Application of Raman spectroscopy to the evaluation of F-actin changes in sea urchin eggs at fertilization.

The actin filaments on the surface of echinoderm oocytes and eggs readily undergo massive reorganization during meiotic maturation and fertilization. In sea urchin eggs, the actin cytoskeletal response to the fertilizing sperm is fast enough to accompany Ca2+ signals and to guide sperm's entry into the egg. Although recent work using live cell imaging technology confirmed changes in the actin polymerization status in fertilized eggs, as was previously shown using light and electron microscopy, it failed to provide experimental evidence of F-actin depolymerization a few seconds after insemination, which is concurrent with the sperm-induced Ca2+ release. In the present study, we applied Raman microspectroscopy to tackle this issue by examining the spectral profiles of the egg's subplasmalemmal regions before and after treating the eggs with actin drugs or fertilizing sperm. At both early (15 s) and late (15 min) time points after fertilization, specific peak shifts in the Raman spectra revealed change in the actin structure, and Raman imaging detected the cytoskeletal changes corresponding to the F-actin reorganization visualized with LifeAct-GFP in confocal microscopy. Our observation suggests that the application of Raman spectroscopy, which does not require microinjection of fluorescent probes and exogenous gene expression, may serve as an alternative or even advantageous method in disclosing rapid subtle changes in the subplasmalemmal actin cytoskeleton that are difficult to resolve.

Altered actin cytoskeleton in ageing eggs of starfish affects fertilization process.

Integrity of oocytes is of pivotal interest in the medical and zootechnical practice of in vitro fertilization. With time, oocytes undergo deterioration in quality, and ageing oocytes often exhibit compromised competence in fertilization and the subsequent embryonic development. With ageing oocytes and eggs of starfish (Astropecten aranciacus), we addressed the issue by examining changes of the subcellular structure and their performance at fertilization. Ageing eggs were simulated in two different experimental paradigms: i) oocytes were overmatured by 6 hours stimulation with 1-methyladenine (1-MA); ii) oocytes were removed from the gonad and maintained in seawater for 24 or 48 h before applying the hormonal stimulation (1-MA, 70 min). These eggs were compared with normally matured eggs (stimulated after isolation from the gonad with 1-MA for 70 min) with respect to the sperm-induced intracellular Ca2+ signaling and the structural changes of the egg surface. The cytoskeletal and ultrastructural differences in these eggs were assessed by confocal and transmission electron microscopy, respectively. In the two categories of ageing eggs, we have found remarkable structural modifications of the actin cytoskeleton and the cortical vesicles beneath the plasma membrane. At fertilization, these ageing eggs manifested an altered pattern of intracellular Ca2+ release, aberrant actin dynamics, and increased rate of polyspermy often despite full elevation of the fertilization envelope. Taken together, our results highlight the importance of spatio-temporal regulation of the actin cytoskeleton in the cortex of the eggs, and we postulate that the status of the actin cytoskeleton is one of the major determinants of the oocyte quality that ensures successful monospermic fertilization.

Sodium-mediated fast electrical depolarization does not prevent polyspermic fertilization in Paracentrotus lividus eggs.

During sea urchins fertilization, the activating spermatozoon triggers a series of physiological changes that transforms the quiescent egg into a dynamic zygote. It has been suggested that several of these egg activation events, e.g. sperm-induced plasma membrane depolarization and the Ca2+-linked cortical reaction, play additional roles to prevent the entry of supernumerary spermatozoa. In particular, the abrupt shift in egg membrane potential at fertilization, which is sustained by a Na+ influx, has been considered as a fast mechanism to block polyspermy. To test the relevance of the Na+-mediated fast electrical block to polyspermy, we fertilized sea urchin eggs in artificial seawater with a low concentration of Na+; nearly all the eggs were still monospermic, as judged by the number of Hoechst 33422-stained sperm. When fertilized in normal seawater, eggs that were pre-incubated in the low Na+ medium exhibited impaired elevation of the fertilization envelope. Nevertheless, these eggs manifested entry of a single spermatozoon, suggesting that the fertilization envelope was not the primary determinant of the block to polyspermy. Furthermore, we showed that the abnormal cleavage patterns displayed by eggs pre-incubated in low Na+, which were often considered a hallmark of polyspermy, were due to the alterations in the cortical actin filaments dynamics following fertilization, and not to the formation of multipolar spindles associated with supernumerary sperm centrosomes. Hence, our results suggested that Paracentrotus lividus eggs do not utilize Na+ to rapidly prevent additional spermatozoa from entering the egg, at variance with the hypothesis of an electrical fast block to polyspermy.

