Different calcium-dependent pathways control fertilisation-triggered glycoside release and the cortical contraction in ascidian eggs

Obtaining gametes and embryos of ascidians

Ascidians are basal chordates that have become increasingly important for understanding chordate evolution. They comprise three orders. In the orders Phlebobranchia and Stolidobranchia, most species freely spawn eggs and sperm, whereas members of the order Aplousobranchia form colonies that brood their eggs and broadcast sperm. In the two free spawning orders, eggs and sperm are easily obtained for in vitro fertilizations. In the third order, slices of colonies yield gametes and embryos of all stages. Methods are described for obtaining gametes, performing fertilizations, and culturing embryos. Also included are methods for removing follicle cells and vitelline coats from oocytes.

Signaling pathways in ascidian oocyte maturation: the roles of cAMP/Epac, intracellular calcium levels, and calmodulin kinase in regulating GVBD

Most mature ascidian oocytes undergo germinal vesicle breakdown (GVBD) when released by the ovary into sea water (SW). Acidic SW blocks this but they can be stimulated by raising the pH, increasing intracellular cAMP levels by cell permeant forms, inhibiting its breakdown or causing synthesis. Boltenia villosa oocytes undergo GVBD in response to these drugs. However, the cAMP receptor protein kinase A (PKA) does not appear to be involved, as oocytes are not affected by the kinase inhibitor H-89. Also, the PKA independent Epac agonist 8CPT-2Me-cAMP stimulates GVBD in acidic SW. GVBD is inhibited in calcium free sea water (CaFSW). The intracellular calcium chelator BAPTA-AM blocks GVBD at 10 µM. GVBD is also inhibited when the ryanodine receptors (RYR) are blocked by tetracaine or ruthenium red but not by the IP(3) inhibitor D-609. However, dimethylbenzanthracene (DMBA), a protein kinase activator, stimulates GVBD in BAPTA, tetracaine or ruthenium red blocked oocytes. The calmodulin kinase inhibitor KN-93 blocks GVBD at 10 µM. This and preceding papers support the hypothesis that the maturation inducing substance (MIS) produced by the follicle cells in response to increased pH causes activation of a G protein which triggers cAMP synthesis. The cAMP then activates an Epac molecule, which causes an increase in intracellular calcium from the endoplasmic reticulum ryanodine receptor. The increased intracellular calcium subsequently activates calmodulin kinase, which causes an increase in cdc25 phosphatase activity, activating MPF and the progression of the oocyte into meiosis.

Calcium signalling in early embryos

The onset of development in most species studied is triggered by one of the largest and longest calcium transients known to us. It is the most studied and best understood aspect of the calcium signals that accompany and control development. Its properties and mechanisms demonstrate what embryos are capable of and thus how the less-understood calcium signals later in development may be generated. The downstream targets of the fertilization calcium signal have also been identified, providing some pointers to the probable targets of calcium signals further on in the process of development. In one species or another, the fertilization calcium signal involves all the known calcium-releasing second messengers and many of the known calcium-signalling mechanisms. These calcium signals also usually take the form of a propagating calcium wave or waves. Fertilization causes the cell cycle to resume, and therefore fertilization signals are cell-cycle signals. In some early embryonic cell cycles, calcium signals also control the progress through each cell cycle, controlling mitosis. Studies of these early embryonic calcium-signalling mechanisms provide a background to the calcium-signalling events discussed in the articles in this issue.

Calcium-responsive contractility during fertilization in sea urchin eggs

Fertilization triggers a reorganization of oocyte cytoskeleton, and in sea urchins, there is a dramatic increase in cortical F-actin. However, the role that myosin II plays during fertilization remains largely unexplored. Myosin II is localized to the cortical cytoskeleton both before and after fertilization and to examine myosin II contractility in living cells, Lytechinus pictus eggs were observed by time-lapse microscopy. Upon sperm binding, a cell surface deflection traversed the egg that was followed by and dependent on the calcium wave. The calcium-dependence of surface contractility could be reproduced in unfertilized eggs, where mobilization of intracellular calcium in unfertilized eggs under compression resulted in a marked contractile response. Lastly, inhibition of myosin II delayed absorption of the fertilization cone, suggesting that myosin II not only responds to the same signals that activate eggs but also participates in the remodeling of the cortical actomyosin cytoskeleton during the first zygotic cell cycle.

