The wave of activation current in the Xenopus egg

Spontaneous Overactivation of Xenopus Frog Eggs Triggers Necrotic Cell Death

The excessive activation of frog eggs, referred to as overactivation, can be initiated by strong oxidative stress, leading to expedited calcium-dependent non-apoptotic cell death. Overactivation also occurs spontaneously, albeit at a low frequency, in natural populations of spawned frog eggs. Currently, the cytological and biochemical events of the spontaneous process have not been characterized. In the present study, we demonstrate that the spontaneous overactivation of Xenopus frog eggs, similarly to oxidative stress- and mechanical stress-induced overactivation, is characterized by the fast and irreversible contraction of the egg's cortical layer, an increase in egg size, the depletion of intracellular ATP, a drastic increase in the intracellular ADP/ATP ratio, and the degradation of M phase-specific cyclin B2. These events manifest in eggs in the absence of caspase activation within one hour of triggering overactivation. Importantly, substantial amounts of ATP and ADP leak from the overactivated eggs, indicating that plasma membrane integrity is compromised in these cells. The rupture of the plasma membrane and acute depletion of intracellular ATP explicitly define necrotic cell death. Finally, we report that egg overactivation can occur in the frog's genital tract. Our data suggest that mechanical stress may be a key factor promoting egg overactivation during oviposition in frogs.

Geometry-structure models for liquid crystal interfaces, drops and membranes: wrinkling, shape selection and dissipative shape evolution

We review our recent contributions to anisotropic soft matter models for liquid crystal interfaces, drops and membranes, emphasizing validations with experimental and biological data, and with related theory and simulation literature. The presentation aims to illustrate and characterize the rich output and future opportunities of using a methodology based on the liquid crystal-membrane shape equation applied to static and dynamic pattern formation phenomena. The geometry of static and kinetic shapes is usually described with dimensional curvatures that co-mingle shape and curvedness. In this review, we systematically show how the application of a novel decoupled shape-curvedness framework to practical and ubiquitous soft matter phenomena, such as the shape of drops and tactoids and bending of evolving membranes, leads to deeper quantitative insights than when using traditional dimensional mean and Gaussian curvatures. The review focuses only on (1) statics of wrinkling and shape selection in liquid crystal interfaces and membranes; (2) kinetics and dissipative dynamics of shape evolution in membranes; and (3) computational methods for shape selection and shape evolution; due to various limitations other important topics are excluded. Finally, the outlook follows a similar structure. The main results include: (1) single and multiple wavelength corrugations in liquid crystal interfaces appear naturally in the presence of surface splay and bend orientation distortions with scaling laws governed by ratios of anchoring-to-isotropic tension energy; adding membrane elasticity to liquid crystal anchoring generates multiple scales wrinkling as in tulips; drops of liquid crystals encapsulates in membranes can adopt, according to the ratios of anchoring/tension/bending, families of shapes as multilobal, tactoidal, and serrated as observed in biological cells. (2) Mapping the liquid crystal director to a membrane unit normal. The dissipative shape evolution model with irreversible thermodynamics for flows dominated by bending rates, yields new insights. The model explains the kinetic stability of cylinders, while spheres and saddles are attractors. The model also adds to the evolving understanding of outer hair cells in the inner ear. (3) Computational soft matter geometry includes solving shape equations, trajectories on energy and orientation landscapes, and shape-curvedness evolutions on entropy production landscape with efficient numerical methods and adaptive approaches.

Fertility decline in female mosquitoes is regulated by the orco olfactory co-receptor

Female Aedes aegypti mosquitoes undergo multiple rounds of reproduction, known as gonotrophic cycles. These cycles span the period from blood meal intake to oviposition. Understanding how reproductive success is maintained across gonotrophic cycles allows for the identification of molecular targets to reduce mosquito population growth. Odorant receptor co-receptor (orco) encodes a conserved insect-specific transmembrane ion channel that complexes with tuning odorant receptors (ORs) to form a functional olfactory receptor. orco expression has been identified in the male and female mosquito germline, but its role is unclear. We report an orco-dependent, maternal effect reduction in fertility after the first gonotrophic cycle. This phenotype was removed by CRISPR-Cas9 reversion of the orco mutant locus. Eggs deposited by orco mutant females are fertilized but the embryos reveal developmental defects, reduced hatching, and changes in ion channel signaling gene transcription. We present an unexpected role for an olfactory receptor pathway in mosquito reproduction.

Sperm MMP-2 is indispensable for fast electrical block to polyspermy at fertilization in Xenopus tropicalis

Sperm matrix metalloproteinase-2 (MMP-2) is necessary for frog fertilization. Monospermy is ensured by a fast, electrical block to polyspermy mediated by a positive fertilization potential. To determine the role of the MMP-2 hemopexin domain (HPX) in a fast block to polyspermy during fertilization of the frog, Xenopus tropicalis, we prepared mutant frogs deficient in mmp2 gene using the transcription activator-like effector nuclease method. mmp2 ΔHPX (-/-) sperm without MMP-2 protein were able to fertilize wild-type (WT; +/+) eggs. However, polyspermy occurred in some eggs. The mutant sperm generated a normal fertilization potential amounting to 10 mV, and were able to fertilize eggs at 10 mV, at which WT sperm never fertilized. Sensitivity during voltage-dependent fertilization decreased in mutant sperm. This study demonstrates for the first time that the genetic alteration of the MMP-2 molecule in sperm causes polyspermy during fertilization of a monospermic species. Our findings provide reliable evidence that sperm MMP-2 is indispensable for the fast, electrical block to polyspermy during Xenopus fertilization.

How to make a good egg!: The need for remodeling of oocyte Ca(2+) signaling to mediate the egg-to-embryo transition

The egg-to-embryo transition marks the initiation of multicellular organismal development and is mediated by a specialized Ca(2+) transient at fertilization. This explosive Ca(2+) signal has captured the interest and imagination of scientists for many decades, given its cataclysmic nature and necessity for the egg-to-embryo transition. Learning how the egg acquires the competency to generate this Ca(2+) transient at fertilization is essential to our understanding of the mechanisms controlling egg and the transition to embryogenesis. In this review we discuss our current knowledge of how Ca(2+) signaling pathways remodel during oocyte maturation in preparation for fertilization with a special emphasis on the frog oocyte as additional reviews in this issue will touch on this in other species.

