Cortical granule translocation is microfilament mediated and linked to meiotic maturation in the sea urchin oocyte

Influence of ovarian hyperstimulation and ovulation induction on the cytoskeletal dynamics and developmental competence of oocytes

This study was undertaken to determine the effects of gonadotrophin on cytoskeletal dynamics and embryo development and its role in improving the retrieval of developmentally competent oocytes. Female golden hamsters were injected with human chorionic gonadotrophin (hCG; 5-, 7.5- or 15-IU) on the day 4 of estrus, pregnant mare serum gonadotrophin (PMSG; 5-, 7.5- or 15-IU) on the day 1 of estrus, or 15-IU hCG at 56 hr post-15-IU PMSG injection in any cycle except estrus. Increasing the hCG dose decreased not only retrieval rate of 2-cell embryo but development to blastocyst after subsequent in vitro culture. Whereas, although increasing the PMSG dose induced increasing the number of 2-cell embryo and blastocyst, 15-IU PMSG injection caused retardation of development to blastocyst. No 2-cell embryos were retrieved by injecting both PMSG and hCG. The injections of 15-IU hCG and 7.5- or 15-IU PMSG inhibited the proliferation of trophectodermal and inner cell mass cells, respectively. Gonadotrophin injection didn't influence microtubular spindle formation, but 5- or 15-IU hCG, 15-IU PMSG, or PMSG and hCG injections induced aberrant cortical granule (CG) and microfilament distribution. After 15-IU hCG or PMSG and hCG injections, fewer oocytes had enriched cortical actin domains, and the expression of alpha-, beta- and gamma-actin genes was greatly increased. In conclusion, a high dose of gonadotrophins alters the microfilament and CG distribution, which in turn reduces the developmental competence of oocytes. Injecting a reduced dose of PMSG to initiate ovarian hyperstimulation without triggering ovulation contributes to the efficient retrieval of developmentally competent oocytes.

Antagonists of myosin light chain kinase and of myosin II inhibit specific events of egg activation in fertilized mouse eggs

Although recent studies have demonstrated the importance of calcium/calmodulin (Ca(2+)/CAM) signaling in mammalian fertilization, many targets of Ca(2+)/CAM have not been investigated and represent potentially important regulatory pathways to transduce the Ca2+ signal that is responsible for most events of egg activation. A well-established Ca(2+)/CAM-dependent enzyme is myosin light chain kinase (MYLK2), the downstream target of which is myosin II, an isoform of myosin known to be important in cytokinesis. In fertilized mouse eggs, established inhibitors of MYLK2 and myosin II were investigated for their effects on events of egg activation. The MYLK2 antagonist, ML-7, did not decrease the activity of Ca(2+)/CAM protein kinase II or the elevation of intracellular Ca2+, and it did not delay the onset of Ca2+ oscillations. In contrast, ML-7 inhibited second polar body (PB) formation in a dose-dependent manner and reduced cortical granule (CG) exocytosis by a mean of approximately 50%. The myosin II isoform-specific inhibitor, blebbistatin, had similar inhibitory effects. Although both antagonists had no effect on anaphase onset, they inhibited second PB formation by preventing spindle rotation before telophase II and normal contractile ring constriction. To our knowledge, this is the first report that MYLK2 and myosin II are involved in regulating the position of the meiotic spindle, formation of the second PB, and CG exocytosis. The present results suggest that MYLK2 is one of a family of CAM-dependent proteins that act as multifunctional regulators and transduce the Ca2+ signal at fertilization.

