Ganglioside-enriched microdomains define an oolemma that is functionally polarized with respect to fertilizability in the mouse

Serotonin Signaling through Lipid Membranes

Serotonin (5-HT) is a vital modulatory neurotransmitter responsible for regulating most behaviors in the brain. An inefficient 5-HT synaptic function is often linked to various mental disorders. Primarily, membrane proteins controlling the expression and activity of 5-HT synthesis, storage, release, receptor activation, and inactivation are critical to 5-HT signaling in synaptic and extra-synaptic sites. Moreover, these signals represent information transmission across membranes. Although the lipid membrane environment is often viewed as fairly stable, emerging research suggests significant functional lipid-protein interactions with many synaptic 5-HT proteins. These protein-lipid interactions extend to almost all the primary lipid classes that form the plasma membrane. Collectively, these lipid classes and lipid-protein interactions affect 5-HT synaptic efficacy at the synapse. The highly dynamic lipid composition of synaptic membranes suggests that these lipids and their interactions with proteins may contribute to the plasticity of the 5-HT synapse. Therefore, this broader protein-lipid model of the 5-HT synapse necessitates a reconsideration of 5-HT's role in various associated mental disorders.

Has the concept of polyspermy prevention been invented in the laboratory?

There is no evidence, nor need, for a fast block to polyspermy in animal oocytes. The idea that oocytes have evolved a mechanism to allow the entry of one spermatozoon and repel all others has, however, gained consensus over the last century. The main culprit is the sea urchin, which has been used for over a century in in vitro studies of the fertilization process. Images of sea urchin oocytes with thousands of sperm attached to the surface are commonplace in textbooks and appeal to the nature of the reader implying an intriguing surface mechanism of sperm selection despite these oocytes being fixed for photography (Figure ). The abundance of gametes in this marine invertebrate and the ease of experimentation have given us the possibility to elucidate many aspects of the mechanism of fertilization, but has also led to ongoing controversies in reproductive biology, one being polyspermy prevention. Kinetic experiments by Rothschild and colleagues in the 1950s led to the hypothesis of a fast partial block to polyspermy in sea urchin oocytes that reduced the probability of a second spermatozoon from entering the oocyte by 1/20th. In the 1970s, Jaffe and colleagues suggested, with circumstantial evidence, that this partial block was due to the sperm-induced depolarization of the oocyte plasma membrane. However, the fate of supernumerary spermatozoa is determined well before the plasma membrane of the oocyte depolarizes. Transmembrane voltage does not serve to regulate sperm entry. Scholastic texts have inadvertently promulgated this concept across the animal kingdom with no logical correlation or experimentation and, as of today, a molecular mechanism to regulate sperm entry in oocytes has not been identified.

Aging-Related Ovarian Failure and Infertility: Melatonin to the Rescue

Aging has a major detrimental effect on the optimal function of the ovary with changes in this organ preceding the age-related deterioration in other tissues, with the middle-aged shutdown leading to infertility. Reduced fertility and consequent inability to conceive by women in present-day societies who choose to have children later in life leads to increased frustration. Melatonin is known to have anti-aging properties related to its antioxidant and anti-inflammatory actions. Its higher follicular fluid levels relative to blood concentrations and its likely synthesis in the oocyte, granulosa, and luteal cells suggest that it is optimally positioned to interfere with age-associated deterioration of the ovary. Additionally, the end of the female reproductive span coincides with a significant reduction in endogenous melatonin levels. Thus, the aims are to review the literature indicating melatonin production in mitochondria of oocytes, granulosa cells, and luteal cells, identify the multiple processes underlying changes in the ovary, especially late in the cessation of the reproductive life span, summarize the physiological and molecular actions of melatonin in the maintenance of normal ovaries and in the aging ovaries, and integrate the acquired information into an explanation for considering melatonin in the treatment of age-related infertility. Use of supplemental melatonin may help preserve fertility later in life and alleviate frustration in women delaying childbearing age, reduce the necessity of in vitro fertilization–embryo transfer (IVF-ET) procedures, and help solve the progressively increasing problem of non-aging-related infertility in women throughout their reproductive life span. While additional research is needed to fully understand the effects of melatonin supplementation on potentially enhancing fertility, studies published to date suggest it may be a promising option for those struggling with infertility.

Tetraspanin Cd9b plays a role in fertility in zebrafish

In mice, CD9 expression on the egg is required for efficient sperm-egg fusion and no effects on ovulation or male fertility are observed in CD9 null animals. Here we show that cd9b knockout zebrafish also appear to have fertility defects. In contrast to mice, fewer eggs were laid by cd9b knockout zebrafish pairs and, of the eggs laid, a lower percentage were fertilised. These effects could not be linked to primordial germ cell numbers or migration as these were not altered in the cd9b mutants. The decrease in egg numbers could be rescued by exchanging either cd9b knockout partner, male or female, for a wildtype partner. However, the fertilisation defect was only rescued by crossing a cd9b knockout female with a wildtype male. To exclude effects of mating behaviour we analysed clutch size and fertilisation using in vitro fertilisation techniques. Number of eggs and fertilisation rates were significantly reduced in the cd9b mutants suggesting the fertility defects are not solely due to courtship behaviours. Our results indicate that CD9 plays a more complex role in fish fertility than in mammals, with effects in both males and females.

