Nitric oxide gates fertilization channels in ascidian oocytes through nicotinamide nucleotide metabolism

Novel Insights on Nitric Oxide Synthase and NO Signaling in Ascidian Metamorphosis

Nitric oxide (NO) is a pivotal signaling molecule involved in a wide range of physiological and pathological processes. We investigated NOS/NO localization patterns during the different stages of larval development in the ascidia Ciona robusta and evidenced a specific and temporally controlled pattern. NOS/NO expression starts in the most anterior sensory structures of the early larva and progressively moves towards the caudal portion as larval development and metamorphosis proceeds. We here highlight the pattern of NOS/NO expression in the central and peripheral nervous system of Ciona larvae which precisely follows the progression of neural signals of the central pattern generator necessary for the control of the movements of the larva towards the substrate. This highly dynamic localization profile perfectly matches with the central role played by NO from the first phase of settlement induction to the next control of swimming behavior, adhesion to substrate and progressive tissue resorption and reorganization of metamorphosis itself.

Nitric-oxide generation induced by metals plays a role in their accumulation by Phallusia nigra hemocytes

Ascidians are good monitors for assessing water quality, since they filter large volumes of water; however, little is known about how xenobiotics, including metals, can affect ascidian hemocytes. Metals can be either toxic or beneficial to health, inducing many different responses. The response mechanism depends on the class of metals to which organisms are exposed: essential, nonessential, and borderline. To analyze the influence of metals from different classes on the protective mechanisms of an ascidian, we investigated the production of nitric oxide (NO) after exposure to various concentrations of Mg, Mn and Pb over different time periods. We also determined the amounts of each metal in the hemocytes. Our results indicated that especially Pb could stimulate NO production. Although Pb induced the highest NO production, cell viability was not severely altered in all Pb concentrations and time periods. Ascidians might serve as biomonitor for Pb, since their vanadocytes accumulate Pb.

Adverse effect of antifouling compounds on the reproductive mechanisms of the ascidian Ciona intestinalis

Fertilization and embryo development that occur in sea water are sensitive to xenobiotics from anthropogenic sources. In this work, we evaluated the influence of two antifouling biocides, tributyltin (TBT) and diuron, on the reproductive mechanisms of the marine invertebrate Ciona intestinalis. By using electrophysiological techniques, we examined the impact of these compounds on the electrical properties of the mature oocytes and of events occurring at fertilization. With different toxicity assays, we studied the effect of the two biocides on the gametes by evaluating fertilization rate and embryo development. Results show that sodium (Na⁺) currents were significantly reduced by either of the two biocides, whereas conductance was significantly increased. The fertilization current frequency and amplitude, fertilization rate and larval development were affected only by TBT. This study suggests that: (i) the two biocides affect either the electrical properties of the oocyte plasma membrane and the reproductive success representing a risk factor for the survival of the species exposed to environmental pollution; (ii) the ascidian Ciona intestinalis may represent a good model organism to test toxicity of marine pollutants. Possible mechanisms of action of the two biocides are discussed.

Regulatory roles of nitric oxide during larval development and metamorphosis in Ciona intestinalis

Metamorphosis in the ascidian Ciona intestinalis is a very complex process which converts a swimming tadpole to an adult. The process involves reorganisation of the body plan and a remarkable regression of the tail, which is controlled by caspase-dependent apoptosis. However, the endogenous signals triggering apoptosis and metamorphosis are little explored. Herein, we report evidence that nitric oxide (NO) regulates tail regression in a dose-dependent manner, acting on caspase-dependent apoptosis. An increase or decrease of NO levels resulted in a delay or acceleration of tail resorption, without affecting subsequent juvenile development. A similar hastening effect was induced by suppression of cGMP-dependent NO signalling. Inhibition of NO production resulted in an increase in caspase-3-like activity with respect to untreated larvae. Detection of endogenously activated caspase-3 and NO revealed the existence of a spatial correlation between the diminution of the NO signal and caspase-3 activation during the last phases of tail regression. Real-time PCR during development, from early larva to early juveniles, showed that during all stages examined, NO synthase (NOS) is always more expressed than arginase and it reaches the maximum value at late larva, the stage immediately preceding tail resorption. The spatial expression pattern of NOS is very dynamic, moving rapidly along the body in very few hours, from the anterior part of the trunk to central nervous system (CNS), tail and new forming juvenile digestive organs. NO detection revealed free diffusion from the production sites to other cellular districts. Overall, the results of this study provide a new important link between NO signalling and apoptosis during metamorphosis in C. intestinalis and hint at novel roles for the NO signalling system in other developmental and metamorphosis-related events preceding and following tail resorption.

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.

