the time of ovulation. To examine the relationship between the developmental stage of oocytes and their positions in the ovary of vertebrate medaka during oogenesis, the stage of oocyte development was determined from the diameter of the oocytes and the cellular morphological characteristics, such as the germinal vesicle and micropyle at the animal pole and attaching filaments at the vegetal pole, and the positions of developing oocytes in the ovary in all sections were observed. Furthermore, to investigate the characteristics of the dorsal ovarian epithelium to which the oocytes attach themselves, the dorsal and vegetal ovarian epithelia were observed. The dorsal ovarian epithelium invaginated in spots. When all serial sections of the oocytes were observed, all oocytes at stages I-VIII were attached to the dorsal ovarian epithelium, regardless of whether it invaginated or not.

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.

Cytoplasmic flows in starfish oocytes are fully determined by cortical contractions

Cytoplasmic flows are an ubiquitous feature of biological systems, in particular in large cells, such as oocytes and eggs in early animal development. Here we show that cytoplasmic flows in starfish oocytes, which can be imaged well with transmission light microscopy, are fully determined by the cortical dynamics during surface contraction waves. We first show that the dynamics of the oocyte surface is highly symmetric around the animal-vegetal axis. We then mathematically solve the Stokes equation for flows inside a deforming sphere using the measured surface displacements as boundary conditions. Our theoretical predictions agree very well with the intracellular flows quantified by particle image velocimetry, proving that during this stage the starfish cytoplasm behaves as a simple Newtonian fluid on the micrometer scale. We calculate the pressure field inside the oocyte and find that its gradient is too small as to explain polar body extrusion, in contrast to earlier suggestions. Myosin II inhibition by blebbistatin confirms this conclusion, because it diminishes cell shape changes and hydrodynamic flow, but does not abolish polar body formation.

As already noted by Aristotle, life is motion. On the molecular scale, thermal motion leads to diffusive transport. On cellular scales, however, diffusion starts to become inefficient, due to the general property of random walks that their spatial excursions grow less than linear with time. Therefore more directed transport processes are needed on cellular scales, including transport by molecular motors or by hydrodynamic flows. This is especially true for oocytes and eggs in early animal development, which often have to be large in order to store sufficient amounts of nutrients. Here we use starfish oocytes as a convenient model system to investigate the nature and function of cytoplasmic flows in early development. These cells are very large and optically transparent, and therefore ideal for live cell imaging that here we combine with image processing and mathematical modelling. This approach allows us to demonstrate that the experimentally observed cytoplasmic flows during early development are a direct consequence of surface contraction waves that deform the soft and contractile eggs. Additionally we show that despite its microscopic complexity, the cytoplasm behaves like a Newtonian fluid on the cellular scale. Our findings impose strong physical limits on the potential biological function of these flows and suggest that also other cellular systems that are soft and contractile might experience large cytoplasmic flows upon cell shape changes, for example during cell migration or division.

A cdk1 gradient guides surface contraction waves in oocytes

Surface contraction waves (SCWs) in oocytes and embryos lead to large-scale shape changes coupled to cell cycle transitions and are spatially coordinated with the cell axis. Here, we show that SCWs in the starfish oocyte are generated by a traveling band of myosin II-driven cortical contractility. At the front of the band, contractility is activated by removal of cdk1 inhibition of the RhoA/RhoA kinase/myosin II signaling module, while at the rear, contractility is switched off by negative feedback originating downstream of RhoA kinase. The SCW's directionality and speed are controlled by a spatiotemporal gradient of cdk1-cyclinB. This gradient is formed by the release of cdk1-cyclinB from the asymmetrically located nucleus, and progressive degradation of cyclinB. By combining quantitative imaging, biochemical and mechanical perturbations with mathematical modeling, we demonstrate that the SCWs result from the spatiotemporal integration of two conserved regulatory modules, cdk1-cyclinB for cell cycle regulation and RhoA/Rok/NMYII for actomyosin contractility.Surface contraction waves (SCWs) are prominent shape changes coupled to cell cycle transitions in oocytes. Here the authors show that SCWs are patterned by the spatiotemporal integration of two conserved modules, cdk1-cyclinB for cell cycle regulation and RhoA/Rok/NMYII for actomyosin contractility.