Boar spermadhesin AWN: Novel insights in its binding behavior and localization on sperm

As a major spermadhesin first found in the seminal plasma of boars, AWN is described to fulfil a variety of reproduction related tasks. Although being the best investigated boar spermadhesin, information about its interaction with membranes is inconsistent. In this regard, previous reports locate AWN either inside or on the surface of sperm cells and at different regions, depending on the method and antibody used. Here, we localize native AWN in/on epididymal, ejaculated, capacitated and acrosome-reacted boar sperm using epifluorescence and electron microscopy, as well as an analysis of potential lipid binding partners of native and recombinant AWN. By applying a custom-made anti-AWN antibody, localization of AWN in the equatorial segment of ejaculated, capacitated and acrosome-reacted boar sperm was discovered. Electron microscopy showed that AWN is localized both on the sperm surface and on the cytoplasmic side of the plasma membrane, and in close vicinity to the nuclear and both acrosomal membranes of sperm. Analysis of epididymal sperm indicated migration of AWN from the retral postacrosomal part to the equatorial segment during the epididymal passage. In contrast to hypotheses claiming a specific association of AWN to phosphatidylethanolamine and in line with our previous study describing an interaction with phosphatidic acid, the current results show a rather electrostatically-driven binding mechanism of AWN to negative lipids. In conclusion, this work provides new insights into the arrangement of AWN in the equatorial segment that suggest a possible role in sperm-oocyte fusion.

Nuclear-to-cytoplasmic ratios of 1PN and 2PN zygotes after in vitro fertilization of mouse oocytes

Numerous studies have reported comparisons of the nuclear-to-cytoplasmic (NC) ratio during mitosis. However, little information is known about how the pronuclear size is regulated and determined at the end of meiosis II in mammalian zygotes. The present study aims to analyze the NC ratio of female and male pronuclei, and also to compare the size of single pronuclei using photographs that were obtained during experiments to create chimeric hermaphrodites from 2-cell oocytes. The volume of both the female and the male pronucleus was found to correlate with the volume of the oocyte cytoplasm. The NC ratio of the male pronucleus was greater than that of the female pronucleus. The NC ratio of the average volume of the female and male pronuclei was greater than the NC ratio of the mononucleate oocytes. The occurrence of 1PN oocytes was significantly higher when the volume of cytoplasm was lower than the cut-off value. These results indicated that the NC ratio is retained during pronuclear formation. A higher NC ratio in male compared with the female pronucleus indicated structural and/or molecular difference between the two pronuclei. 1PN formation may occur when sperm enters close to the MII spindle.

Free cholesterol and cholesterol esters in bovine oocytes: Implications in survival and membrane raft organization after cryopreservation

Part of the damage caused by cryopreservation of mammalian oocytes occurs at the plasma membrane. The addition of cholesterol to cell membranes as a strategy to make it more tolerant to cryopreservation has been little addressed in oocytes. In order to increase the survival of bovine oocytes after cryopreservation, we proposed not only to increase cholesterol level of oocyte membranes before vitrification but also to remove the added cholesterol after warming, thus recovering its original level. Results from our study showed that modulation of membrane cholesterol by methyl-β-cyclodextrin (MβCD) did not affect the apoptotic status of oocytes and improved viability after vitrification yielding levels of apoptosis closer to those of fresh oocytes. Fluorometric measurements based on an enzyme-coupled reaction that detects both free cholesterol (membrane) and cholesteryl esters (stored in lipid droplets), revealed that oocytes and cumulus cells present different levels of cholesterol depending on the seasonal period. Variations at membrane cholesterol level of oocytes were enough to account for the differences found in total cholesterol. Differences found in total cholesterol of cumulus cells were explained by the differences found in both the content of membrane cholesterol and of cholesterol esters. Cholesterol was incorporated into the oocyte plasma membrane as evidenced by comparative labeling of a fluorescent cholesterol. Oocytes and cumulus cells increased membrane cholesterol after incubation with MβCD/cholesterol and recovered their original level after cholesterol removal, regardless of the season. Finally, we evaluated the effect of vitrification on the putative raft molecule GM1. Cholesterol modulation also preserved membrane organization by maintaining ganglioside level at the plasma membrane. Results suggest a distinctive cholesterol metabolic status of cumulus-oocyte complexes (COCs) among seasons and a dynamic organizational structure of cholesterol homeostasis within the COC. Modulation of membrane cholesterol by MβCD improved survival of bovine oocytes and preserved integrity of GM1-related rafts after vitrification.