Disassembly of Subplasmalemmal Actin Filaments Induces Cytosolic Ca2+ Increases in Astropecten aranciacus Eggs.

BACKGROUND/AIMS: Eggs of all animal species display intense cytoplasmic Ca2+ increases at fertilization. Previously, we reported that unfertilized eggs of Astropecten aranciacus exposed to an actin drug latrunculin A (LAT-A) exhibit similar Ca2+ waves and cortical flashes after 5-10 min time lag. Here, we have explored the molecular mechanisms underlying this unique phenomenon. METHODS: Starfish eggs were pretreated with various agents such as other actin drugs or inhibitors of phospholipase C (PLC), and the changes of the intracellular Ca2+ levels were monitored by use of Calcium Green in the presence or absence of LAT-A. The concomitant changes of the actin cytoskeleton were visualized with fluorescent F-actin probes in confocal microscopy. RESULTS: We have shown that the LAT-A-induced Ca2+ increases are related to the disassembly of actin flaments: i) not only LAT-A but also other agents depolymerizing F-actin (i.e. cytochalasin B and mycalolide B) induced similar Ca2+ increases, albeit with slightly lower efficiency; ii) drugs stabilizing F-actin (i.e. phalloidin and jasplakinolide) either blocked or significantly delayed the LAT-A-induced Ca2+ increases. Further studies utilizing pharmacological inhibitors of PLC (U-73122 and neomycin), dominant negative mutant of PLC-É£, specific sequestration of PIP2 (RFP-PH), InsP3 uncaging, and quantitation of endogenous InsP3 all indicated that LAT-A induces Ca2+ increases by stimulating PLC rather than sensitizing InsP3 receptors. In support of the idea, it bears emphasis that LAT-A timely increased intracellular contents of InsP3 with concomitant decrease of PIP2 levels in the plasma membrane. CONCLUSION: Taken together, our results suggest that suboolemmal actin filaments may serve as a scaffold for cell signaling and modulate the activity of the key enzyme involved in intracellular Ca2+ signaling.

Maturation and fertilization of echinoderm eggs: Role of actin cytoskeleton dynamics.

Starfish and sea urchin are excellent models to study the mechanisms that regulate oocyte maturation and egg activation. Hormonal stimulation of starfish oocytes and their following interaction with spermatozoa induce rapid changes of F-actin and Ca2+ increases which are prerequisites for normal fertilization and development. Fully grown oocytes isolated from the gonads of starfish contain a large nucleus (∼60-70 µm) (termed germinal vesicle, GV), which is arrested at the first prophase of meiosis. If inseminated, these immature oocytes are penetrated by additional spermatozoa. However, starfish oocytes naturally shed into the sea have already initiated the (meiotic) maturation and are normally fertilized between GV breakdown and the extrusion of the first polar body. This is considered the optimum period to ensure monospermic instead of polyspermic fertilization. By contrast, sea urchin eggs are fertilized only after being fully matured, i.e., at the end of the two meiotic divisions. Here, we provide a comparative review of the role of the actin cytoskeleton in oocyte maturation and fertilization in starfish and sea urchin. It has become increasingly evident that the exquisite regulation of the cortical F-actin is involved in nearly all aspects of the molecular events taking place during the progression of meiotic maturation and fertilization.

Calcium and actin in the saga of awakening oocytes.