Modified whole-mount in situ hybridisation and immunohistochemistry protocols without removal of the vitelline membrane in the appendicularian Oikopleura dioica

The appendicularian Oikopleura dioica is a planktonic chordate that retains a tadpole shape throughout its life. Its simple and transparent body, invariant cell lineages, fast development and available genome and transcriptome resources make it a promising model organism for research in developmental biology. However, large-scale analysis of gene expression in O. dioica is limited owing to the laborious and time-consuming process of manual removal of the vitelline membrane, because devitellinisation of pre-hatching embryos causes failure of normal development. Therefore, in this study, modified procedures were developed for whole-mount in situ hybridisation (WISH) and immunohistochemistry (WIHC). This protocol enables rapid mRNA or protein detection without a manual devitellination step for each specimen. The critical procedure is brief treatment of the vitelline membrane of living embryos with 0.05% actinase E before fixation. Two minutes of treatment was optimal for the penetration of antisense RNA probes and antibodies through the vitelline membrane. This WISH protocol was applicable for chromogenic and fluorescent tyramide signal amplification reactions. Using the new protocol, we found eight genes with tissue-specific expression in the tail muscle, trunk epidermis, heart, pharynx, oesophagus, stomach or gill openings of developing larvae. This procedure also allowed for the detection of exogenous FLAG-tagged histone-enhanced green fluorescent protein by WIHC using anti-FLAG antibody. This study provides a useful and convenient tool for studying spatial and temporal gene expression patterns in this simple chordate model and should facilitate handling large amounts of genetic data from transcriptome-based approaches and other techniques such as treatments with chemical inhibitors.

Eccentric position of the germinal vesicle and cortical flow during oocyte maturation specify the animal-vegetal axis of ascidian embryos

The animal-vegetal (A-V) axis is already set in unfertilized eggs. It plays crucial roles to coordinate germ-layer formation. However, how the A-V axis is set has not been well studied. In ascidians, unfertilized eggs are already polarized along the axis in terms of cellular components. The polarization occurs during oocyte maturation. Oocytes within the gonad have the germinal vesicle (GV) close to the future animal pole. When the GVs of full-grown oocytes were experimentally translocated to the opposite pole by centrifugal force, every aspect that designates A-V polarity was reversed in the eggs and embryos. This was confirmed by examining the cortical allocation of the meiotic spindle, position of the polar body emission, polarized distribution of mitochondria and postplasmic/PEM mRNA, direction of the cortical flow during oocyte maturation, cleavage pattern, and germ-layer formation during embryogenesis. Therefore, the eccentric position of the GV triggers subsequent polarizing events and establishes the A-V axis in eggs and embryos. We emphasize important roles of the cortical flow. This is the first report in which the A-V axis was experimentally and completely reversed in animal oocytes before fertilization.

Rho-GTPase effector ROCK phosphorylates cofilin in actin-meditated cytokinesis during mouse oocyte meiosis

During oocyte meiosis, a spindle forms in the central cytoplasm and migrates to the cortex. Subsequently, the oocyte extrudes a small body and forms a highly polarized egg; this process is regulated primarily by actin. ROCK is a Rho-GTPase effector that is involved in various cellular functions, such as stress fiber formation, cell migration, tumor cell invasion, and cell motility. In this study, we investigated possible roles for ROCK in mouse oocyte meiosis. ROCK was localized around spindles after germinal vesicle breakdown and was colocalized with cytoplasmic actin and mitochondria. Disrupting ROCK activity by RNAi or an inhibitor resulted in cell cycle progression and polar body extrusion failure. Time-lapse microscopy showed that this may have been due to spindle migration and cytokinesis defects, as chromosomes segregated but failed to extrude a polar body and then realigned. Actin expression at oocyte membranes and in cytoplasm was significantly decreased after these treatments. Actin caps were also disrupted, which was confirmed by a failure to form cortical granule-free domains. The mitochondrial distribution was also disrupted, which indicated that mitochondria were involved in the ROCK-mediated actin assembly. In addition, the phosphorylation levels of Cofilin, a downstream molecule of ROCK, decreased after disrupting ROCK activity. Thus, our results indicated that a ROCK-Cofilin-actin pathway regulated meiotic spindle migration and cytokinesis during mouse oocyte maturation.