Egg activation in physiological polyspermy

Fertilization is indispensable not only for restoring diploid genomes but also for the initiation of early embryonic cell cycles in sexual reproduction. While most animals exhibit monospermy, which is ensured by polyspermy blocks to prevent the entry of extra sperm into the egg at fertilization, several animals exhibit physiological polyspermy, in which the entry of several sperm is permitted but only one sperm nucleus participates in the formation of a zygote nucleus. Polyspermy requires that the sperm transmit the egg activation signal more slowly, thus allowing the egg to accept several sperm. An increase in intracellular Ca(2+) concentration induced by the fertilizing sperm is both necessary and sufficient for egg activation in polyspermy. Multiple small Ca(2+) waves induced by several fertilizing sperm result in a long-lasting Ca(2+) rise, which is a characteristic of polyspermic amphibian eggs. We introduced a novel soluble sperm factor for egg activation, sperm-specific citrate synthase, into polyspermic newt eggs to cause Ca(2+) waves. Citrate synthase may perform dual functions: as an enzyme in mitochondria and as a Ca(2+)-inducing factor in egg cytoplasm. We also discuss the close relationship between the mode of fertilization and the Ca(2+) rise at egg activation and consider changes in this process through evolution in vertebrates.

Topological singularities and symmetry breaking in development

The review presents a topological interpretation of some morphogenetic events through the use of well-known mathematical concepts and theorems. Spatial organization of the biological fields is analyzable in topological terms. Topological singularities inevitably emerging in biological morphogenesis are retained and transformed during pattern formation. It is the topological language that can provide strict and adequate description of various phenomena in developmental and evolutionary transformations. The relationship between local and global orders in metazoan development, i.e., between local morphogenetic processes and integral developmental patterns, is established. A topological inevitability of some developmental events through the use of classical topological concepts is discussed. This methodology reveals a topological imperative as a certain set of topological rules that constrains and directs embryogenesis. A breaking of spatial symmetry of preexisting pattern plays a critical role in biological morphogenesis in development and evolution.

Membrane depolarization increases membrane PtdIns(4,5)P2 levels through mechanisms involving PKC βII and PI4 kinase

In a previous study, we showed that membrane depolarization induced elevation of membrane phosphatidylinositol 4,5-bisphosphates (PtdIns(4,5)P(2), also known as PIP(2)) and subsequently increased the KCNQ2/Q3 currents expressed in Xenopus oocytes through increased PI4 kinase activity. In this study, the underlying mechanism for this depolarization-induced enhancement of PIP(2) synthesis was further investigated. Our results indicate that activation of protein kinase C (PKC) isozyme βII was responsible for the enhanced PIP(2) synthesis. We found that phorbol-12-myristate, 13-acetate (PMA), an activator of PKC, mimicked the effects of the membrane depolarization by increasing KCNQ2/Q3 activity, elevating membrane PIP(2) levels and increasing activity of PI4 kinase β. Furthermore, membrane depolarization enhanced PKC activity. The effects of both depolarization and PMA were blocked by a PKC inhibitor or PI4 kinase β RNA interference. Further results demonstrate that the depolarization selectively activated the PKC βII isoform and enhanced its interaction with PI4 kinase β. These results reveal that the depolarization-induced elevation of membrane PIP(2) is through activation of PKC and the subsequent increased activity of PI4 kinase β.

Full-term mouse development by abolishing Zn2+-dependent metaphase II arrest without Ca2+ release

In vertebrates, a rise in intracellular free Ca2+ (Ca2+i) levels during fertilization initiates second metaphase (mII) exit and the developmental programme. The Ca2+ rise has long been considered to be crucial for development, but verifying this contribution would benefit from defining its role during fertilization. Here, we delineate the role of Ca2+ release during mII exit in wild-type mouse eggs and show that it is dispensable for full-term development. Exit from mII can be induced by Zn2+-specific sequestration without Ca2+ release, eliciting Cyclin B degradation in a manner dependent upon the proteasome pathway and intact microtubules, but not accompanied by degradation of the meiotic regulator Emi2. Parthenogenotes generated by Zn2+ sequestration developed in vitro with normal expression of Ca2+-sensitive genes. Meiotic exit induced by either Ca2+ oscillations or a single Ca2+ rise in oocytes containing a signaling-deficient sperm resulted in comparable developmental rates. In the absence of Ca2+ release, full-term development occurred ∼50% less efficiently, but at readily detectable rates, with the birth of 27 offspring. These results show in intact mouse oocytes that Zn2+ is essential for mII arrest and suggest that triggering meiotic exit is the sole indispensable developmental role of Ca2+ signaling in mammalian fertilization.

Two phases of calcium requirement during starfish meiotic maturation

During meiosis in oocytes of the starfish, Asterina pectinifera, a Ca(2+) transient has been observed. To clarify the role of Ca(2+) during oocyte maturation in starfish, an intracellular Ca(2+) blocker, TMB-8, was applied. The oocyte maturation induced by 1-methyladenine (1-MA) was blocked by 100 microM TMB-8. Reinitiation of meiosis with germinal vesicle breakdown (GVBD) and the following chromosome condensation did not take place. Maturation-promoting factor (MPF) activity did not increase and GVBD and chromosome condensation did not occur. Ca(2+) transient observed immediately after 1-MA application in control oocytes was also blocked by TMB-8. When calyculin A, which activate the MPF directly, was applied to the oocytes instead of 1-MA in seawater containing 100 microM TMB-8, GVBD and chromosome condensation were blocked. Cytoplasmic transplantation studies confirmed that MPF was activated, although TMB-8 blocked GVBD. These results show that TMB-8 blocked the increase of MPF activity induced by 1-MA and the process of active MPF inducing GVBD and subsequent chromosome condensation. Together with the above phenomena, it is conceivable that there are two phases of Ca(2+) requirement during starfish oocyte maturation. These are the activation of MPF, moreover, GVBD, and the subsequent chromosome condensation.