The interaction of the spermatozoon with the egg at fertilization remains one of the most fascinating mysteries of life. Much of our scientific knowledge on fertilization comes from studies on sea urchin and starfish, which provide plenty of gametes. Large and transparent, these eggs have served as excellent model systems for studying egg activation and embryo development in seawater, a plain natural medium. Starfish oocytes allow the study of the cortical, cytoplasmic and nuclear changes during the meiotic maturation process, which can also be triggered in vitro by hormonal stimulation. These morphological and biochemical changes ensure successful fertilization of the eggs at the first metaphase. On the other hand, sea urchin eggs are fertilized after the completion of meiosis, and are particularly suitable for the study of sperm-egg interaction, early events of egg activation, and embryonic development, as a large number of mature eggs can be fertilized synchronously. Starfish and sea urchin eggs undergo abrupt changes in the cytoskeleton and ion fluxes in response to the fertilizing spermatozoon. The plasma membrane and cortex of an egg thus represent "excitable media" that quickly respond to the stimulus with the Ca(2+) swings and structural changes. In this article, we review some of the key findings on the rapid dynamic rearrangements of the actin cytoskeleton in the oocyte/egg cortex upon hormonal or sperm stimulation and their roles in the modulation of the Ca(2+) signals and in the control of monospermic fertilization.

Assisted yes, but where do we draw the line?

In a recent report in Reproductive Biomedicine Online by Ebner et al., a comprehensive multi-centre study was presented on the use of a calcium ionophore, A23187, to artificially activate oocytes from patients who had poor fertilization rates in previous cycles. Under physiological conditions, the calcium increase in oocytes at activation is caused by influx and release from specific stores and ion channels, and has precise temporal, quantitative and spatial patterns. Calcium ionophores may release Ca(2+) in an uncontrolled fashion from intracellular stores that would not normally be involved in the activation process. Ionophores, including A23187, have a multitude of effects on cell homeostasis, not yet defined in oocytes, that may have long-term effects, for example on gene expression. We suspect that the successful births reported by Ebner et al. are a result of the overriding influence of the injected spermatozoa, rather than the effect of the ionophore; nevertheless, such an invasive non-physiological approach to assisted reproduction techniques is worrying, especially as epigenetic effects may result in future generations.

Ca²âº influx-linked protein kinase C activity regulates the ß-catenin localization, micromere induction signalling and the oral-aboral axis formation in early sea urchin embryos.

Sea urchin embryos initiate cell specifications at the 16-cell stage by forming the mesomeres, macromeres and micromeres according to the relative position of the cells in the animal-vegetal axis. The most vegetal cells, micromeres, autonomously differentiate into skeletons and induce the neighbouring macromere cells to become mesoendoderm in the ß-catenin-dependent Wnt8 signalling pathway. Although the underlying molecular mechanism for this progression is largely unknown, we have previously reported that the initial events might be triggered by the Ca2+ influxes through the egg-originated L-type Ca2+ channels distributed asymmetrically along the animal-vegetal axis and through the stretch-dependent Ca2+channels expressed specifically in the micromere at the 4th cleavage. In this communication, we have examined whether one of the earliest Ca2+ targets, protein kinase C (PKC), plays a role in cell specification upstream of ß-catenin. To this end, we surveyed the expression pattern of ß-catenin in early embryos in the presence or absence of the specific peptide inhibitor of Hemicentrotus pulcherrimus PKC (HpPKC-I). Unlike previous knowledge, we have found that the initial nuclear entrance of ß-catenin does not take place in the micromeres, but in the macromeres at the 16-cell stage. Using the HpPKC-I, we have demonstrated further that PKC not only determines cell-specific nucleation of ß-catenin, but also regulates a variety of cell specification events in the early sea urchin embryos by modulating the cell adhesion structures, actin dynamics, intracellular Ca2+ signalling, and the expression of key transcription factors.

Early events of fertilization in sea urchin eggs are sensitive to actin-binding organic molecules.