Activation of ADF/cofilin by phosphorylation-regulated Slingshot phosphatase is required for the meiotic spindle assembly in Xenopus laevis oocytes

We identify Xenopus ADF/cofilin (XAC) and its activator, Slingshot phosphatase (XSSH), as key regulators of actin dynamics essential for spindle microtubule assembly during Xenopus oocyte maturation. Phosphorylation of XSSH at multiple sites within the tail domain occurs just after germinal vesicle breakdown (GVBD) and is accompanied by dephosphorylation of XAC, which was mostly phosphorylated in immature oocytes. This XAC dephosphorylation after GVBD is completely suppressed by latrunculin B, an actin monomer-sequestering drug. On the other hand, jasplakinolide, an F-actin-stabilizing drug, induces dephosphorylation of XAC. Effects of latrunculin B and jasplakinolide are reconstituted in cytostatic factor-arrested extracts (CSF extracts), and XAC dephosphorylation is abolished by depletion of XSSH from CSF extracts, suggesting that XSSH functions as an actin filament sensor to facilitate actin filament dynamics via XAC activation. Injection of anti-XSSH antibody, which blocks full phosphorylation of XSSH after GVBD, inhibits both meiotic spindle formation and XAC dephosphorylation. Coinjection of constitutively active XAC with the antibody suppresses this phenotype. Treatment of oocytes with jasplakinolide also impairs spindle formation. These results strongly suggest that elevation of actin dynamics by XAC activation through XSSH phosphorylation is required for meiotic spindle assembly in Xenopus laevis.

Stabilization of actin filaments prevents germinal vesicle breakdown and affects microtubule organization in Xenopus oocytes

In Xenopus oocytes, extremely giant nuclei, termed germinal vesicles, contain a large amount of actin filaments most likely for mechanical integrity. Here, we show that microinjection of phalloidin, an F-actin-stabilizing drug, prevents the germinal vesicle breakdown (GVBD) in oocytes treated with progesterone. These nuclei remained for more 12 h after control oocytes underwent GVBD. Immunostaining showed significant elevation of actin in the remaining nuclei and many actin filament bundles in the cytoplasm. Furthermore, microtubules formed unusual structures in both nuclei and cytoplasm of phalloidin-injected oocytes stimulated by progesterone. Cytoplasmic microtubule arrays and intranuclear microtubules initially formed in phalloidin-injected oocytes as control oocytes exhibited white maturation spots; these structures gradually disappeared and finally converged upon intranuclear short bundles when control oocytes completed maturation. In contrast, treatment of oocytes with jasplakinolide, a cell membrane-permeable actin filament-stabilizing drug, did not affect GVBD. This drug preferentially induced accumulation of actin filaments at the cortex without any increase in cytoplasmic actin staining. Based on these results, intranuclear and cytoplasmic actin filament dynamics appear to be required for the completion of GVBD and critically involved in the regulation of microtubule assembly during oocyte maturation in Xenopus laevis.

Cytoplasmic localization and reorganization in ascidian eggs: role of postplasmic/PEM RNAs in axis formation and fate determination

Localization of maternal molecules in eggs and embryos and cytoplasmic movements to relocalize them are fundamental for the orderly cellular and genetic processes during early embryogenesis. Ascidian embryos have been known as 'mosaic eggs' because of their autonomous differentiation abilities based on localized cell fate determinants. This review gives a historical overview of the concept of cytoplasmic localization, and then explains the key features such as ooplasmic movements and cell lineages that are essential to grasp the process of ascidian development mediated by localized determinant activities. These activities are partly executed by localized molecules named postplasmic/PEM RNAs, originating from approximately 50 genes, of which the muscle determinant, macho-1, is an example. The cortical domain containing these RNAs is relocalized to the posterior-vegetal region of the egg by cytoskeletal movements after fertilization, and plays crucial roles in axis formation and cell fate determination. The cortical domain contains endoplasmic reticulum and characteristic granules, and gives rise to a subcellular structure called the centrosome-attracting body (CAB), in which postplasmic/PEM RNAs are highly concentrated. The CAB is responsible for a series of unequal partitionings of the posterior-vegetal cytoplasmic domain and the postplasmic/PEM RNAs at the posterior pole during cleavage. Some components of this domain, which is rich in granules, are eventually inherited by prospective germline cells with particular postplasmic/PEM RNAs such as vasa. The postplasmic/PEM RNAs are classified into two groups according to their final cellular destinations and localization pathways. Localization of these RNAs is regulated by specific nucleotide sequences in the 3' untranslated regions (3'UTRs).

