Phospholipid utilization in ascidian Ciona intestinalis spermatozoa during swimming

Fatty Acid and Lipid Profiles with Emphasis on n-3 Fatty Acids and Phospholipids from Ciona intestinalis

In order to establish Ciona intestinalis as a new bioresource for n-3 fatty acids-rich marine lipids, the animal was fractionated into tunic and inner body tissues prior to lipid extraction. The lipids obtained were further classified into neutral lipids (NL), glycolipids (GL) and phospholipids (PL) followed by qualitative and quantitative analysis using GC-FID, GC-MS, (1)H NMR, 2D NMR, MALDI-TOF-MS and LC-ESI-MS methods. It was found that the tunic and inner body tissues contained 3.42-4.08% and 15.9-23.4% of lipids respectively. PL was the dominant lipid class (42-60%) irrespective of the anatomic fractions. From all lipid fractions and classes, the major fatty acids were 16:0, 18:1n-9, C20:1n-9, C20:5n-3 (EPA) and C22:6n-3 (DHA). The highest amounts of long chain n-3 fatty acids, mainly EPA and DHA, were located in PL from both body fractions. Cholestanol and cholesterol were the dominant sterols together with noticeable amounts of stellasterol, 22 (Z)-dehydrocholesterol and lathosterol. Several other identified and two yet unidentified sterols were observed for the first time from C. intestinalis. Different molecular species of phosphatidylcholine (34 species), sphingomyelin (2 species), phosphatidylethanolamine (2 species), phosphatidylserine (10 species), phosphatidylglycerol (9 species), ceramide (38 species) and lysophospholipid (5 species) were identified, representing the most systematic PL profiling knowledge so far for the animal. It could be concluded that C. intestinalis lipids should be a good alternative for fish oil with high contents of n-3 fatty acids. The lipids would be more bioavailable due to the presence of the fatty acids being mainly in the form of PL.

Lipid accumulation and metabolism in polychaete spermatogenesis: Role of the large discoidal lipoprotein

In most oviparous animals, lipoprotein-mediated lipid transport plays an important role in the nutrient supply for the oocyte. In male gametes, lipids are used as energy substrates in spermatozoa but nothing is yet known about their origin and metabolism throughout spermatogenesis. The lipid profiles analyzed from different stages of male germ cell development in the marine annelid Nereis virens were found to undergo a dramatic change from primary triacylglycerides at the beginning of germ cell development to cholesterol and phospholipids at the end of development as demonstrated by HPLC with evaporative light scattering detection and mass spectrometry. The uptake of a large discoidal lipoprotein into the developing germ cells could be demonstrated by fluorescence labeling and electron microscopic techniques as well as by the presence of a lipoprotein receptor in the germ cells, thus establishing its role in lipid supply. The incorporated lipoprotein discs were found to be stored as intact complexes indicating that they are not readily degraded upon endocytotic uptake. The change in lipid composition during germ cell development reflects their metabolic activity, especially in spermatogonia. The high concentration of lipids maintained by spermatogonia during the early phase of gametogenesis seems to be required for the later rapid processes of meiosis and spermatocyte differentiation. At times when peak demand of lipids arises for membrane synthesis and increased metabolism, this may be met more efficiently by a rapid on-site mobilization of lipids instead of an external supply.

Remarkable longevity of dilute sperm in a free-spawning colonial ascidian

Many benthic marine invertebrates reproduce by releasing sperm into the sea (free-spawning), but the amount of time that sperm are viable after spawning may have different consequences for fertilization, depending on the type of free-spawner. In egg-broadcasting marine organisms, gamete age is usually assumed to be irrelevant because of the low probability of contact between dilute sperm and egg. However, direct dilution effects might be reduced in egg-brooding free-spawners that filter dilute sperm out of the water column, and sperm longevity may play a role in facilitating fertilization in these taxa. We investigated the effects of time, temperature, and mixing on the viability of naturally released sperm of the colonial ascidian Botryllus schlosseri. Our data indicate that B. schlosseri sperm have a functional life span that is considerably longer than those of the sperm of many other marine invertebrate taxa (half-life of approximately 16 to 26 h), are able to fertilize eggs at extremely low external sperm concentrations (ca. 10(1) sperm ml(-1)), and have a longevity that varies with temperature. It is possible that such prolonged sperm longevity may be achieved by reductions in motility, reactivation of quiescent sperm by chemical cues, or intermittent swimming.