Nitric oxide in marine invertebrates: a comparative perspective

Since the discovery of the biological effects of nitric oxide (NO) more than two decades ago, NO has been identified as an important physiological modulator and a messenger molecule in mammals. Parallel to these studies, evidence that has accumulated in recent years has revealed that the NO signalling pathway is spread throughout the entire phylogenetic scale, being increasingly found in lower organisms, ranging from Chordata to Mollusca. The present review attempts to provide a survey of current knowledge of the genesis and possible roles of NO and the related signalling pathway in marine invertebrates, with special emphasis on Sepia, a choice dictated by the increasing appreciation of cephalopods as most valuable model systems for studies of NO biology and the present expectation for new exciting insights into as yet little explored segments of NO biology.

Bioactive aldehydes from diatoms block the fertilization current in ascidian oocytes

The effects of bioactive aldehydes from diatoms, unicellular algae at the base of the marine food web, were studied on fertilization and early development processes of the ascidian Ciona intestinalis. Using whole-cell voltage clamp techniques, we show that 2-trans-4-trans-decadienal (DD) and 2-trans-4-cis-7-cis-decatrienal (DT) inhibited the fertilization current which is generated in oocytes upon interaction with the spermatozoon. This inhibition was dose-dependent and was accompanied by inhibition of the voltage-gated calcium current activity of the plasma membrane. DD and DT did not inhibit the subsequent contraction of the cortex. Moreover, DD specifically acted as a fertilization channel inhibitor since it did not affect the steady state conductance of the plasma membrane or gap junctional (GJ) communication within blastomeres of the embryo. On the other hand, DD did affect actin reorganization even though the mechanism of action on actin filaments differed from that of other actin blockers. Possibly this effect on actin reorganization was responsible for the subsequent teratogenic action on larval development. The effect of DD was reversible if oocytes were washed soon after fertilization indicating that DD may specifically target certain fertilization mechanisms. Thus, diatom reactive aldehydes such as DD may have a dual effect on reproductive processes, influencing primary fertilization events such as gating of fertilization channels and secondary processes such as actin reorganization which is responsible for the segregation of cell lineages. These findings add to a growing body of evidence on the antiproliferative effects of diatom-derived aldehydes. Our results also report, for the first time, on the action of a fertilization channel blocker in marine invertebrates.

Simultaneous Measurement of Intracellular Nitric Oxide and Free Calcium Levels in Chordate Eggs Demonstrates That Nitric Oxide Has No Role at Fertilization

At fertilization in sea urchin, the free radical nitric oxide (NO) has recently been suggested to cause the intracellular Ca(2+) rise responsible for egg activation. The authors suggested that NO could be a universal activator of eggs and the present study was set up to test this hypothesis. Intracellular NO and Ca(2+) levels were monitored simultaneously in eggs of the mouse or the urochordate ascidian Ascidiella aspersa. Eggs were either fertilized or sperm extracts microinjected. Sperm-induced Ca(2+) rises were not associated with any global, or local, change in intracellular NO, although we were able to detect NO produced by the addition of a NO donor. Furthermore, the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester had no effect on sperm-induced Ca(2+) release but did block completely ionomycin-induced NO synthase activation. Therefore, we suggest that the current data provide evidence that NO has no role in the fertilization of these two chordate eggs.

Nitric oxide synthase in filariae: demonstration of nitric oxide production by embryos in Brugia malayi and Acanthocheilonema viteae

The radical gas nitric oxide (NO) is synthesized by nitric oxide synthase (NOS) from l-arginine and molecular oxygen. Nitric oxide is an important signaling molecule in invertebrate and vertebrate systems. Previously we have shown that NOS is localized to more tissues in Brugia malayi than has been reported in Ascaris suum. In this paper, we analyze the distribution of NOS in Acanthocheilonema viteae, a filarial nematode that differs from B. malayi in that A. viteae females release microfilariae without a sheath. A. viteae is also one of a few filarial parasites without the Wolbachia intracellular endosymbiont. By use of a specific antibody, NOS was demonstrated in extracts of A. viteae and Dirofilaria immitis. The localization pattern of NOS in A. viteae was similar to that seen in B. malayi, with the enzyme localized to the body wall muscles of both sexes, developing spermatozoa, intrauterine sperm, and early embryos. By use of DAF-2, a fluorescent indicator specific for nitric oxide, the embryos of B. malayi and A. viteae were demonstrated to produce NO ex utero. The near identical staining patterns seen in A. viteae and B. malayi argue that NO is not produced by Wolbachia, nor is it produced by the nematodes in response to the infection. Localization of NOS to the sperm of filarial nematodes suggests a role for NO during fertilization as has been described for sea urchin and ascidian fertilization. Demonstration of the activity of embryonic NOS supports our earlier hypothesis that NO is a signaling molecule during embryogenesis in filarial nematodes.

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.

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.

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.