Nutritive phagocyte incubation chambers provide a structural and nutritive microenvironment for germ cells of Strongylocentrotus droebachiensis, the green sea urchin

Here we characterize the germinal epithelia of both sexes of Strongylocentrotus droebachiensis, the green sea urchin, throughout its annual gametogenic cycle, using light and electron microscopy and cytochemistry. In both sexes, germinal epithelia include two interacting cellular populations: nutritive phagocytes (NPs) and germ cells. After spring spawning, NPs accumulate nutrients; amitotic oogonia and often mitotic spermatogonia occur in clusters beneath NPs; and subsequent gametogenic stages are residual or absent. During the summer, NP nutrients are mobilized for use in vitellogenesis by residual primary oocytes or to support limited spermatogenesis. In addition, some residual primary oocytes may degenerate and be phagocytized by NPs. Significant nutrient mobilization from NPs and substantial gonial cell mitoses (indicative of new gametogenesis) occur in the fall. In both sexes, all of these changes are facilitated by NPs that form basal incubation chambers near the gonadal wall and within which germ cells are surrounded by nutrients released from the NPs. In females, germ cells at several stages of gametogenesis may be housed in separate chambers in the same NP. Primary oocytes also carry out jelly coat formation, meiosis, and cortical granule translocation within NP incubation chambers. In males, many NPs cooperate to provide large continuous chambers that contain spermatogenic cells at diverse stages. In both sexes these chambers persist throughout the year and isolate gametogenesis from the gonadal lumen. NPs become slender and shorten as their nutrients are depleted. Ova or spermatozoa are stored in the gonadal lumen. Post-spawning, NPs phagocytize differentiated germ cells while simultaneously enclosing intact gonial and residual gametogenic cells in basal chambers near the gonadal wall. In light of our observations, we suggest investigating proteins that may be important in the structural, phagocytic, and nutritive functions of NPs and for which corresponding genes have already been identified in the genome of S. purpuratus, the closely related purple sea urchin.

Activation of multidrug efflux transporter activity at fertilization in sea urchin embryos (Strongylocentrotus purpuratus)

This study presents functional and molecular evidence for acquisition of multidrug transporter-mediated efflux activity as a consequence of fertilization in the sea urchin. Sea urchin eggs and embryos express low levels of efflux transporter genes with homology to the multidrug resistance associated protein (mrp) and permeability glycoprotein (p-gp) families of ABC transporters. The corresponding efflux activity is low in unfertilized eggs but is dramatically upregulated within 25 min of fertilization; the expression of this activity does not involve de novo gene expression and is insensitive to inhibitors of transcription and translation indicating activation of pre-existing transporter protein. Our study, using specific inhibitors of efflux transporters, indicates that the major activity is from one or more mrp-like transporters. The expression of activity at fertilization requires microfilaments, suggesting that the transporters are in vesicles and moved to the surface after fertilization. Pharmacological inhibition of mrp-mediated efflux activity with MK571 sensitizes embryos to the toxic compound vinblastine, confirming that one role for the efflux transport activity is embryo protection from xenobiotics. In addition, inhibition of mrp activity with MK571 alone retards mitosis indicating that mrp-like activity may also be required for early cell divisions.

A Rho-signaling pathway mediates cortical granule translocation in the sea urchin oocyte

Cortical granules are secretory vesicles of the egg that play a fundamental role in preventing polyspermy at fertilization. In the sea urchin egg, they localize directly beneath the plasma membrane forming a compact monolayer and, upon fertilization, undergo a Ca(2+)-dependent exocytosis. Cortical granules form during early oogenesis and, during maturation, translocate from the cytosol to the oocyte cortex in a microfilament-mediated process. We tested the hypothesis that these cortical granule dynamics were regulated by Rho, a GTPase of the Ras superfamily. We observed that Rho is synthesized early in oogenesis, mainly in a soluble form. At the end of maturation, however, Rho associates with cortical granules. Inhibition of Rho with the C3 transferase from C. botulinum blocks cortical granule translocation and microfilaments undergo a significant disorganization. A similar effect is observed by GGTI-286, a geranylgeranyl transferase inhibitor, suggesting that the association of Rho with the cortical granules is indispensable for its function. In contrast, the anchorage of the cortical granules in the cortex, as well as their fusion at fertilization, are Rho-independent processes. We conclude that Rho association with the cortical granules is a critical regulatory step in their translocation to the egg cortex.

