Some properties of the membrane currents underlying the fertilization potential in sea urchin eggs

Sources, distribution and effects of rare earth elements in the marine environment: Current knowledge and research gaps

Rare earth elements and yttrium (REY) are critical elements for a wide range of applications and consumer products. Their growing extraction and use can potentially lead to REY and anthropogenic-REY chemical complexes (ACC-REY) being released in the marine environment, causing concern regarding their potential effects on organisms and ecosystems. Here, we critically review the scientific knowledge on REY sources (geogenic and anthropogenic), factors affecting REY distribution and transfer in the marine environment, as well as accumulation in- and effects on marine biota. Further, we aim to draw the attention to research gaps that warrant further scientific attention to assess the potential risk posed by anthropogenic REY release. Geochemical processes affecting REY mobilisation from natural sources and factors affecting their distribution and transfer across marine compartments are well established, featuring a high variability dependent on local conditions. There is, however, a research gap with respect to evaluating the environmental distribution and fate of REY from anthropogenic sources, particularly regarding ACC-REY, which can have a high persistence in seawater. In addition, data on organismal uptake, accumulation, organ distribution and effects are scarce and at best fragmentary. Particularly, the effects of ACC-REY at organismal and community levels are, so far, not sufficiently studied. To assess the potential risks caused by anthropogenic REY release there is an urgent need to i) harmonise data reporting to promote comparability across studies and environmental matrices, ii) conduct research on transport, fate and behaviour of ACC-REY vs geogenic REY iii) deepen the knowledge on bioavailability, accumulation and effects of ACC-REY and REY mixtures at organismal and community level, which is essential for risk assessment of anthropogenic REY in marine ecosystems.

Starfish oocytes of A. pectinifera reveal marked differences in sperm-induced electrical and intracellular calcium changes during oocyte maturation and at fertilization

Although changes in membrane potential and intracellular Ca²⁺ (Ca_i ) during fertilization in starfish oocytes have been known for long time, little is known precisely about how and what kind of channels are involved during oocyte maturation and in fertilization, and how the mechanisms of changes in Ca_i in oocytes develop during oocyte maturation. Since in starfish, oocyte maturation-inducing hormone, 1-methyladenine (1MA) is well known, we took advantage of it to investigate the developmental process of channel-function and changes in Ca_i in three different developmental stages using 1MA. Sperm-induced membrane current at voltage clamp and changes in Ca_i in starfish oocytes, Asterina pectinifera, were examined in stages of immature, partly mature (a state in 15-20 min after sufficient concentration, 1 µM of 1MA addition, or 30-40 min exposure to subthreshold concentration of 1MA), and mature oocytes (MO). We found some immature and many partly MOs showed fluctuating responses in membrane current, membrane potential, and corresponding changes in Ca_i , which are distinct from those in MOs. The responses in immature and partly MOs indicate physiologically characteristic responses of insufficient changes in Ca_i and its corresponding electrical responses at the partial developmental stage during maturation. Our data should shed light on the mechanism of egg activation and oocyte maturation in terms of examining membrane current and corresponding changes in Ca_i .

Gadolinium perturbs expression of skeletogenic genes, calcium uptake and larval development in phylogenetically distant sea urchin species

Chelates of Gadolinium (Gd), a lanthanide metal, are employed as contrast agents for magnetic resonance imaging and are released into the aquatic environment where they are an emerging contaminant. We studied the effects of environmentally relevant Gd concentrations on the development of two phylogenetically and geographically distant sea urchin species: the Mediterranean Paracentrotus lividus and the Australian Heliocidaris tuberculata. We found a general delay of embryo development at 24h post-fertilization, and a strong inhibition of skeleton growth at 48h. Total Gd and Ca content in the larvae showed a time- and concentration-dependent increase in Gd, in parallel with a reduction in Ca. To investigate the impact of Gd on the expression of genes involved in the regulation of skeletogenesis, we performed comparative RT-PCR analysis and found a misregulation of several genes involved in the skeletogenic and left-right axis specification gene regulatory networks. Species-specific differences in the biomineralization response were evident, likely due to differences in the skeletal framework of the larvae and the amount of biomineral produced. Our results highlight the hazard of Gd for marine organisms.