We previously demonstrated that many aspects of the intracellular Ca(2+) increase in fertilized eggs of starfish are significantly influenced by the state of the actin cytoskeleton. In addition, the actin cytoskeleton appeared to play comprehensive roles in modulating cortical granules exocytosis and sperm entry during the early phase of fertilization. In the present communication, we have extended our work to sea urchin which is believed to have bifurcated from the common ancestor in the phylogenetic tree some 500 million years ago. To corroborate our earlier findings in starfish, we have tested how the early events of fertilization in sea urchin eggs are influenced by four different actin-binding drugs that promote either depolymerization or stabilization of actin filaments. We found that all the actin drugs commonly blocked sperm entry in high doses and significantly reduced the speed of the Ca(2+) wave. At low doses, however, cytochalasin B and phalloidin increased the rate of polyspermy. Overall, certain aspects of Ca(2+) signaling in these eggs were in line with the morphological changes induced by the actin drugs. That is, the time interval between the cortical flash and the first Ca(2+) spot at the sperm interaction site (the latent period) was significantly prolonged in the eggs pretreated with cytochalasin B or latrunculin A, whereas the Ca(2+) decay kinetics after the peak was specifically attenuated in the eggs pretreated with jasplakinolide or phalloidin. In addition, the sperm interacting with the eggs pretreated with actin drugs often generated multiple Ca(2+) waves, but tended to fail to enter the egg. Thus, our results indicated that generation of massive Ca(2+) waves is neither indicative of sperm entry nor sufficient for cortical granules exocytosis in the inseminated sea urchin eggs, whereas the structure and functionality of the actin cytoskeleton are the major determining factors in the two processes.

Antibody against the actin-binding protein depactin attenuates Ca2+ signaling in starfish eggs.

Being present in starfish oocytes, the cofilin/ADF (actin-depolymerizing factor) family protein depactin severs actin filaments. Previously, we reported that exogenous cofilin microinjected into starfish eggs significantly augmented the Ca(2+) release in response to inositol 1,4,5-trisphosphate (InsP3) or fertilizing sperm, raising the possibility that intracellular Ca(2+) signaling could be modulated by the actin cytoskeleton. In this communication, we have targeted the endogenous depactin by use of the specific antibody that was raised against its actin-binding domain. The anti-depactin antibody microinjected into the starfish oocytes and eggs effectively altered the structure of the actin cytoskeleton, and significantly delayed the meiotic progression induced by 1-methyladenine. When microinjected into the mature eggs, the anti-depactin antibody markedly reduced the amplitude of the Ca(2+) response in a dose-dependent manner, corroborating the results of our previous study with cofilin. In addition, the eggs microinjected with the anti-depactin antibody displayed reduced rate of successful elevation of the fertilization envelope and an elevated tendency of polyspermic interaction. Taken together, our data suggest that the actin cytoskeleton is implicated not only in meiotic maturation and intracellular Ca(2+) signaling, but also in the fine regulation of gametes interaction and cortical granules exocytosis.

Dithiothreitol Affects the Fertilization Response in Immature and Maturing Starfish Oocytes.

Immature starfish oocytes isolated from the ovary are susceptible to polyspermy due to the structural organization of the vitelline layer covering the oocyte plasma membrane, as well as the distribution and biochemical properties of the actin cytoskeleton of the oocyte cortex. After the resumption of the meiotic cycle of the oocyte triggered by the hormone 1-methyladenine, the maturing oocyte reaches fertilizable conditions to be stimulated by only one sperm with a normal Ca2+ response and cortical reaction. This cytoplasmic ripening of the oocyte, resulting in normal fertilization and development, is due to the remodeling of the cortical actin cytoskeleton and germinal vesicle breakdown (GVBD). Since disulfide-reducing agents such as dithiothreitol (DTT) are known to induce the maturation and GVBD of oocytes in many species of starfish, we analyzed the pattern of the fertilization response displayed by Astropecten aranciacus oocytes pre-exposed to DTT with or without 1-MA stimulation. Short treatment of A. aranciacus immature oocytes with DTT reduced the rate of polyspermic fertilization and altered the sperm-induced Ca2+ response by changing the morphology of microvilli, cortical granules, and biochemical properties of the cortical F-actin. At variance with 1-MA, the DTT treatment of immature starfish oocytes for 70 min did not induce GVBD. On the other hand, the DTT treatment caused an alteration in microvilli morphology and a drastic depolymerization of the cortical F-actin, which impaired the sperm-induced Ca2+ response at fertilization and the subsequent embryonic development.