NAADP triggers the fertilization potential in starfish oocytes

In invertebrates oocytes or eggs, the fertilization or activation potential establishes the fast electrical block to polyspermy and, in some species, provides the Ca2+ influx which contributes to the following intracellular Ca2+ wave. In echinoderms, the molecule triggering the activation potential is still unknown. The aim of this study was to assess whether nicotinic acid-adenine dinucleotide phosphate (NAADP) elicited the fertilization potential in starfish oocytes. The changes in membrane potential induced by the sperm were measured in oocytes held at a low resting potential, so that the Ca2+-action potential was inactivated and only the initial slower depolarization caused by the sperm could be studied. Decreasing extracellular Na+ concentration did not prevent the onset of the fertilization potential, while removal of external Ca2+ abolished it. The pre-incubation with SK&F 96365 and verapamil and the pre-injection of BAPTA inhibited the fertilization potential, while the injection of heparin only reduced its duration. The biophysical and pharmacological properties of the sperm-elicited depolarization were similar to those displayed by the NAADP-activated Ca2+-mediated current recently described in starfish oocytes. Indeed, the desensitization of NAADP-receptors prevented the onset of the fertilization potential. Taken together, these data suggest that NAADP could trigger the fertilization potential in starfish oocytes.

Cdc42-dependent actin polymerization during compensatory endocytosis in Xenopus eggs

The actin filament (F-actin) cytoskeleton associates dynamically with the plasma membrane and is thus ideally positioned to participate in endocytosis. Indeed, a wealth of genetic and biochemical evidence has confirmed that actin interacts with components of the endocytic machinery, although its precise function in endocytosis remains unclear. Here, we use 4D microscopy to visualize the contribution of actin during compensatory endocytosis in Xenopus laevis eggs. We show that the actin cytoskeleton maintains exocytosing cortical granules as discrete invaginated compartments, such that when actin is disrupted, they collapse into the plasma membrane. Invaginated, exocytosing cortical granule compartments are directly retrieved from the plasma membrane by F-actin coats that assemble on their surface. These dynamic F-actin coats seem to drive closure of the exocytic fusion pores and ultimately compress the cortical granule compartments. Active Cdc42 and N-WASP are recruited to exocytosing cortical granule membranes before F-actin coat assembly and coats assemble by Cdc42-dependent, de novo actin polymerization. Thus, F-actin may power fusion pore resealing and function in two novel endocytic capacities: the maintenance of invaginated compartments and the processing of endosomes.

Voltage-clamp study of the activation currents and fast block to polyspermy in the egg of Xenopus laevis

Voltage-clamped mature, jelly-intact Xenopus eggs were used to carefully examine the ionic currents crossing the plasma membrane before, during, and after fertilization. The bulk of the fertilization current was transient, of large amplitude, and reversed at the predicted Cl- reversal potential. However, the large amplitude fertilization current was preceded by a small, step-like increase in holding current. This small increase in holding current is referred to in this paper as Ion to acknowledge its qualitative similarity to the Ion current previously described in the sea urchin. It was observed in both fertilized and artificially activated eggs, and was found to be unaffected by 10 mm tetra-ethyl ammonium (TEA), a concentration found to block K+ currents in Rana pipiens. Current-voltage relationships are presented for the large fertilization potential, and show that the fertilization currents have a marked outward rectification and are voltage sensitive. These properties are in contrast to the total lack of rectification and slight voltage sensitivity seen before or after the fertilization currents. The time required for sperm to fertilize the egg was found to be voltage dependent with a relatively more depolarized voltage requiring a longer time for fertilization to occur. The percentage of eggs blocked with varying potential levels was determined and this information was fitted to a modified Boltzmann equation having a midpoint of -9 mV.

Induction of maturation-promoting factor during Xenopus oocyte maturation uncouples Ca(2+) store depletion from store-operated Ca(2+) entry

During oocyte maturation, eggs acquire the ability to generate specialized Ca(2+) signals in response to sperm entry. Such Ca(2+) signals are crucial for egg activation and the initiation of embryonic development. We examined the regulation during Xenopus oocyte maturation of store-operated Ca(2+) entry (SOCE), an important Ca(2+) influx pathway in oocytes and other nonexcitable cells. We have previously shown that SOCE inactivates during Xenopus oocyte meiosis. SOCE inactivation may be important in preventing premature egg activation. In this study, we investigated the correlation between SOCE inactivation and the Mos-mitogen-activated protein kinase (MAPK)-maturation-promoting factor (MPF) kinase cascade, which drives Xenopus oocyte maturation. SOCE inactivation at germinal vesicle breakdown coincides with an increase in the levels of MAPK and MPF. By differentially inducing Mos, MAPK, and MPF, we demonstrate that the activation of MPF is necessary for SOCE inactivation during oocyte maturation. In contrast, sustained high levels of Mos kinase and the MAPK cascade have no effect on SOCE activation. We further show that preactivated SOCE is not inactivated by MPF, suggesting that MPF does not block Ca(2+) influx through SOCE channels, but rather inhibits coupling between store depletion and SOCE activation.

Injection of a sperm extract triggers egg activation in the newt Cynops pyrrhogaster

Unfertilized eggs of the newt Cynops pyrrhogaster are arrested at the second meiotic metaphase. The primary signal for egg activation is a transient increase in [Ca2+](i), which is triggered by the fertilizing sperm and propagates over the egg cortex as a Ca2+ wave. We injected an extract of Cynops sperm (SE) into unfertilized eggs and induced a wave-like [Ca2+](i) increase which resulted in activation and resumption of meiosis. The SE-injected eggs showed degradation of cyclin B1 and DNA replication. When SE was boiled or treated with proteinase K before injection, it was unable to cause egg activation. Preinjection of Ca2+ -chelator BAPTA before SE injection inhibited egg activation. These results indicate that a heat-labile and proteinaceous factor in the sperm cytoplasm induces a transient increase in [Ca2+](i) which is required for egg activation. Injection of IP3 into unfertilized eggs caused an increase in [Ca2+](i) and egg activation, but injection of cADP-ribose did not. These results support the hypothesis that Ca2+ release at fertilization occurs via IP3 receptors.