Effects of exposure to gadolinium on the development of geographically and phylogenetically distant sea urchins species

Gadolinium (Gd), a metal of the lanthanide series used as contrast agent for magnetic resonance imaging, is released into the aquatic environment. We investigated the effects of Gd on the development of four sea urchin species: two from Europe, Paracentrotus lividus and Arbacia lixula, and two from Australia, Heliocidaris tuberculata and Centrostephanus rodgersii. Exposure to Gd from fertilization resulted in inhibition or alteration of skeleton growth in the plutei. The similar morphological response to Gd in the four species indicates a similar mechanism underlying abnormal skeletogenesis. Sensitivity to Gd greatly varied, with the EC50 ranging from 56 nM to 132 μM across the four species. These different sensitivities highlight the importance of testing toxicity in several species for risk assessment. The strong negative effects of Gd on calcification in plutei, together with the plethora of marine species that have calcifying larvae, indicates that Gd pollution is urgent issue that needs to be addressed.

PLCγ, G-protein of the Gαq type and cADPr pathway are associated to trigger the fertilization Ca2+ signal in the sea urchin egg

In all species, fertilization triggers in the egg a rapid and transient increase of intracellular free calcium (Cai), but how this signal is generated following sperm and egg interaction has not been clearly characterised yet. In sea urchin, a signalling pathway involving tyrosine kinase and PLCγ has been proposed to be at the origin of the fertilization Cai signal. We report here that injection of src homology-2 (SH2) domains of the sea urchin PLCγ inhibits in a competitive manner the endogenous PLCγ, alters both the amplitude and duration of the fertilization Cai wave, but does not abrogate it. Our results suggest that PLCγ acts in conjunction with a cADPr pathway and G-proteins of the Gαq type to trigger the fertilization Cai wave, and reinforce a crucial role for PLCγ at mitosis and cytokinesis.

Differential expression of membrane conductances underlies spontaneous event initiation by rostral midline neurons in the embryonic mouse hindbrain

Spontaneous activity is expressed in many developing CNS structures and is crucial in correct network development. Previous work using [Ca(2+)](i) imaging showed that in the embryonic mouse hindbrain spontaneous activity is initiated by a driver population, the serotonergic neurons of the nascent raphe. Serotonergic neurons derived from former rhombomere 2 drive 90% of all hindbrain events at E11.5. We now demonstrate that the electrical correlate of individual events is a spontaneous depolarization, which originates at the rostral midline and drives events laterally. Midline events have both a rapid spike and a large plateau component, while events in lateral tissue comprise only a smaller amplitude plateau. Lateral cells have a large resting conductance and are highly coupled via neurobiotin-permeant gap junctions, while midline cells are significantly less gap junction-coupled and uniquely express a T-type Ca(2+) channel. We propose that the combination of low resting conductance and expression of T-type Ca(2+) current is permissive for midline neurons to acquire the initiator or driver phenotype, while cells without these features cannot drive activity. This demonstrates that expression of specific conductances contributes to the ability to drive spontaneous activity in a developing network.

Calcium at fertilization and in early development

Fertilization calcium waves are introduced, and the evidence from which we can infer general mechanisms of these waves is presented. The two main classes of hypotheses put forward to explain the generation of the fertilization calcium wave are set out, and it is concluded that initiation of the fertilization calcium wave can be most generally explained in invertebrates by a mechanism in which an activating substance enters the egg from the sperm on sperm-egg fusion, activating the egg by stimulating phospholipase C activation through a src family kinase pathway and in mammals by the diffusion of a sperm-specific phospholipase C from sperm to egg on sperm-egg fusion. The fertilization calcium wave is then set into the context of cell cycle control, and the mechanism of repetitive calcium spiking in mammalian eggs is investigated. Evidence that calcium signals control cell division in early embryos is reviewed, and it is concluded that calcium signals are essential at all three stages of cell division in early embryos. Evidence that phosphoinositide signaling pathways control the resumption of meiosis during oocyte maturation is considered. It is concluded on balance that the evidence points to a need for phosphoinositide/calcium signaling during resumption of meiosis. Changes to the calcium signaling machinery occur during meiosis to enable the production of a calcium wave in the mature oocyte when it is fertilized; evidence that the shape and structure of the endoplasmic reticulum alters dynamically during maturation and after fertilization is reviewed, and the link between ER dynamics and the cytoskeleton is discussed. There is evidence that calcium signaling plays a key part in the development of patterning in early embryos. Morphogenesis in ascidian, frog, and zebrafish embryos is briefly described to provide the developmental context in which calcium signals act. Intracellular calcium waves that may play a role in axis formation in ascidian are discussed. Evidence that the Wingless/calcium signaling pathway is a strong ventralizing signal in Xenopus, mediated by phosphoinositide signaling, is adumbrated. The central role that calcium channels play in morphogenetic movements during gastrulation and in ectodermal and mesodermal gene expression during late gastrulation is demonstrated. Experiments in zebrafish provide a strong indication that calcium signals are essential for pattern formation and organogenesis.