Polarizing animal cells via mRNA localization in oogenesis and early development

The localization of mRNAs in developing animal cells is essential for establishing cellular polarity and setting up the body plan for subsequent development. Cellular and molecular mechanisms by which maternal mRNAs are localized during oogenesis have been extensively studied in Drosophila and Xenopus. In contrast, evidence for mechanisms used in the localization of mRNAs encoded by developmentally important genes has also been accumulating in several other organisms. This offers the opportunity to unravel the fundamental mechanisms of mRNA localization shared among many species, as well as unique mechanisms specifically acquired or retained by animals based on their developmental needs. In addition to maternal mRNAs, the localization of zygotically expressed mRNAs in the cells of cleaving embryos is also important for early development. In this review, mRNA localization dynamics in the oocytes/eggs of Drosophila and Xenopus are first summarized, and evidence for localized mRNAs in the oocytes/eggs and cleaving embryos of other organisms is then presented.

Bi-polarized translation of ascidian maternal mRNA determinant pem-1 associated with regulators of the translation machinery on cortical Endoplasmic Reticulum (cER)

Polarized cortical mRNA determinants such as maternal macho-1 and pem-1 in ascidians, like budding yeast mating factor ASH1 reside on the cER-mRNA domain a subdomain of cortical Endoplasmic Reticulum(ER) and are translated in its vicinity. Using high resolution imaging and isolated cortical fragments prepared from eggs and embryos we now find that macho-1 and pem-1 RNAs co-localize with phospho-protein regulators of translation initiation (MnK/4EBP/S6K). Translation of cortical pem-1 RNA follows its bi-polarized relocalization. About 10 min after fertilization or artificial activation with a calcium ionophore, PEM1 protein is detected in the vegetal cortex in the vicinity of pem-1 RNA. About 40 min after fertilization-when pem-1 RNA and P-MnK move to the posterior pole-PEM1 protein remains in place forming a network of cortical patches anchored at the level of the zygote plasma membrane before disappearing. Cortical PEM1 protein is detected again at the 4 cell stage in the posterior centrosome attracting body (CAB) region where the cER-mRNA domain harboring pem-1/P-MnK/P-4EBP/P-S6K is concentrated. Bi-polarized PEM1 protein signals are not detected when pem-1 morpholinos are injected into eggs or zygotes or when MnK is inhibited. We propose that localized translation of the pem-1 RNA determinant is triggered by the fertilization/calcium wave and that the process is controlled by phospho-protein regulators of translation initiation co-localized with the RNA determinant on a sub-domain of the cortical Endoplasmic Reticulum.

Localization and anchorage of maternal mRNAs to cortical structures of ascidian eggs and embryos using high resolution in situ hybridization

In several species, axis formation and tissue differentiation are the result of developmental cascades which begin with the localization and translation of key maternal mRNAs in eggs. Localization and anchoring of mRNAs to cortical structures can be observed with high sensitivity and resolution by fluorescent in situ hybridization coupled with labeling of membranes and macromolecular complexes. Oocytes and embryos of ascidians--marine chordates closely related to vertebrates--are ideal models to understand how maternal mRNAs pattern the simple ascidian tadpole. More than three dozen cortically localized maternal mRNAs have been identified in ascidian eggs. They include germ cell markers such as vasa or pem-3 and determinants of axis (pem-1), unequal cleavage (pem-1), and muscle cells (macho-1). High resolution localization of mRNAs, proteins, and lipids in whole eggs and embryos and their cortical fragments shows that maternal mRNA determinants pem-1 and macho-1 are anchored to cortical endoplasmic reticulum and segregate with it into small posterior somatic cells. In contrast, mRNAs such as vasa are associated with granular structures which are inherited by the same somatic cells plus adjacent germ cell precursors. In this chapter, we provide detailed protocols for simultaneous localization of mRNAs and proteins to determine their association with cellular structures in eggs and embryos. Using preparations of isolated cortical fragments with intact membranous structures allows unprecedented high resolution analysis and identification of cellular anchoring sites for key mRNAs. This information is necessary for understanding the mechanisms for localizing mRNAs and partitioning them into daughter cells after cleavage.