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.

Signaling pathways in ascidian oocyte maturation: Effects of various inhibitors and activators on germinal vesicle breakdown

The Ascidiacea, the invertebrate chordates, includes three orders; the Stolidobranchia is the most complex. Until the present study, the onset of oocyte maturation (germinal vesicle breakdown) had been investigated in only a single pyurid (Halocynthia roretzi), in which germinal vesicle breakdown (GVBD) begins when the oocyte contacts seawater (SW); nothing was known about internal events. This study strongly suggests the importance of protein phosphorylation in this process. Herdmania pallida (Pyuridae) functions like H. roretzi; GVBD occurs in SW. Oocytes of Cnemidocarpa irene (Styelidae) do not spontaneously undergo GVBD in SW but must be activated. Herdmania oocytes are inhibited from GVBD by pH 4 SW and subsequently activated by mastoparan (G-protein activator), A23187 (Ca2+ ionophore) or dimethylbenzanthracene (tyrosine kinase activator). This requires maturation promoting factor (MPF) activity; cyclin-dependent kinase inhibitors roscovitine and olomoucine are inhibitory. It also entails dephosphorylation as demonstrated by the ability of the phosphatase inhibitor vitamin K3 to inhibit GVBD. GVBD is also inhibited by the tyrosine kinase inhibitors tyrphostin A23 and genistein, and LY-294002, a phosphatidylinositol-3-kinase inhibitor previously shown to inhibit starfish GVBD. LY-294002 inhibits strongly when activation is by mastoparan or ionophore but not when activated by dimethylbenzanthracene (DMBA). The DMBA is hypothesized to phosphorylate a phosphatase directly or indirectly causing secondary activation, bypassing inhibition.

Polyspermy prevention in marine invertebrates

In marine invertebrates, as in most other organisms, normal development requires that only one sperm nucleus joins with the egg nucleus at fertilization. The principal mechanisms employed are (1) prevention of sperm-egg plasma membrane fusion and (2) modifications of the egg extracellular coat to prevent sperm binding and/or penetration. In a third strategy, fertilization is polyspermic, but only one sperm nucleus fuses with the egg nucleus. Other factors such as gamete density during spawning, chemotaxis, and localized sites for sperm entry may also affect the numbers of sperm reaching the egg.

Sperm surface proteases in ascidian fertilization

Ascidian eggs are surrounded by a noncellular layer and two cellular layers, which are penetrated by sperm. Three sperm surface proteases are essential for fertilization of eggs from the stolidobranch ascidian Halocynthia: spermosin, acrosin, and the proteasome. In the phlebobranch Ciona, a chymotrypsin-like protease and the proteasome are essential in fertilization. Sperm from the phlebobranch ascidians Phallusia mammillata, Ascidia (=Phallusia) nigra, and Ascidia columbiana, all express spermosin, acrosin, and the proteasomal chymotrypsin activities on their surfaces. Chymostatin blocks cleavage in phlebobranchs, but inhibitors of spermosin and acrosin only delay it by several minutes. Protease inhibitors have little effect upon sperm binding in Phallusia but strongly affect the rate of sperm passage through the vitelline coat. Peptide substrates and inhibitors to spermosin and acrosin cause a significant decline in the number of eggs undergoing pre-meiotic contractions at 3 min after fertilization. Thus while chymotrypsin activity is essential for penetration of the vitelline coat, spermosin and acrosin both function to increase the rate of fertilization. A crucial step in the divergence of the phlebobranchs and stolidobranchs may have been the conversion of spermosin and acrosin to essential proteases in the stolidobranchs, or, perhaps, their essential function was lost in the evolution of phlebobranchs. Aplousobranch ascidians are all colonial with very small zooids. Sperm from Aplidium californicum, Aplidium solidum (Polyclinidae), and Distaplia occidentalis (Holozoidae) have acrosin and chymotrypsin activities but lack spermosin activity. This enzyme is also missing from sperm of colonial phlebobranch and stolidobranch ascidians, suggesting that spermosin is not necessary for small zooids with internal fertilization.