Differential contribution of cytoplasmic Ca2+ and Ca2+ influx to gamete fusion and egg activation in maize

In multicellular organisms, gamete fusion triggers a set of events, collectively known as egg activation, that leads to the development of a new individual. Every species that has been studied shows at least one rise in cytoplasmic Ca2+ concentration ([Ca2+]Cyt) after gamete fusion which is believed to be involved in activation. Yet the source and regulation of this Ca2+ signal and the way it is transduced inside the zygote are controversial. In higher plants, in vitro fertilization (IVF) has enabled the description of a rise in [Ca2+]Cyt (ref. 4) that is sufficient for activation, and of a Ca2+ influx that spreads as a wavefront from the fusion site The relationship between these two responses is unknown. Using a new combination of methods that simultaneously monitor the extracellular flux with a Ca2+-vibrating probe, and [Ca2+]Cyt by widefield imaging, we directly determined that the Ca2+ influx precedes the [Ca2+]Cyt elevation by 40-120 s. In addition, results from experiments using the Ca2+-channel inhibitor gadolinium (Gd3+) suggest that the Ca2+ influx may be necessary for sperm incorporation. We also present evidence for a putative sperm-dependent Gd3+-insensitive localized Ca2+ influx confined to the fusion point.

Calcium influx mediates the voltage-dependence of sperm entry into sea urchin eggs

Sperm entry was monitored in voltage-clamped sea urchin eggs following insemination in a variety of artificial seawaters. In regular seawater, maintaining the membrane potential at increasingly negative values progressively inhibits sperm entry. Reducing [Ca(2+)](o) relieves the inhibition, shifting the sperm entry vs voltage relationship toward more negative potentials. Raising [Ca(2+)](o) shifts the relationship in the other direction. Large changes in [Na(+)](o) or [Mg(2+)](o) do not affect sperm entry although changing [Na(+)](o) dramatically changes the currents following sperm attachment. Applying one of seven different calcium channel blockers or replacing Ca(2+) with Ba(2+) or Sr(2+) or microinjecting calcium chelators into the cytoplasm relieves the block to sperm entry at negative potentials. We conclude that the block to sperm entry at negative potentials is mediated by calcium which crosses the membrane and acts at an intracellular site.

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.

Dieldrin modifies the hydrolysis of PIP2 and decreases the fertilization rate in Bufo arenarum oocytes

Carbachol treatment in Bufo arenarum oocytes decreases the radioactivity in [32P]PIP2 in the following 20 min after stimulation and increases the [3H]glycerol labeling of 1,2-DAG at 1 min of stimulation. On the contrary, in Dieldrin treated oocytes carbachol stimulation produces an increase in [32P]PIP2 labeling without changes in [3H]1,2-DAG radioactivity. The sustained hydrolysis of PIP2 observed in Control oocytes is necessary to generate the intracellular second messengers which initiate the fertilization pathway. The lack of response to muscarinic stimulation in Dieldrin treated oocytes, may be associated with an early activation of PIP2-PLC by the insecticide, producing a depletion of the PIP2 pool previous to the stimulation with carbachol. These changes take place simultaneously with a decrease in the ability of Bufo arenarum oocytes to be fertilized in vitro, suggesting a correlation between impairment in the PIP2 cascade and a decrease in the fertilization rate.

Is buffering capacity the principle role of the jelly coat in Bufo arenarum fertilization?

1. Dejellied Bufo arenarum oocytes can be fertilized in Ringer-Phosphate buffer with the same efficiency as jellied (control) oocytes. 2. Ringer-Phosphate buffer at pH = 7.4 not only provides buffering capacity but also the delta pH necessary for the acrosomal reaction. 3. The use of Ringer-TRIS buffer at pH = 7.4 does not render as good as Ringer-Phosphate buffer results, in terms of fertilization percentages. 4. Insemination of oocytes in Ringer-TRIS buffer interferes with early development.