The Effect of Acidic and Alkaline Seawater on the F-Actin-Dependent Ca2+ Signals Following Insemination of Immature Starfish Oocytes and Mature Eggs.

In starfish, the addition of the hormone 1-methyladenine (1-MA) to immature oocytes (germinal vesicle, GV-stage) arrested at the prophase of the first meiotic division induces meiosis resumption (maturation), which makes the mature eggs able to respond to the sperm with a normal fertilization response. The optimal fertilizability achieved during the maturation process results from the exquisite structural reorganization of the actin cytoskeleton in the cortex and cytoplasm induced by the maturing hormone. In this report, we have investigated the influence of acidic and alkaline seawater on the structure of the cortical F-actin network of immature oocytes of the starfish (Astropecten aranciacus) and its dynamic changes upon insemination. The results have shown that the altered seawater pH strongly affected the sperm-induced Ca2+ response and the polyspermy rate. When immature starfish oocytes were stimulated with 1-MA in acidic or alkaline seawater, the maturation process displayed a strong dependency on pH in terms of the dynamic structural changes of the cortical F-actin. The resulting alteration of the actin cytoskeleton, in turn, affected the pattern of Ca2+ signals at fertilization and sperm penetration.

Fertilization in echinoderms.

For more than 150 years, echinoderm eggs have served as overly favored experimental model systems in which to study fertilization. Sea urchin and starfish belong to the same phylum and thus share many similarities in their fertilization patterns. However, several subtle but fundamental differences do exist in the fertilization of sea urchin and starfish, reflecting their phylogenetic bifurcation approximately 500 million years ago. In this article we review some of the seminal and recent findings that feature similarities and differences in sea urchin and starfish at fertilization.

Species-Specific Gamete Interaction during Sea Urchin Fertilization: Roles of the Egg Jelly and Vitelline Layer.

In sea urchins, the sequence of the cellular and molecular events characterizing the fertilization process has been intensively studied. We have learned that to activate the egg, the fertilizing sperm must undergo morphological modifications (the acrosome reaction, AR) upon reaching the outer gelatinous layer enveloping the egg (egg jelly), which triggers the polymerization of F-actin on the sperm head to form the acrosomal process. The AR exposes bindin, an adhesive sperm protein essential for the species-specific interaction with the cognate receptor on the egg vitelline layer. To investigate the specific roles of the egg jelly and vitelline layer at fertilization of sea urchin eggs, Paracentrotus lividus eggs were incubated in acidic seawater, which removes the egg jelly, i.e., experimental conditions that should prevent the occurrence of the AR, and inseminated in the same medium. At variance with the prevailing view, our results have shown that these dejellied P. lividus eggs can still interact with sperm in acidic seawater, albeit with altered fertilization responses. In particular, the eggs deprived of the vitelline layer reacted with multiple sperm but with altered Ca2+ signals. The results have provided experimental evidence that the plasma membrane, and not the vitelline layer, is where the specific recognition between gametes occurs. The vitelline layer works in unfertilized eggs to prevent polyspermy.

Regulation of the Actin Cytoskeleton-Linked Ca2+ Signaling by Intracellular pH in Fertilized Eggs of Sea Urchin.