ADP-ribose gates the fertilization channel in ascidian oocytes

We report an ion channel in the plasma membrane of unfertilized oocytes of the ascidian Ciona intestinalis that is directly gated by the second messenger ADP-ribose. The ion channel is permeable to Ca2+ and Na+ and is characterized by a reversal potential between 0 and +20 mV and a unitary conductance of 140 pS. Preinjection of the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) or antagonists of intracellular Ca2+ release channels into oocytes did not inhibit the ADP-ribose current, demonstrating that the channel is activated in a Ca2+-independent manner. Both the fertilization current and the current induced by the injection of nicotinamide nucleotides are blocked by nicotinamide, suggesting that the ADP-ribose channel is activated at fertilization in a nicotinamide-sensitive manner. These data suggest that ascidian sperm trigger the hydrolysis of nicotinamide nucleotides in the oocyte to ADP-ribose and that this mechanism is responsible for the production of the fertilization current.

Ion channels and early development of neural cells

In this review we underscore the merits of using voltage-dependent ion channels as markers for neuronal differentiation from the early stages of uncommitted embryonic blastomeres. Furthermore, a fairly large part of the review is devoted to the descriptions of the establishment of a simple model system for neural induction derived from the cleavage-arrested eight-cell ascidian embryo by pairing a single ectodermal with a single vegetal blastomere as a competent and an inducer cell, respectively. The descriptions are focused particularly on the early developmental processes of various ion channels in neuronal and other excitable membranes observed in this extraordinarily simple system, and we compare these results with those in other significant and definable systems for neural differentiation. It is stressed that this simple system, for which most of the electronic and optical methods and various injection experiments are applicable, may be useful for future molecular physiological studies on the intracellular process of differentiation of the early embryonic cells. We have also highlighted the importance of suppressive mechanisms for cellular differentiation from the experimental results, such as epidermal commitment of the cleavage-arrested one-cell Halocynthia embryos or suppression of epidermal-specific transcription of inward rectifier channels by neural induction signals. It was suggested that reciprocal suppressive mechanisms at the transcriptional level may be one of the key processes for cellular differentiation, by which exclusivity of cell types is maintained.

Non-specific currents at fertilisation in sea urchin oocytes

Using the whole-cell voltage-clamp technique to clamp sea urchin oocytes we show that the fertilising spermatozoon triggers an inward current of -521 +/- 56.7 pA (n = 8) at activation. Simultaneously, the plasma membrane depolarises and the conductance increases from 23.4 +/- 1.4 to 40.6 +/- 1.2 nS (n = 8). The I/V curve for the peak activation current is linear and the current reverses between 0 and +20 mV, suggesting a non-specific ion current. Since injection of inositol triphosphate induced an inward current of -1062 +/- 314 pA (n = 4), and the current was inhibited by preloading oocytes with the calcium chelator BAPTA, the non-specific activation current in sea urchin appears to be calcium dependent.

The intracellular calcium increase at fertilization in Urechis caupo oocytes: activation without waves

The intracellular Ca2+ (Cai) increase at fertilization of the marine worm Urechis caupo (Echiura) was studied with conventional and confocal epifluorescence microscopy in oocytes microinjected with calcium green dextran or dually labeled with the calcium-insensitive dye tetramethylrhodamine dextran. Calcium green fluorescence was also measured with a photomultiplier system while the oocyte membrane potential was recorded and manipulated. The results show that Cai rises simultaneously around the oocyte cortex and peaks slightly later in the nucleoplasm. The Cai rise coincides with the initiation of the fertilization potential and we conclude that it is due primarily to external Ca2+ entering through the voltage-gated Ca2+ action potential channels that open during the fertilization potential because: (1) current clamping the oocyte membrane potential to positive values in the absence of sperm produces a similar Cai increase, (2) external Ca2+ is required, (3) and the confocal images are consistent with this mechanism. External application of sperm acrosomal peptide (P23) also caused a Cai increase that was inhibited in the presence of CoCl2. Cai and pHi (measured with BCECF dextran) were manipulated in experiments employing microinjection of BAPTA (to chelate Cai), external application of NH4Cl (to increase pHi) and CoCl2 (to block Ca2+ channels), and fertilization of eggs in pH 7 seawater (Cai increase without pHi increase). The results showed that increases in both Cai and pHi are required for GVBD; neither alone is sufficient. However, although nuclear and cytoplasmic Ca2+ levels tended to parallel each other in oocytes fertilized at pH 7, and during the initial Cai response in oocytes fertilized at pH 8, there was a disproportionate fluorescence increase in the nucleoplasm of the latter prior to GVBD which could not be explained by any artifact we tested, suggesting there may be a selective increase in nuclear Ca2+ associated with GVBD. Finally, electrophysiological experiments with BAPTA-injected oocytes showed that the opening of the fertilization potential Na+ channels was Ca2+-independent, (although they did not close at the normal time). These and earlier results suggest that Urechis sperm may activate oocytes by interacting directly with the Na+ channels or associated receptors.

Calcium and related channels in fertilization and early development ofFucus

Unfertilized eggs ofFucus serratusare primed to respond rapidly to the fertilizing sperm. The unfertilized egg plasma membrane is excitable due to the presence of voltage-regulated Ca2+and K+channels. Sperm-egg interaction elicits a fertilization potential as the first observable fertilization event. It is speculated that sperm-gated Na+channels are responsible for the initial depolarization phase, leading to opening of Ca2+channels, allowing Ca2+influx and further depolarizing the membrane to the threshold for outward K+channels. K+efflux repolarizes the membrane and the zygote plasmalemma quickly becomes dominated by a large K+conductance. The involvement of Ca2+in axis formation and fixation is not clear. Ca2+carries a proportion of the inward current at the future rhizoid pole and asymmetric45Ca influx has been detected in polarizing zygotes. However, there is no requirement for external Ca2+in axis fixation. In contrast, Ca2+influx is required for expression of polarity and rhizoid growth. New developments in patch clamping can now enable localized areas of the plasma membrane in polarized cells to be studied. So far, both inward and outward single channel currents have been observed in the growing rhizoid tip, most probably carrying Cl-and K+respectively. These channels can be related to the currents identified by previous studies using the extracellular vibrating probe.