Thimerosal reveals calcium-induced calcium release in unfertilised sea urchin eggs

The fertilisation calcium wave in sea urchin eggs triggers the onset of development. The wave is an explosive increase in intracellular free calcium concentration ([Ca2+i]) that begins at the point of sperm entry and crosses the egg in about 20 s. Thimerosal is a sulphydryl reagent that sensitises calcium release from intracellular stores in a variety of cell types. Treatment of unfertilised eggs with thimerosal causes a slow increase [Ca2+i] that results eventually in a large, spontaneous calcium transient and egg activation. At shorter times after thimerosal treatment, egg activation and the calcium transient can be triggered by calcium influx through voltage-gated calcium channels, a form of calcium-induced/calcium release (CICR). Thimerosal treatment also reduces the latency of the fertilisation calcium response and increases the velocity of the fertilisation wave. These results indicate that thimerosal can unmask CICR in sea urchin eggs and suggest that the ryanodine receptor channel based CICR may contribute to explosive calcium release during the fertilisation wave.

The fertilization potential and associated membrane potential oscillations during the resumption of meiosis in the egg of the ascidian Phallusia mammillata

The fertilization potential in Phallusia mammillata consisted of an initial rapid depolarization. This initial sperm-triggered depolarization was followed by a phase of membrane depolarization which was of either long or short duration, depending on the eggs. When of long duration, the phase of membrane depolarization was divided into two periods: the first one began with a plateau (Em = +20.2 +/- 1.1 mV; duration = 1.7 +/- 0.14 min) which was followed by a series of membrane potential oscillations (n = 3.1 +/- 0.25) lasting 2.4 +/- 0.2 min. The second period also began as a plateau (Em = approximately 0 mV; duration = 3.40 +/- 0.20 min) which was followed by a series of oscillations (n = 11.5 +/- 0.5) lasting 11.8 +/- 0.6 min, followed by a membrane repolarization. The second series of oscillations often continued rising from the resting potential value. In the eggs displaying a short duration of membrane depolarization, the second period of depolarization was shortened (lasting only 3.5 +/- 0.5 min) since it lacked the second plateau. In addition it displayed a smaller number of oscillations (n = 4.7 +/- 0.6). As a consequence of this shortening, the membrane repolarized sooner. After repolarization, the membrane displayed several potential oscillations that started from the repolarization level. Regardless of the length of the depolarized plateau phases, the total number of membrane oscillations and the time period during which they occurred were constant. Eggs displaying a long depolarization phase had 15.9 +/- 0.6 oscillations in a 19.5 +/- 0.6 min interval, while eggs having a short depolarization phase had 16.0 +/- 0.8 oscillations in a 18.1 +/- 0.3 min interval. The time period during which the potential oscillations occurred corresponded remarkably well with the time of the meiotic divisions: the formation of the first polar body was detected about 80 sec after the end of the first series of oscillations; the second polar body was extruded about 85 sec after the last membrane oscillation occurred.

Sperm-induced currents at fertilization in sea urchin eggs injected with EGTA and neomycin

Membrane currents were measured in single voltage-clamped sea urchin eggs (Lytechinus pictus and Lytechinus variegatus) that were injected with either EGTA or neomycin and inseminated. Although egg activation and the fertilization calcium wave were prevented by injection of either of these compounds, sperm attached and still elicited inward currents. Sperm-induced currents in EGTA-injected eggs had an abrupt onset, quickly reached a maximum, and then slowly declined in amplitude. Sperm incorporation occurred readily in EGTA-injected eggs. Similar results were obtained with another calcium chelator, BAPTA. In neomycin-injected eggs, sperm-induced currents generally had an abrupt onset and, in contrast to EGTA-injected eggs, the currents usually cut off rapidly. Sperm failed to enter the neomycin-injected eggs and the duration of sperm-induced currents in neomycin-injected eggs was markedly dependent upon the voltage-clamp holding potential, with shorter duration currents occurring at -70 than at -20 mV. The lability of the initial interaction between sperm and egg at negative holding potentials may explain why activation often fails when the egg membrane is voltage clamped at these potentials (Lynn et al., Dev. Biol. 128, 305-323, 1988).

Multiple intracellular signals coordinate structural dynamics in the sea urchin egg cortex at fertilization

Fertilization of the sea urchin egg is accompanied by a sequence of structural changes in the egg cortex that include exocytosis, endocytosis, and microvillar growth. This architectural reorganization is coordinated by two intracellular signals: a rapid, transient rise in cytosolic free calcium and a slower, longer lasting increase in cytoplasmic pH. In this report we provide ultrastructural views of these events in quick-frozen eggs and discuss their relationship to the calcium and pH signals.