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.

Liposomes fuse with sperm cells and induce activation by delivery of impermeant agents

Sperm cell activation is a critical step in fertilization. To directly investigate the cell signaling events leading to sperm activation it is necessary to deliver membrane impermeant agents into the cytoplasm. In this study, the use of liposomes as possible agent-loading vectors was examined using (1) the octadecylrhodamine B (R18) and NBD phosphatidylethanolamine (NBD DHPE)/rhodamine phosphatidylethanolamine (rhod DHPE) fusion assays in bulk samples, (2) membrane transfer of fluorescence from liposome membranes labeled with R18 and rhodamine-tagged phosphatidylethanolamine (TRITC DHPE), and (3) lumenal transfer of impermeant calcium ions from liposomes to sperm cells, a process that stimulated sperm cell activation. Intermediate-sized unilamellar liposomes (98.17+/-15.34 nm) were prepared by the detergent-removal technique using sodium cholate as the detergent and a phosphatidylcholine/phosphatidylethanolamine/cholesterol (2:1:1 mole ratio) lipid composition. In the R18 fusion assays, self-quenching increased logarithmically with increasing concentrations of R18 in the liposome membranes; addition of unlabeled sperm to R18-labeled liposomes lead to a rapid release of self-quenching. In the NBD DHPE/rhod DHPE resonance energy transfer (RET) fusion assay, RET was rapidly reduced under similar conditions. In addition, individual sperm became fluorescent when TRITC DHPE-labeled liposomes were incubated with unlabeled sperm cells. Incubation of sperm cells with empty liposomes did not significantly affect sperm cell activation and did not alter cell morphology. However, incubation with Ca (10 mM)-loaded liposomes resulted in a time-dependent increase in sperm cell activation (7.5-fold over controls after 15 min). We conclude that liposomes can be used for direct loading of membrane-impermeant agents into sea squirt sperm cell cytoplasm, and that delivery occurs via fusion and content intermixing.

Fertilization in the sea: are the hazards of broadcast spawning avoided when free-spawned sperm fertilize retained eggs?

In broadcast-spawning marine animals, rapid dilution and short lifespan of sperm following release may impose severely localized patterns of mating. Partial or total failure of external fertilization due to sperm limitation appears commonplace. However, it is not clear to what extent the restrictive kinetics of fertilization in water also constrain mating in animals that release sperm but retain their eggs for fertilization.
The compound ascidian Diplosoma listerianum liberates sperm that are dispersed to other colonies and taken in prior to internal cross-fertilization. The fertile lifespan of sperm was found to be long (half-life ca . 8 hours), and a substantial number of fertilizations occurred with 24-hour-old sperm. Fertilizations were obtained from sperm concentrations that would typically produce little or no external fertilization. In a separate experiment, a very small piece of D. listerianum (dry weight less than 2 mg) sired abundant progeny throughout a 3840 l tank. Paternity of progeny in these experiments was confirmed by molecular markers. The same markers were used to extend, to over seven weeks, the known maximum period of storage of exogenous sperm prior to fertilization in this species.
The production of only a few thousand sperm at a time by each zooid, poor synchronization of release between zooids, and the existence of many well-spaced exhalant openings in large colonies suggest that D. listerianum is incapable of generating a dense plume of sperm, even close to the source. It is suggested that, unlike external fertilization, successful internal cross-fertilization in D. listerianum is not dependent upon the interception of a dense cloud of gametes just released by a near neighbour. It seems instead that dilute, long-lived sperm can be extracted efficiently from seawater by this suspension feeder, potentially over a period of time. This capability, and other features of the life history, make it unlikely that sperm limitation is an acute problem in this species and comparable taxa, a conclusion with potential significance for expected patterns of mating, sex allocation and gamete attributes in sessile aquatic invertebrates. Variance in reproductive success between individuals due to differences in fertilization rate may be much lower than in broadcast spawners exhibiting external fertilization.