Actin microfilaments guide the polarized transport of nuclear pore complexes and the cytoplasmic dispersal of Vasa mRNA during GVBD in the ascidian Halocynthia roretzi

Meiosis reinitiation starts with the germinal vesicle breakdown (GVBD) within the gonad before spawning. Here, we have extended our previous observations and identified the formation of conspicuous actin bundles emanating from the germinal vesicle (GV) during its breakdown in the ascidian Halocynthia roretzi. Time-lapse video recordings and fluorescent labelling of microfilaments (MFs) indicate that these microfilamentous structures invariantly elongate towards the vegetal hemisphere at the estimated speed of 20 mum/min. Interestingly, the nuclear pore complex protein Nup153 accumulates at the vegetal tip of actin bundles. To determine if these structures play a role in the formation of the germ plasm, we have analyzed the localization pattern of Vasa transcript in maturing oocytes and early embryos. We found that Hr-Vasa mRNA, one of Type II postplasmic/PEM mRNAs, changes from a granular and perinuclear localization to an apparent uniform cytoplasmic distribution during oocyte maturation, and then concentrate in the centrosome-attracting body (CAB) by the eight-cell stage. In addition, treatments with Latrunculin B, but not with Nocodazole, blocked the redistribution of Nup153 and Hr-Vasa mRNA, suggesting that these mechanisms are both actin-dependant. We discuss the pleiotropic role played by MFs, and the relationship between nuclear pores, maternal Vasa mRNA and germ plasm in maturing ascidian oocytes.

Three distinct RNA localization mechanisms contribute to oocyte polarity establishment in the cnidarian Clytia hemisphaerica

Egg animal-vegetal polarity in cnidarians is less pronounced than in most bilaterian species, and its normal alignment with the future embryonic axis can be disturbed by low-speed centrifugation. We have analyzed the development of oocyte polarity within the transparent and autonomously functioning gonads of Clytia medusae, focusing on the localization of three recently identified maternal mRNAs coding for axis-directing Wnt pathway regulators. Animal-vegetal polarity was first detectable in oocytes committed to their final growth phase, as the oocyte nucleus (GV) became positioned at the future animal pole. In situ hybridization analyses showed that during this first, microtubule-dependent polarization event, CheFz1 RNA adopts a graded cytoplasmic distribution, most concentrated around the GV. CheFz3 and CheWnt3 RNAs adopt their polarized cortical localizations later, during meiotic maturation. Vegetal localization of CheFz3 RNA was found to require both microtubules and an intact gonad structure, while animal localization of CheWnt3 RNA was microtubule independent and oocyte autonomous. The cortical distribution of both these RNAs was sensitive to microfilament-disrupting drugs. Thus, three temporally and mechanistically distinct RNA localization pathways contribute to oocyte polarity in Clytia. Unlike the two cortical RNAs, CheFz1 RNA was displaced in fertilized eggs upon centrifugation, potentially explaining how this treatment re-specifies the embryonic axis.

A new model for asymmetric spindle positioning in mouse oocytes

An oocyte matures into an egg by extruding half of the chromosomes in a small polar body. This extremely asymmetric division enables the oocyte to retain sufficient storage material for the development of the embryo after fertilization. To divide asymmetrically, mammalian oocytes relocate the spindle from their center to the cortex. In all mammalian species analyzed so far, including human, mouse, cow, pig, and hamster, spindle relocation depends on filamentous actin (F-actin). However, even though spindle relocation is essential for fertility, the involved F-actin structures and the mechanism by which they relocate the spindle are unknown. Here we show in live mouse oocytes that spindle relocation requires a continuously reorganizing cytoplasmic actin network nucleated by Formin-2 (Fmn2). We found that the spindle poles were enriched in activated myosin and pulled on this network. Inhibition of myosin activation by myosin light chain kinase (MLCK) stopped pulling and spindle relocation, indicating that myosin pulling creates the force that drives spindle movement. Based on these results, we propose the first mechanistic model for asymmetric spindle positioning in mammalian oocytes and validate five of its key predictions experimentally.

Building the cell: design principles of cellular architecture

The astounding structural complexity of a cell arises from the action of a relatively small number of genes, raising the question of how this complexity is achieved. Self-organizing processes combined with simple physical constraints seem to have key roles in controlling organelle size, number, shape and position, and these factors then combine to produce the overall cell architecture. By examining how these parameters are controlled in specific cell biological examples we can identify a handful of simple design principles that seem to underlie cellular architecture and assembly.