A soluble sperm factor gates Ca(2+)-activated K+ channels in human oocytes

PURPOSE

Our goal was to study the activation current in physiologically competent metaphase II human oocytes, i.e., not previously exposed to spermatozoa or aged in vitro, and, in particular, to determine whether a soluble sperm factor triggers a fertilization current comparable to that observed with intact spermatozoa and to characterize the current involved.

METHODS

The whole-cell voltage-clamp technique was used on spare metaphase II oocytes, obtained with patient consent from IVF programs. In this configuration a soluble fraction from human spermatozoa was microinjected, and the current recorded.

RESULTS

Metaphase II human oocytes generate bell-shaped outward currents of 400-1000 pA (X = 706 +/- 322; n = 10), following injection of a cytosolic extract from human spermatozoa. The amount of sperm extract injected was less than 10% of the total oocyte volume and was equivalent to 1-10 spermatozoa. A similar current was generated following exposure to 20 microM of the calcium ionophore A23187 (n = 10). The steady-state conductance of the oocyte increased from 10 to 19.8 nS (n = 10) following injection of the sperm factor and from 5.3 to 27.7 nS following ionophore exposure. Both sperm factor- and ionophore-induced currents were reduced in amplitude when the unfertilized oocyte was preexposed to 25-75 microM iberiotoxin (n = 8) and eliminated at a concentration of 100 microM iberiotoxin.

CONCLUSIONS

The data support the hypothesis of a soluble sperm factor involved in the activation of human oocytes and shows that the initial activation response in the human oocyte is the gating of Ca(2+)-activated K+ channels.

Local alteration of cortical actin in Xenopus eggs by the fertilizing sperm

Rhodamine phalloidin (Rph) staining was used to examine the microfilament organization of the Xenopus laevis egg cortex during the early stages of fertilization. Unactivated eggs possessed a cytochalasin B (CB)-insensitive Rph-stained matrix that was reorganized upon egg activation and diminished in the presence of CB. Xenopus laevis sperm caused a temporary local increase in Rph staining on the Xenopus cortex. In CB-treated eggs, the local increases of cortical Rph staining later changed to a Rph-free area. These temporary local increases of cortical Rph staining were also observed when Notophthalmus viridescens sperm fertilized Xenopus and Rana pipiens eggs, and were followed by the appearance of concentric rings of stained and unstained areas. Our data suggest that Xenopus and Notophthalmus sperm have activities that can both organize and disrupt the cortical filamentous actin of the Xenopus egg.

Polarity and reorganization of the endoplasmic reticulum during fertilization and ooplasmic segregation in the ascidian egg

During the first cell cycle of the ascidian egg, two phases of ooplasmic segregation create distinct cytoplasmic domains that are crucial for later development. We recently defined a domain enriched in ER in the vegetal region of Phallusia mammillata eggs. To explore the possible physiological and developmental function of this ER domain, we here investigate its organization and fate by labeling the ER network in vivo with DiIC16(3), and observing its distribution before and after fertilization in the living egg. In unfertilized eggs, the ER-rich vegetal cortex is overlaid by the ER-poor but mitochondria-rich subcortical myoplasm. Fertilization results in striking rearrangements of the ER network. First, ER accumulates at the vegetal-contraction pole as a thick layer between the plasma membrane and the myoplasm. This accompanies the relocation of the myoplasm toward that region during the first phase of ooplasmic segregation. In other parts of the cytoplasm, ER becomes progressively redistributed into ER-rich and ER-poor microdomains. As the sperm aster grows, ER accumulates in its centrosomal area and along its astral rays. During the second phase of ooplasmic segregation, which takes place once meiosis is completed, the concentrated ER domain at the vegetal-contraction pole moves with the sperm aster and the bulk of the myoplasm toward the future posterior side of the embryo. These results show that after fertilization, ER first accumulates in the vegetal area from which repetitive calcium waves are known to originate (Speksnijder, J. E. 1992. Dev. Biol. 153:259-271). This ER domain subsequently colocalizes with the myoplasm to the presumptive primary muscle cell region.

Endogenous angiotensin II receptors in Xenopus oocytes and eggs

Angiotensin II (AII) induced strongly desensitizing oscillatory Cl- inward currents in both follicle-enclosed and collagenase-treated Xenopus oocytes. The AII response was abolished by EGTA and attenuated by pertussis toxin. Treatment of oocytes with collagenase transiently reduced both the ratio of oocytes responsive to AII and the amplitude of AII responses, followed by restoration to original levels in 3-4 days. The response to adrenaline, which is mediated by endogenous beta-adrenoceptors in follicle cells, however, was irreversibly abolished by collagenase treatment. These results suggest that endogenous current-mediating AII receptors in oocytes are coupled with phosphatidylinositol hydrolysis and localized in the oocyte or in a cellular structure distinct from that for endogenous beta-adrenoceptors. Progesterone-matured Xenopus eggs also responded to AII, and this AII-induced depolarization resembled the fertilization potential in the eggs, suggesting a possible role of AII receptors in processes of fertilization or growth of the eggs.

Polarity of the surface and cortex of the amphibian egg from fertilization to first cleavage

The anuran egg is polarized along its animal-vegetal axis and becomes bilaterally symmetrical before first cleavage. Functional sperm entry is regionally restricted to the animal hemisphere of the egg, and functional sperm entry does not occur after egg activation. This regional and functional restriction in sperm entry correlates with the presence of long, slender microvilli and with the presence of the filamentous component of the glycocalyx. After sperm fusion, the egg undergoes activation, including a depolarization of the membrane potential and exocytosis of granules in the cortex. Both of these activation responses are the result of a propagated increase in intracellular calcium. The egg's ability to undergo a propagated activation response develops after germinal vesicle breakdown and depends on the development of the cortical endoplasmic reticulum. Once activated, the radial symmetric egg acquires bilateral symmetry due to a rotation of the egg cortex relative to the inner cytoplasm. A transient array of parallel microtubules forms near the vegetal cortex and may be part of the motor driving the cortical rotation.