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.

Activation of follicle cell surface phospholipase by tyrosine kinase dependent pathway is an essential event in Ascidian fertilization

Eggs of Ascidia ceratodes and Phallusia mammillata block polyspermy by releasing a phosphatidylinositol-linked glycosidase from the follicle cell and egg surface that binds to and blocks all unoccupied sperm binding sites on the vitelline coat. Release of this glycosidase is thought to be under the control of a membrane-bound phospholipase. To elucidate the mechanism of phospholipase activation, intact eggs and isolated follicle cells are activated by either sperm or the tyrosine kinase activator 9, 10-dimethyl-1,2-benzanthracene (DMBA). Both treatments caused release of comparable quantities of glycosidase activity, the earliest event following fertilization. A corresponding increase in phospholipase activity accompanied this glycosidase release. The tyrosine kinase inhibitor genistein blocked release by DMBA at concentrations as low as 1 microM, but had no effect on sperm-induced release even when used up to 100 microM. Tyrphostin A23, another tyrosine kinase inhibitor, when used at 200 microM blocked glycosidase release and decreased phospholipase activity following both DMBA activation and fertilization. Western blot analysis probing for phosphotyrosine content of disrupted intact eggs with their follicle cells revealed the absence of a band in tyrphostin-treated eggs corresponding to a 40 kDa protein that was present in both unfertilized and fertilized egg samples. Based on these results, we propose that phosphorylation of specific tyrosine residues is necessary for phospholipase activation and is sufficient to trigger subsequent glycosidase release.

Comparative biology of calcium signaling during fertilization and egg activation in animals

During animal fertilizations, each oocyte or egg must produce a proper intracellular calcium signal for development to proceed normally. As a supplement to recent synopses of fertilization-induced calcium responses in mammals, this paper reviews the spatiotemporal properties of calcium signaling during fertilization and egg activation in marine invertebrates and compares these patterns with what has been reported for other animals. Based on the current database, fertilization causes most oocytes or eggs to generate multiple wavelike calcium oscillations that arise at least in part from the release of internal calcium stores sensitive to inositol 1,4,5-trisphosphate (IP3). Such calcium waves are modulated by upstream pathways involving oolemmal receptors and/or soluble sperm factors and in turn regulate calcium-sensitive targets required for subsequent development. Both "protostome" animals (e.g., mollusks, annelids, and arthropods) and "deuterostomes" (e.g., echinoderms and chordates) display fertilization-induced calcium waves, IP3-mediated calcium signaling, and the ability to use a combination of external calcium influx and internal calcium release. Such findings fail to support the dichotomy in calcium signaling modes that had previously been proposed for protostomes vs deuterostomes and instead suggest that various features of fertilization-induced calcium signals are widely shared throughout the animal kingdom.

The passage of Ca2+ and fluorescent markers between the sperm and egg after fusion in the mouse

Mouse sperm-egg fusion was examined using two-photon and confocal microscopy. A delay of several minutes occurred between the first observable event of fusion (which was the diffusion of Ca2+-sensitive dyes from egg into sperm) and any change in egg cytoplasmic Ca2+. When indo-1 dextran was used to obtain ratiometric two-photon images, there was no detectable local increase in egg cytoplasmic Ca2+ near the site of sperm fusion. However, the sperm underwent a Ca2+ transient which appeared to be coincident with the egg cytoplasm Ca2+ transient, which suggested that there was a high permeability pathway for Ca2+ between egg and sperm. To exclude this pathway from providing trigger Ca2+ for the egg transient, we reduced bathing [Ca2+] to approx. 18 microM and 13nM (with EGTA). In these conditions the first egg Ca2+ transient was not prevented, which makes an obligatory role for extracellular Ca2+ in the initiation of the egg Ca2+ transient unlikely. Both FITC-albumin (70 kDa) and 10 kDa dextran-linked Ca2+ indicators were able to diffuse into the sperm from the egg. In addition, phycoerythrin (240 kDa) rapidly diffused into the sperm shortly after fusion (but before any changes in Ca2+ occurred). This suggests that the ‘pore(s)’ that form during sperm-egg fusion must be at least 8 nm in diameter. These data are compatible with the idea that a diffusible sperm protein could trigger the observed changes in intracellular Ca2+ in the egg, but do not exclude the possibility that other second messengers are generated during sperm-egg fusion.