Calcium and fertilization: the beginning of life

The explosive increase in Ca2+ that occurs in the cytosol at fertilization is brought about by the activation of Ca2+-release channels in the intracellular stores. Inositol 1,4,5-trisphosphate (InsP3) is traditionally considered to be the messenger that initiates the increase and spreading of the activating Ca2+ wave. In line with this hypothesis, recent evidence suggests that the penetrating sperm delivers into mammalian eggs a novel isoform of phospholipase C (PLC), which promotes the formation of InsP3. By contrast, data from echinoderms studies indicate that the newly discovered second messenger nicotinic adenine dinucleotide phosphate (NAADP) promotes an initial, localized increase in Ca2+, which is then followed by the InsP3-mediated globalization of the Ca2+ wave. The mechanism by which the interacting sperm triggers the production of NAADP and subsequently that of InsP3 remains obscure.

Susceptibility of Bovine Germinal Vesicle-Stage Oocytes from Antral Follicles to Direct Effects of Heat Stress In Vitro1

Delineation of maternal versus direct effects of heat stress in reducing development at the germinal vesicle (GV) stage is challenging, because oocytes spontaneously resume meiosis after removal from antral follicles. The use of S-roscovitine (inhibitor of p34(cdc2)/cyclin B kinase) to hold bovine oocytes at the GV stage without compromising early embryo development was previously validated in our laboratory. The objective of the present study was to assess the direct effects of an elevated temperature commonly seen in heat-stressed dairy cows on cumulus-oocyte complexes (COCs) held at the GV stage using 50 microM S-roscovitine. During roscovitine culture, GV-stage COCs (antral follicle diameter, 3-8 mm) were cultured at 38.5 or 41 degrees C. Thereafter, oocytes were removed from roscovitine medium and allowed to undergo in vitro maturation, fertilization, and culture. Zona pellucida hardening (solubility to 0.5% pronase), nuclear stage (Hoechst 33342), cortical granule type (lens culinaris agglutinin-fluorescein isothiocyanate [FITC]), and early embryo development were evaluated. Culture of GV-stage COCs at 41 degrees C increased the proportion that had type III cortical granules and reduced the proportion that progressed to metaphase II after in vitro maturation. Effects of 41 degrees C on zona pellucida hardening, fertilization (penetration, sperm per oocyte, pronuclear formation, and monospermic and putative embryos), and cleavage of putative zygotes were not noted. However, culture of GV-stage COCs at 41 degrees C for 6 h decreased the proportion of 8- to 16-cell embryos, whereas 41 degrees C for 12 h reduced blastocyst development. In summary, antral follicle COCs are susceptible to direct effects of elevated body temperature, which may account in part for reduced fertility in heat-stressed cows.

The Major Yolk Protein of Sea Urchins Is Endocytosed by a Dynamin-Dependent Mechanism1

Sea urchin oocytes grow to 10 times their original size during oogenesis by both synthesizing and importing a specific repertoire of proteins to drive fertilization and early embryogenesis. During the vitellogenic growth period, the major yolk protein (MYP), a transferrin-like protein, is synthesized in the gut, transported into the ovary, and actively endocytosed by the oocytes. Here, we begin to dissect this mechanism by first testing the hypothesis that MYP endocytosis is dynamin-dependent. We have identified a sea urchin dynamin cDNA that is highly similar in amino acid sequence, structure, and size to mammalian dynamin I: it contains an N-terminal GTPase domain, a pleckstrin-homology domain, and a C-terminal proline-rich domain. Sea urchin dynamin is enriched at the cortex of oocytes and colocalizes to MYP endocytic vesicles at the oocyte periphery. To test for a functional relationship between MYP endocytosis and dynamin, we used a dominant-negative human dynamin I mutant protein containing an alteration within the GTPase domain (hDyn(K44A)) to specifically compete for dynamin function. Using a fluorescent MYP construct to follow its endocytosis solely, as well as a general endocytosis marker, we demonstrate that the disruption of dynamin function significantly reduces MYP uptake but does not affect fluid-phase endocytosis. Using this specific biochemical approach, we are able to separate distinct pathways of endocytosis during oogenesis and learn that dynamin-mediated endocytosis is responsible for MYP endocytosis but not fluid-phase uptake.