Guanosine 5'-thiotriphosphate may stimulate phosphoinositide messenger production in sea urchin eggs by a different route than the fertilizing sperm

We show that microinjecting guanosine-5'-thiotriphosphate (GTP gamma S) into unfertilized sea urchin eggs generates an intracellular free calcium concentration [( Ca]i) transient apparently identical in magnitude and duration to the calcium transient that activates the egg at fertilization. The GTP gamma S-induced transient is blocked by prior microinjection of the inositol trisphosphate (InsP3) antagonist heparin. GTP gamma S injection also causes stimulation of the egg's Na+/H+ antiporter via protein kinase C, even in the absence of a [Ca]i increase. These data suggest that GTP gamma S acts by stimulating the calcium-independent production of the phosphoinositide messengers InsP3 and diacylglycerol (DAG). However, the fertilization [Ca]i transient is not affected by heparin, nor can the sperm cause calcium-independent stimulation of protein kinase C. It seems that the bulk of InsP3 and DAG production at fertilization is triggered by the [Ca]i transient, not by the sperm itself. GDP beta S, a G-protein antagonist, does not affect the fertilization [Ca]i transient. Our findings do not support the idea that signal transduction at fertilization operates via a G-protein linked directly to a plasma membrane sperm receptor.

Membrane conductance patterns during fertilization are sperm dependent in two sea urchin species

The influence of the egg and sperm on the conductance changes at fertilization in the sea urchin were investigated through cross-fertilization of two Hawaiian species, Tripneustes gratilla and Pseudoboletia indiana. The current-voltage (I-V) relation, measured in voltage-clamped eggs at intervals over the period 2-16 min following the rise to a positive membrane potential that signals sperm attachment, differs significantly in the two species. The magnitude of the conductance change depends on the species of the fertilizing sperm in both homologous and heterologous crosses. This supports the hypothesis that currents during this period arise from sperm membrane channels incorporated into the egg at sperm-egg fusion. Measurements of conductance during the first 90 sec, which includes the period of the major inward current correlated with cortical granule breakdown and elevation of the fertilization envelope, showed that the magnitude and timing of the maximum current also differed in the two species. This conductance change presumably involves an activation of egg membrane channels initiated by the sperm and would be expected to be characteristic of the egg species. However, in cross-fertilized eggs the magnitude and timing of the conductance change over this period also depends on the species of the sperm with little identifiable egg contribution, indicating that the fertilizing sperm can modulate the egg response to influence these events.

Voltage clamp studies of fertilization in sea urchin eggs. II. Current patterns in relation to sperm entry, nonentry, and activation

Following attachment of a sperm to the surface of a sea urchin egg clamped at a membrane potential (Vm) more positive than +17 mV, no changes in membrane conductance can be detected, the sperm does not enter egg, and no morphological changes can be detected. At Vm from +17 to -100 mV three characteristically different types of current profiles are observed: Type I are activation currents in eggs penetrated by a sperm. These have three phases, which occur in all eggs clamped at Vm from +17 to -20 mV and in decreasing percentages at clamped Vm more negative than -20 mV (to -75 mV). Complete fertilization envelopes are elevated, relatively large mound-shaped fertilization cones form, and the eggs develop to normal embryos. Type II are sperm transient currents in eggs not penetrated by a sperm, the eggs otherwise remaining in the unfertilized state. These transients are simpler and shorter than type I currents, and are observed only at clamped Vm more negative than -20 mV. Type III are modified activation currents in eggs not penetrated by a sperm. These have three phases, are observed only at clamped Vm more negative than -20 mV, and are the only type of activation current seen at clamped Vm more negative than -75 mV. Complete fertilization envelopes are elevated, the fertilization cones are small and filament-like, and the eggs fail to cleave. We conclude that (a) the sperm transient currents (type II) and phase 1 of the activation currents (type I and III) are similar events generated by a sperm-initiated localized conductance increase, (b) the abrupt decrease of current which terminates the sperm transients and phase 1 of type III currents results from a turnoff of the sperm-induced conductance increase and signals that the sperm will not enter the egg, and (c) the occurrence of phase 2 during an electrophysiological response induced by a sperm indicates that the egg is activating.