A localized zone of increased conductance progresses over the surface of the sea urchin egg during fertilization

Although activation of a sea urchin egg by sperm leads to three phases of membrane conductance increase in the egg, the mechanism by which the sperm causes these conductance changes is not known. We used the loose patch clamp technique to localize the conductance changes in voltage clamped eggs. A patch of the egg's membrane was isolated from the bath by pressing the loose patch clamp pipette against the egg surface. Sperm added to the bath attached to the surface of the egg in a region other than at the isolated membrane patch. During phase 1 of the activation current, no changes of the membrane conductance were detected. At the time of, and subsequent to the onset of phase 2, large currents recorded between the interior of the patch pipette and the bath were attributed to changes of the seal resistance between the surface of the egg and the pipette. A local change of membrane conductance was observed during phase 2 despite the changes of seal resistance. During phase 2, the large amplitude and short duration of the local membrane conductance increase relative to the membrane, conductance increase for the whole egg during phase 2 indicated that the conductance increase occurred over the entire surface of the egg, but not simultaneously. The time when the peak conductance for the membrane patch occurred, relative to the time of onset for phase 2 in the whole egg, depended on the distance, measured in a straight line, between the site of sperm attachment and the tip of the pipette. These data indicate that the localized conductance increase progressed over the surface of the egg from the site of sperm attachment to the opposite pole of the egg. It is proposed that the local conductance increase, the cortical reaction, and the change of seal resistance are all evoked by a common cytoplasmic message that progresses throughout the cytoplasm of the egg from the site of sperm attachment to the opposite pole of the egg.

The four dimensions of calcium signalling in Xenopus oocytes

This review focuses on the inositol phosphate/Ca2+ signalling pathway in Xenopus oocytes. The known characteristics of the individual elements of this cascade--from the membrane receptors to the intracellular Ca2+ stores--will be covered. Based on this knowledge, a simple model will then try to account for the behaviour of the newly recognized oscillations of free intracellular Ca2+ and propagated Ca2+ waves. Finally, some of the potential physiological functions of the inositol phosphate pathway will be summarized. Although there is no systematic attempt to contrast the findings in the oocyte to those in other cells, the readers of this journal will not fail to notice a high degree of similarity. Although this may seem unexciting at first, it suggests that the inositol phosphate signalling pathway may be strikingly conserved across species.

The effects of inositol trisphosphates and inositol tetrakisphosphate on Ca2+ release and Cl- current pattern in the Xenopus laevis oocyte

We report that Ins(1,3,4,5)P4 releases calcium from intracellular stores of intact Xenopus laevis oocytes, as indicated by two different techniques, Ca2(+)-sensitive microelectrodes and a fura-2 imaging system. Ins(1,3,4,5)P4 releases only 20% as much Ca2+ as the same amount of Ins(1,4,5)P3. This effect is not due to the conversion of the injected Ins(1,3,4,5)P4 to Ins(1,4,5)P3, which is known to release Ca2+, because the amount of [3H]Ins(1,3,4,5)P4 that is converted to Ins(1,4,5)P3 is extremely small, as determined using HPLC. Examination of the different current patterns induced by Ins(1,4,5)P3 and Ins(1,3,4,5)P4, when injected into voltage-clamped oocytes, provided further evidence that the Ins(1,3,4,5)P4 was not being converted back to Ins(1,4,5)P3. We investigated the effects of four compounds, three inositol trisphosphates (Ins(1,4,5)P3, Ins(2,4,5)P3, and Ins(1,3,4)P3), and Ins(1,3,4,5)P4, on Cl- current conductance in order to examine (1) the possible role of Ins(1,3,4,5)P4 in cell activation and (2) the relationships between intracellular Ca2+ and the activation of Cl- currents. Immature stage VI Xenopus laevis oocytes were voltage-clamped and injected with Ins(1,4,5)P3, Ins(2,4,5)P3, and Ins(1,3,4)P3. Ins(1,4,5)P3 and Ins(2,4,5)P3 triggered Ca2(+)-dependent Cl- currents, but Ins(1,3,4)P3 did not trigger currents nor did it release intracellular Ca2+. Ins(2,4,5)P3 was fourfold less effective at inducing the immediate Cl- current pulse than Ins(1,4,5)P3. The Cl- current pattern was quite dependent on the amount of Ins(1,4,5)P3 injected into the oocyte. Low amounts of Ins(1,4,5)P3 triggered only an immediate single Cl- current pulse, whereas large amounts triggered the immediate single pulse, followed by a quiescent period, followed by oscillating Cl- currents. In contrast to the response of Ins(1,4,5)P3, injection of Ins(1,3,4,5)P4 triggered only oscillating Cl- currents whose magnitude, but not pattern, was dependent on the amount injected into the cell. The currents generated by Ins(1,3,4,5)P4 resemble the oscillating Cl- currents triggered by large amounts of Ins(1,4,5)P3 and Ins(2,4,5)P3. Ins(1,3,4,5)P4, unlike Ins(1,4,5)P3 and Ins(2,4,5)P3, rarely caused an immediate Cl- current pulse, but caused an immediate release of calcium. Therefore, we suggest that the oscillating currents are only indirectly dependent on calcium. These [Ca2+]i and conductance measurements suggest that both Ins(1,4,5)P3 and Ins(1,3,4,5)P4 have roles in intracellular Ca2+ regulation.

Changes in intracellular free calcium activity in Xenopus eggs following imposed intracellular pH changes using weak acids and weak bases

The aim of this work was to determine the potential relationships between rises in intracellular pH (pHi) and intracellular free calcium activity (Cai2+) during cell activation in Xenopus eggs. To do this, we used two weak bases, NH4Cl and procaine, and a weak acid, CO2, and measured Cai2+ variations in response to these imposed pHi variations. NH4Cl and procaine increased Cai2+ in both unactivated and activated eggs. Procaine was found to alkalinize the egg cytoplasm, whereas the other weak base, NH4Cl, acidified the egg cytoplasm. On the other hand, CO2 was found to acidify the cytoplasm and to substantially decrease Cai2+, also in unactivated and activated eggs. In addition, CO2 triggered an increase in the conductance of the plasma membrane to Cl- ions, similarly to what had been found previously with weak bases (Charbonneau, M. (1989) Cell Differ. Develop. 26, 39-52). These Cl- channels, similarly to the sperm-triggered Cl- channels during the fertilization potential, are supposed to be Ca2(+)-sensitive. Therefore, the changes in Ca2+ observed in response to CO2 do not seem to be responsible for the opening of these Cl- channels, which would rather be triggered by an increase in Cai2+ localized near the plasma membrane. We conclude therefore that weak acids and bases represent appropriate tools for studying cytosolic Ca2+ homeostasis, but not for dissecting the complex pathways involved in signal transduction.