Calcium signaling by cyclic ADP-ribose, NAADP, and inositol trisphosphate are involved in distinct functions in ascidian oocytes

ADP-ribosyl cyclase catalyzes the synthesis of two structurally and functionally different Ca2+ releasing molecules, cyclic ADP-ribose (cADPR) from beta-NAD and nicotinic acid-adenine dinucleotide phosphate (NAADP) from beta-NADP. Their Ca2+-mobilizing effects in ascidian oocytes were characterized in connection with that induced by inositol 1,4,5-trisphosphate (InsP3). Fertilization of the oocyte is accompanied by a decrease in the oocyte Ca2+ current and an increase in membrane capacitance due to the addition of membrane to the cell surface. Both of these electrical changes could be induced by perfusion, through a patch pipette, of nanomolar concentrations of cADPR or its precursor, beta-NAD, into unfertilized oocytes. The changes induced by beta-NAD showed a distinctive delay consistent with its enzymatic conversion to cADPR. The cADPR-induced changes were inhibited by preloading the oocytes with a Ca2+ chelator, indicating the effects were due to Ca2+ release induced by cADPR. Consistently, ryanodine (at high concentration) or 8-amino-cADPR, a specific antagonist of cADPR, but not heparin, inhibited the cADPR-induced changes. Both inhibitors likewise blocked the membrane insertion that normally occurred at fertilization consistent with it being mediated by a ryanodine receptor. The effects of NAADP were different from those of cADPR. Although NAADP induced a similar decrease in the Ca2+ current, no membrane insertion occurred. Moreover, pretreatment of the oocytes with NAADP inhibited the post-fertilization Ca2+ oscillation while cADPR did not. A similar Ca2+ oscillation could be artificially induced by perfusing into the oocytes a high concentration of InsP3 and NAADP could likewise inhibit such an InsP3-induced oscillation. This work shows that three independent Ca2+ signaling pathways are present in the oocytes and that each is involved in mediating distinct changes associated with fertilization. The results are consistent with a hierarchical organization of Ca2+ stores in the oocyte.

Effects of wheat germ agglutinin on tunicate egg activation and fertilization: is there a plasma membrane sperm receptor system on Ascidia ceratodes eggs?

Little work has been carried out on the sperm recognition systems present on the egg plasma membrane. Here it is shown that wheat germ agglutinin (WGA) interferes with the sperm-interacting system on the plasma membrane of eggs of the ascidian, Ascidia ceratodes. The WGA activates the dechorionated egg, indicating that a plasma membrane sugar residue can be directly tied to egg activation. Low concentrations of this lectin do not activate the eggs, but reduce fertilizability. This observation suggests that the WGA binding site might be part of a sperm reception-activation complex in the plasma membrane. While WGA also affects sperm binding to the chorion, the mechanisms of sperm interaction at the plasma membrane and chorion show different sensitivities to lectins, sugars and enzymes.

Evidence for cell surface and internal phospholipase activity in ascidian eggs

Upon fertilization, ascidian eggs release a cell surface glycosidase used in the block to polyspermy and undergo cortical contractions resulting from increased intracellular calcium levels. The glycosidase is released by fertilization, calcium ionophores or added phospholipase C (PLC) activity. The PLC inhibitor D609 blocks glycosidase release. Intact Ascidia ceratodes eggs cleave 4-methylumbelliferyl-phospho-choline when it is added to seawater. This yields highly fluorescent 4-methylumbelliferone. Authentic phospholipase C but not phospholipase D can cleave this substrate. Thus, the authors believe that cleavage of the substrate is specific for PLC activity. Eggs incubated in the fluorogenic substrate after having been washed and detergent extracted were not fluorescent. Therefore the substrate failed to enter intact cells. Glycosidase release and PLC activity were stimulated by ionomycin. Octylglucoside or Triton X-100 extracts of ascidian eggs had two forms of phospholipase activity as shown by ion affinity chromatography: PL1 eluting at 0.25 mol/L NaCl and PL2 eluting at 0.6 mol/L NaCl. The PL1 appeared to be isolated as a single protein. When surface proteins were labeled with non-penetrating biotin and were subsequently reacted with streptavidin, half of the PLC activity bound. This demonstrates that half the ascidian egg PLC activity is located on the surface of either the egg or follicle cell, and half is located within the egg.