Regulatory contribution of heterotrimeric G-proteins to oocyte maturation in the sea urchin

Regulation of animal oocyte maturation is hypothesized to involve heterotrimeric G-proteins. It is difficult to test this hypothesis though without knowing what G-proteins are present in these cells and where are they localized. We set out to test the hypothesis that G-proteins regulate maturation in the sea urchin oocyte by identifying resident G-proteins in oocytes and eggs, and then investigating their function. We find four families of G-protein alpha-subunits (Galphai, Galphaq, Galphas, and Galpha12) present in both oocytes and eggs of the sea urchin. Three of them, Galphai, Galphaq, and Galphas are present on the plasma membrane of the oocyte, while the fourth is located on cytoplasmic vesicles. Upon oocyte maturation, these proteins remain in eggs, and continue to be expressed in embryonic tissues. To test the functional contribution of the G-proteins to the regulation of oocyte maturation, we employ specific intervening reagents, including antibodies and competitor peptides to each Galpha subunit, and specific Galpha toxins. We find that Gi is a main candidate for a positive regulator of sea urchin oocyte maturation. These studies provide a foundation to further test specific hypotheses of the G-protein mediated regulation of oocyte maturation, fertilization, and early development in the sea urchin.

CDK1/cyclin B regulation during oocyte maturation in two closely related lugworm species, Arenicola marina and Arenicola defodiens

The molecular mechanisms underlying oocyte maturation in the annelid polychaetes Arenicola marina and Arenicola defodiens were investigated. In both species, a hitherto unidentified hormone triggers synchronous and rapid transition from prophase to metaphase, a maturation process which can be easily reproduced in vitro. Activation of a roscovitine- and olomoucine-sensitive M-phase-specific histone, H1 kinase, occurs during oocyte maturation. Using affinity chromatography on immobilized p9CKShs1, we purified CDK1 and cyclin B from oocyte extracts prepared from both phases and both species. In prophase, CDK1 is present both as an inactive, but Thr161-phosphorylated monomer, and as an inactive (Tyr15-phosphorylated) heterodimer with cyclin B. Prophase to metaphase transition is associated with complete tyrosine dephosphorylation of the cyclin B-associated CDK1, with phosphorylation of cyclin B, and with dramatic activation of the kinase activity of the CDK1/cyclin B complex. We propose that Arenicola oocytes may provide an ideal model system to investigate the acquisition of the ability of oocytes to be fertilized that occurs as oocyte shift from prophase to metaphase, an important physiological event, probably regulated by active CDK1/cyclin B.

A C-type lectin associated and translocated with cortical granules during oocyte maturation and egg fertilization in fish

Oocyte maturation and egg fertilization in both vertebrates and invertebrates are marked by orchestrated cytoplasmic translocation of secretory vesicles known as cortical granules. It is thought that such redistribution of cellular content is critical for asymmetrical cell division during early development, but the mechanism and regulation of the process is poorly understood. Here we report the identification, purification and cDNA cloning of a C-type lectin from oocytes of a freshwater fish species gibel carp (Carassius auratus gibelio). The purified protein has been demonstrated to have lectin activity and to be a Ca(2+)-dependent C-type lectin by hemagglutination activity assay. Immunocytochemistry revealed that the lectin is associated with cortical granules, gradually translocated to the cell surface during oocyte maturation, and discharged to the egg envelope upon fertilization. Interestingly, the lectin becomes phosphorylated on threonine residues upon induction of exocytosis by fertilization and returns to its original state after morula stage of embryonic development, suggesting that this posttranslational modification may represent a critical molecular switch for early embryonic development.