Angiotensin II-induced calcium mobilization in oocytes by signal transfer through gap junctions

Angiotensin II (AII) stimulates rapid increases in the concentration of cytosolic calcium in follicular oocytes from Xenopus laevis. This calcium response was not present in denuded oocytes, indicating that it is mediated by AII receptors on the adherent follicular cells. The endogenous AII receptors differed in their binding properties from mammalian AII receptors expressed on the oocyte surface after injection of rat adrenal messenger RNA. Also, the calcium responses to activation of the amphibian AII receptor, but not the expressed mammalian AII receptor, were blocked reversibly by octanol and intracellular acidification, treatments that inhibit cell coupling through gap junctions. In addition, AII increased the rate of progesterone-induced maturation. Thus, an AII-induced calcium-mobilizing signal is transferred from follicle cells to the oocyte through gap junctions and may play a physiological role in oocyte maturation.

Antispectrin antibodies stain the oocyte nucleus and the site of fertilization channels in the egg of Discoglossus pictus (Anura)

In Discoglossus pictus eggs, only the dimple contains ionic channels active at fertilization; in particular, chloride channels are found in the central portion of the dimple, which is also the site of sperm penetration. Moreover the dimple hosts an imposing cytoskeleton, consisting of a cortical network and bundles of microfilaments extending from the microvilli. Since spectrin cross links actin and is connected through ankyrin to anion transporters in the plasma membrane of erythrocytes as well as to anion channels in other cells, we studied, in D. pictus egg, the relationship between the localization of spectrin and the high polarization of ionic channels and cytoskeletal organization. By means of immunocytochemistry, we localized spectrin exclusively in the egg dimple. In an attempt to trace back the source of spectrin localization, we immunostained sections of D. pictus ovary and localized spectrin in the nuclei of previtellogenic oocytes, where actin is also present. Antispectrin staining remained until germinal vesicle breakdown. By contrast, a cortical localization was found only when the oocytes divided into two hemispheres and into the germinative area (GA), which, after germinal vesicle breakdown, gives rise to the dimple. At this stage the antispectrin signal was particularly strong in the GA. Using Rho-pialloidin, we also established that spectrin is generally present where F-actin is found. However, spectrin and F-actin do not have the same pattern of fluorescence. In conclusion, our data suggest that spectrin may play a role in oocyte and egg polarity. In eggs, it could be instrumental in anchoring to the cytoskeleton membrane proteins such as receptors and ionic channels, including chloride-permeable channels.

Two calcium-activated chloride conductances in Xenopus laevis oocytes permeabilized with the ionophore A23187

1. Currents evoked by elevated intracellular free Ca2+ in Xenopus laevis oocytes were studied using the two-electrode voltage clamp technique. The elevation in Ca2+ concentration was achieved in three ways: by the use of the divalent cation ionophore A23187; by application of Ca2+-mobilizing neurotransmitters serotonin and acetylcholine (ACh); by the entry of Ca2+ through voltage-dependent channels. 2. In most experiments, the membrane was permeabilized to Ca2+ by a 15 min pretreatment with A23187 in a Ca2+-free solution. Exposure of the ionophore-treated oocytes to external Ca2+ elicited an inward current (at holding potentials of -40 to -60 mV). At external Ca2+ concentrations ([Ca2+]) between 0.1 and 1 mM, the current had a time-to-peak of at least 10 s, and slowly decayed over tens of seconds. At [Ca2+] greater than 2 mM, the inward current had two distinct kinetic components, a fast and transient one (Ifast) and a slow one (Islow). 3. The main carrier of the Ca2+-evoked inward current was Cl-. Several data indicate the existence of a tetraethylammonium (TEA)-sensitive K+ conductance. No evidence for a Na+ current was found. 4. The two components of the Ca2+-evoked inward current in ionophore-permeabilized oocytes, and the two components of the current evoked by ACh and serotonin (the latter in oocytes injected with rat brain RNA but untreated with A23187), were blocked by intracellular injection of the Ca2+ chelator, ethyleneglycolbis-(beta-aminoethyl ether)-N,N,N'N'-tetraacetic acid (EGTA). The two components of these currents displayed different sensitivity to Ca2+ buffering; higher doses of EGTA were necessary to inhibit the slow component than the fast one. 5. One to two minutes of treatment with 2 mM-9-anthracene carboxylic acid (9-AC) fully blocked Ca2+-dependent Cl- current evoked by Ca2+ influx through voltage- dependent Ca2+ channels in intact (untreated with A23187) oocytes. In ionophore-treated oocytes, block of Ifast was observed at holding potentials at which the current was outward (i.e. due to Cl- influx); Islow was inhibited only partially. The block of Ca2+-evoked Cl- efflux by 9-AC developed much more slowly and was less potent. to explain these results, the existence of two sites of 9-AC action is proposed. 6. Exposure of the ionophore-permeabilized oocytes to 0.1-0.2 mM [Ca2+] strongly reduced the response to higher concentrations of Ca2+. Ifast displayed stronger Ca2+-dependent inactivation than Islow.(ABSTRACT TRUNCATED AT 400 WORDS)

Fertilization in Discoglossus pictus (Anura). I. Sperm-egg interactions in distinct regions of the dimple and occurrence of a late stage of sperm penetration

The heterogeneity of the egg surface with respect to receptivity to sperm was investigated in Discoglossus pictus; in this species fertilization occurs only in an indentation called the dimple, at the center of the animal hemisphere. Following insemination sperm are seen in the outermost jelly layers and in the lens-shaped jelly plug, converging to the dimple center, D1. A fertilization potential (FP) is recorded 30 sec following insemination. About 30 min after fertilization, when fertilization cones can be detected easily, immotile sperm are found at the center of the cone, where 10 min later they accomplish penetration. After 15 min the cone regresses and the second polar body is extruded. In eggs where the plug was experimentally displaced with respect to the dimple, spermatozoa contacted the sides of the dimple and simple protrusions formed but not cones. Spermatozoa do not elicit a normal FP in these regions but small step depolarizations which may be followed by a gradual rise to a positive plateau potential. Such eggs do not develop. In the protrusions, sperm may be only partially incorporated and the unpenetrated portion appears to degenerate. We conclude that at least two regions exist in the dimple: D1, where the FP is triggered, cones are formed, sperm penetration is fully accomplished and development is initiated; and D2 + D3 where the electrical response is not a normal FP, cones do not form, total sperm penetration does not occur, and development is not initiated.