In sea urchin, the immediate contact of the acrosome-reacted sperm with the egg surface triggers a series of structural and ionic changes in the egg cortex. Within one minute after sperm fuses with the egg plasma membrane, the cell membrane potential changes with the concurrent increases in intracellular Ca2+ levels. The consequent exocytosis of the cortical granules induces separation of the vitelline layer from the egg plasma membrane. While these cortical changes are presumed to prevent the fusion of additional sperm, the subsequent late phase (between 1 and 4 min after fertilization) is characterized by reorganization of the egg cortex and microvilli (elongation) and by the metabolic shift to activate de novo protein and DNA syntheses. The latter biosynthetic events are crucial for embryonic development. Previous studies suggested that the early phase of fertilization was not a prerequisite for these changes in the second phase since the increase in the intracellular pH induced by the exposure of unfertilized sea urchin eggs to ammonia seawater could start metabolic egg activation in the absence of the cortical granule exocytosis. In the present study, we have demonstrated that the incubation of unfertilized eggs in ammonia seawater induced considerable elongations of microvilli (containing actin filaments) as a consequence of the intracellular pH increase, which increased the egg's receptivity to sperm and made the eggs polyspermic at fertilization despite the elevation of the fertilization envelope (FE). These eggs also displayed compromised Ca2+ signals at fertilization, as the amplitude of the cortical flash was significantly reduced and the elevated intracellular Ca2+ level declined much faster. These results have also highlighted the importance of the increased internal pH in regulating Ca2+ signaling and the microvillar actin cytoskeleton during the late phase of the fertilization process.

Hexagonal Voronoi pattern detected in the microstructural design of the echinoid skeleton.

Repeated polygonal patterns are pervasive in natural forms and structures. These patterns provide inherent structural stability while optimizing strength-per-weight and minimizing construction costs. In echinoids (sea urchins), a visible regularity can be found in the endoskeleton, consisting of a lightweight and resistant micro-trabecular meshwork (stereom). This foam-like structure follows an intrinsic geometrical pattern that has never been investigated. This study aims to analyse and describe it by focusing on the boss of tubercles-spine attachment sites subject to strong mechanical stresses-in the common sea urchin Paracentrotus lividus. The boss microstructure was identified as a Voronoi construction characterized by 82% concordance to the computed Voronoi models, a prevalence of hexagonal polygons, and a regularly organized seed distribution. This pattern is interpreted as an evolutionary solution for the construction of the echinoid skeleton using a lightweight microstructural design that optimizes the trabecular arrangement, maximizes the structural strength and minimizes the metabolic costs of secreting calcitic stereom. Hence, this identification is particularly valuable to improve the understanding of the mechanical function of the stereom as well as to effectively model and reconstruct similar structures in view of future applications in biomimetic technologies and designs.

Fertilization and development of Arbacia lixula eggs are affected by osmolality conditions.

The sea urchin Arbacia lixula coexist with Paracentrotus lividus in the Mediterranean, but the two sea urchin species are quite different from each other. Concerning the female gamete, A. lixula eggs are much darker than those of P. lividus due to the characteristic pigmentation. Upon insemination, the fertilization envelope formed by A. lixula eggs is remarkably thinner than that of P. livius eggs, which implies that the cortical organization of the eggs in the two species may be quite different. In this communication, we examined the phenotypic plasticity of A. lixula eggs in the changing osmolality. The plasma membrane, cortical actin cytoskeleton and vesicles are extensively altered in the eggs exposed to 40% seawater for 15 min. When fertilized, the Ca2+ response in these eggs was significantly compromised and the sperm often failed to enter the eggs. Remarkably, the pattern of the Ca2+ response was restored when these eggs were transferring back to the natural seawater before fertilization, while the actin cytoskeleton partially reverted to the original state. Nonetheless, these eggs restored in seawater failed to regain the innate sperm receptivity that allows only one sperm to enter in natural seawater. Thus, the ability to guide monospermic fertilization is lost by water entry into the eggs, and the eggs incorporated either multiple or no sperm. On the other hand, eggs briefly exposed to hypertonic seawater exhibited no evident morphological anomaly. Nonetheless, the monospermic eggs that experienced a brief exposure (15 min) to hypertonic seawater prior to fertilization in natural seawater displayed a subtly altered sperm-induced Ca2+ response and morpho-functional anomaly around the pluteus stage. Our results suggest that A. lixula eggs attain only a limited extent of cytological plasticity, and that the osmolality shock affects the physical nature of the egg surface which in turn affects the developmental programming.