A Rho GTPase controls the rate of protein synthesis in the sea urchin egg

Fertilization of the sea urchin egg triggers a Ca(2+)-dependent cortical granule exocytosis and cytoskeletal reorganization, both of which are accompanied by an accelerated protein synthesis. The signaling mechanisms leading to these events are not completely understood. The possible role of Rho GTPases in sea urchin egg activation was studied using the Clostridium botulinum C3 exotoxin, which specifically ADP-ribosylates Rho proteins and inactivates them. We observed that incubation of eggs with C3 resulted in in situ ADP-ribosylation of Rho. Following fertilization, C3-treated eggs were capable of performing cortical granule exocytosis but not the first cytokinesis. C3 caused in both unfertilized eggs and early embryos alterations in the state of actin polymerization and inhibition of the spindle formation. Moreover, C3 diminished markedly the rate of protein synthesis. These findings suggested that Rho is involved in regulating the acceleration of protein synthesis that accompanies the egg activation by sperm.

Selective transport and packaging of the major yolk protein in the sea urchin

The major yolk protein of sea urchins is an iron-binding, transferrin-like molecule that is made in the adult gut. Its final destination though is the developing oocytes that are embedded in somatic accessory cells and encompassed by two epithelial layers of the ovary. In this study, we address the dynamics of yolk transport, endocytosis, and packaging during the vitellogenic phase of oogenesis in the sea urchin by use of fluorescently labeled major yolk protein (MYP). Incorporation of MYP into the accessory cells of the ovary and its packaging into yolk platelets of developing oocytes is visualized in isolated oocytes, ovary explants, and in whole animals. When MYP is introduced into the coelom of adult females, it is first accumulated by the somatic cells of the ovarian capsule and is then transported to the oocytes and packaged into yolk platelets. This phenomenon is specific for MYP and accurately reflects the endogenous MYP packaging. We find that oocytes cultured in isolation are endocytically active and capable of selectively packaging MYP into yolk platelets. Furthermore, oocytes that packaged exogenous MYP are capable of in vitro maturation, fertilization, and early development, enabling an in vivo documentation of MYP utilization and yolk platelet dynamics. These results demonstrate that the endocytic uptake of yolk proteins in sea urchins does not require a signal from their surrounding epithelial cells and can occur autonomous of the ovary. In addition, these results demonstrate that the entire population of yolk platelets is competent to receive new yolk protein input, suggesting that they are all made simultaneously during oogenesis.

An increase in surface area is not required for cell division in early sea urchin development

Cell division requires an increase in surface area to volume ratio. During early development, surface area can increase, volume can decrease, or surface topography can be optimized to allow for division. While exocytosis is thought to be essential for division [Mol. Biol. Cell 10 (1999), 2735; Proc. Natl. Acad. Sci. USA 99 (2002), 3633], exocytosis doesn't always yield an increase in surface area [Proc. Natl. Acad. Sci. USA 79 (1982), 6712]. We used multiphoton laser scanning microscopy, fluorescence spectroscopy, and electron microscopy to monitor membrane trafficking, surface area, volume, and surface topography during early sea urchin development. Despite extensive membrane trafficking monitored by FM 1-43 fluorescence, we find that the net surface area of the embryo does not change prior to the eight-cell stage. During this period, embryo volume decreases by 15%, and microvilli disappear from interior facing membrane segments. Thus, the first three cell divisions utilize residual membrane liberated by decreasing cytoplasmic volume, and reducing microvilli density on interior facing membranes. Only after the eight-cell stage was a net increase in FM 1-43 fluorescence from the embryo surface detected. Our data suggest that compensatory endocytosis is downregulated after this developmental stage to yield an increase in surface area for cell division.