The two intracellular Ca2+ release channels, ryanodine receptor and inositol 1,4,5-trisphosphate receptor, play different roles during fertilization in ascidians

Fertilization in the ascidians triggers an activation wave of calcium release followed by intracellular calcium oscillations synchronous with periodic membrane potential excursions during the completion of the meiotic cell cycle. Fertilization also causes a fast decrease in the egg plasma membrane depolarization-activated calcium current and a large increase in capacitance thought to represent membrane addition to the egg surface. We have analyzed the temporal and causal relationships between these changes in the eggs of Phallusia mammillata using whole-cell patch-clamp recording while simultaneously imaging calcium with fura-2 dextran. We have defined the role of ryanodine receptor (RyR) and InsP3 receptor (InsP3R) during fertilization and meiosis by looking at the effects of InsP3, cyclic ADP ribose (cADPR), and ryanodine in perfused oocytes. We show that InsP3 (10 microM perfused through the patch pipette) is able to trigger sustained oscillations in intracellular calcium concentration in unfertilized oocytes, resembling those recorded in fertilized egg completing meiosis. In addition the sustained oscillations resulting from InsP3 perfusion in unfertilized oocytes are sufficient to cause the emission of both polar bodies. In contrast, ryanodine or cADPR never trigger detectable calcium signal in perfused oocytes. Instead, nanomolar concentrations of ryanodine or cADPR cause a capacitance change, implying a net insertion of membrane to the oocyte surface, and trigger a fast decrease in the depolarization-activated calcium current. Both changes are similar to the changes in conductance and capacitance naturally observed following fertilization. These effects, although not associated with measurable calcium signals, are abolished by coperfusion of the calcium chelator BAPTA. In contrast to ryanodine or cADPR, sustained perfusion of the oocyte with nanomolar concentrations of InsP3 causes no capacitance change and a slow and moderate decrease in calcium current. Our observations on inseminated patch-clamped eggs further indicate that membrane insertion, which starts 15-20 sec after the onset of the membrane conductance change at fertilization, can be altered by interfering with the RyR. Our results imply that, in ascidians, as in some mammals, RyR and InsP3R play distinct roles during fertilization.

Ascidian eggs block polyspermy by two independent mechanisms: one at the egg plasma membrane, the other involving the follicle cells

Many ascidians live in clumps and usually release sperm before the eggs. Consequently, eggs are often spawned into dense clouds of sperm. Because fertilization by more than a single sperm is lethal, ascidians have evolved at least two successive blocks to polyspermy: the rapid release of a glycosidase that inhibits sperm binding to the vitelline coat (VC) and a subsequent change in membrane potential that prevents supernumerary sperm-egg fusion. This paper shows that (1) these two blocks can be uncoupled by the use of suramin, and (2) most of the glycosidase appears to be from the follicle cells, which are accessory cells on the outside of the egg VC. Phallusia mammillata eggs initially bind numerous sperm but, after the glycosidase is released, only a few additional sperm bind. Intact eggs in 20 microM suramin release glycosidase, but the electrical response is inhibited; sperm swim actively and bind to the VC but fail to penetrate. Suramin treatment is completely reversible; intact eggs exhibit the electrical response an average of 11 minutes after the drug is washed out. Sperm must contact the follicle cells before passing through the VC; eggs with the VC removed and fertilized in the presence of 20 microM suramin show the electrical response 35% of the time, thus VC removal enhances sperm entry. Like the intact eggs, 100% of the naked eggs respond electrically to fertilization after the drug is washed out. Follicle cells that are isolated by calcium magnesium free seawater and then returned to complete seawater release N-acetylglucosaminidase activity in response to sperm. Thus, these eggs have two blocks to polyspermy that operate in sequence: an early first block resulting from enzymatic modification of the VC by N-acetylglucosaminidase released primarily from follicle cells and a second electrical block operating at the egg plasma membrane level and requiring sperm-egg fusion.