The cortical reaction in the egg of Discoglossus pictus: a study of the changes in the endoplasmic reticulum at activation

In Discoglossus pictus previous ultrastructural observations have shown that at the animal dimple, where sperm fuse with the egg, cortical granules (CG), vacuoles, and tightly packed clusters of small cisternae are present. At fertilization the clusters open (i.e., become loose) and give rise to longer cisternae arranged in whorls and chains which migrate toward the plasma membrane. The vacuoles fuse to form cisternae and exocytose along with the CG. In the rest of the egg periphery, while exocytosis occurs, the clusters do not open as a result of activation (C. Campanella, R. Talevi, U. Atripaldi, and L. Quaglia (1986). In "Molecular and Cellular Biology of Fertilization" (J.L. Hedrick, Ed.). Plenum, New York). We have recently conducted electrophysiological studies which have detected inward currents at the dimple center, outward current at the rest of the egg surface, and an eightfold increase in [Ca2+]i which propagates from the site of activation throughout the egg (R. Nuccitelli, D. Kline, W. Busa, R. Talevi, and C. Campanella (1988). Dev. Biol. 130, 120-132). In this paper we have asked whether the anionic current and the Ca2+ increase could be causally related to the changes of the smooth endoplasmic reticulum (SER) at activation. The results obtained by activating the eggs in ion-substituted Ringers indicate that (1) the migration of cisternae is not dependent on the polarity of the activation current crossing the dimple, but is strongly impaired, together with CG exocytosis, by 5 x Cl- Ringer; (2) TMB-8, a drug which partially blocks calcium release (C. Y. Choiu and M. J. Malagodi (1975). Brit. J. Pharmacol. 53, 279-288), partially inhibits opening of cisternae clusters and the formation of an SER network in the dimple. This suggests a causal relationship between the Ca2+ rise and the cluster transformation at activation.

A highly localized activation current yet widespread intracellular calcium increase in the egg of the frog, Discoglossus pictus

Sperm entry in the egg of the painted frog, Discoglossus pictus, occurs only at a specialized region of the animal hemisphere called the animal dimple, a structure not found in other species of frog. An extracellular vibrating electrode was used to measure the activation current to determine if the ion channels that open to generate the fertilization potential are localized in this region. Eggs that were activated by microinjecting inositol-1,4,5-trisphosphate (Ins(1,4,5)P3) exhibited activation potentials very similar to those of fertilized eggs. There was a delay between the time of Ins(1,4,5)P3 injection and the initiation of the activation potential that was proportional to the distance between the site of the activating stimulus and the animal dimple, similar to the delay previously observed in prick-activated eggs (R. Talevi, B. Dale, and C. Campanella (1985). Dev. Biol. 111, 316-323). The delay lasted 30 sec when the stimulus site was 20 degrees (300 micron) from the animal dimple and 14 min when it was 150 degrees C from the dimple. Once the activation potential was initiated, there was an excellent temporal correlation between the time of depolarization and the time of the first detectable current entering the dimple region. This inward current was typically 60 microA/cm2 in amplitude and was found only in the central 200 micron of the dimple region. The outward current was distributed over the remainder of the egg surface and was much smaller in amplitude. The activation current was carried by Cl- efflux in the animal dimple region, and was reduced by DIDS and reversed by high external Cl- or I-. The occurrence of inward current only at the dimple region indicates that Cl- channels which open to produce the activation potential are localized there. Using Ca2+-specific microelectrodes, we found that [Ca2+]i increased from 0.25 to 2 microM following both fertilization and activation and returned to the unactivated level after about 37 min. Immature oocytes of D. pictus were also studied with the vibrating probe and the inward current in these cells was much less localized than that in the activating egg. A steady transcellular current of up to 4 microA/cm2 entered the entire animal hemisphere of the oocyte and exited the vegetal hemisphere.

Ionic currents in morphogenesis

Morphogenetic fields must be generated by mechanisms based on known physical forces which include gravitational forces, mechanical forces, electrical forces, or some combination of these. While it is unrealistic to expect a single force, such as a voltage gradient, to be the sole cause of a morphogenetic event, spatial and temporal information about the electrical fields and ion concentration gradients in and around a cell or embryo undergoing morphogenesis can take us one step further toward understanding the entire morphogenetic mechanism. This is especially true because one of the handful of identified morphogens is Ca2+, an ion that will not only generate a current as it moves, but which is known to directly influence the plasma membrane's permeability to other ions, leading to other transcellular currents. It would be expected that movements of this morphogen across the plasma membrane might generate ionic currents and gradients of both electrical potential and intracellular concentration. Such ionic currents have been found to be integral components of the morphogenetic mechanism in some cases and only secondary components in other cases. My goal in this review is to discuss examples of both of these levels of involvement that have resulted from investigations conducted during the past several years, and to point to areas that are ripe for future investigation. This will include the history and theory of ionic current measurements, and a discussion of examples in both plant and animal systems in which ionic currents and intracellular concentration gradients are integral components of morphogenesis as well as cases in which they play only a secondary role. By far the strongest cases for a direct role of ionic currents in morphogenesis is the polarizing fucoid egg where the current is carried in part by Ca2+ and generates an intracellular concentration gradient of this ion that orients the outgrowth, and the insect follicle in which an intracellular voltage gradient is responsible for the polarized transport from nurse cell to oocyte. However, in most of the systems studied, the experiments to determine if the observed ionic currents are directly involved in the morphogenetic mechanism are yet to be done. Our experience with the fucoid egg and the fungal hypha of Achlya suggest that it is the change in the intracellular ion concentration resulting from the ionic current that is critical for morphogenesis.