Analysis of a sperm surface molecule that binds to a vitelline envelope component of Xenopus laevis eggs

Diversity of sialic acids and sialoglycoproteins in gametes and at fertilization

Sialic acids are a family of 9-carbon monosaccharides with particular physicochemical properties. They modulate the biological functions of the molecules that carry them and are involved in several steps of the reproductive process. Sialoglycoproteins participate in the balance between species recognition and specificity, and the mechanisms of these aspects remain an issue in gametes formation and binding in metazoan reproduction. Sialoglycoproteins form a specific coat at the gametes surface and specific polysialylated chains are present on marine species oocytes. Spermatozoa are submitted to critical sialic acid changes in the female reproductive tract facilitating their migration, their survival through the modulation of the female innate immune response, and the final oocyte-binding event. To decipher the role of sialic acids in gametes and at fertilization, the dynamical changes of enzymes involved in their synthesis and removal have to be further considered.

Expression of Prolactin Receptor on the Surface of Quail Spermatozoa

Prolactin receptor (PRLR) is expressed in a wide variety of tissues and mediates diverse biological actions of prolactin (PRL). In mammals, PRL signaling is thought to be involved not only in the process of spermatogenesis and steroidogenesis in the testis, but also in the survival of ejaculated sperm. In avian species, although the expression of PRLR with several variants in the testis was reported, the role of PRL in testicular function is still unclear. The aim of this study was to examine the expression of PRLR in the testis and mature sperm in quail. It is revealed that PRLR was mainly localized in the round- and elongated-spermatid by immunohistochemical analysis on the testis suggesting that PRL signaling may participate in the spermatogenesis. Western blot analysis confirmed the presence of PRLR in the plasma membrane of the ejaculated sperm (SPML), whereas the size of PRLR in the sperm was smaller than that in the hypothalamus. Moreover, PRLR was detected on the surface of the midpiece and flagellum of sperm by immunostaining. To evaluate the functionality of the sperm PRLR, the dot blot assay was performed to test the binding of pituitary PRL to PRLR in the SPML, and resulted in the detection of specific binding of PRL to the component of SPML, most likely to sperm PRLR. Furthermore, when the ejaculates were incubated with pituitary PRL to investigate the role of PRL on the sperm, the occurrence of spontaneous acrosome reaction was significantly decreased. In addition, the expression of PRL on the surface of utero-vaginal junction of oviduct was detected by immunohistochemistry. These results may suggest a novel system that the interaction between oviductal PRL and sperm PRLR is involved in the maintenance of the fertilizability of the spermatozoa through the prevention of the spontaneous acrosome reaction in Japanese quail.

Transmembrane signal transduction in oocyte maturation and fertilization: focusing on Xenopus laevis as a model animal

Fertilization is a cell biological phenomenon of crucial importance for the birth of new life in a variety of multicellular and sexual reproduction species such as algae, animal and plants. Fertilization involves a sequence of events, in which the female gamete "egg" and the male gamete "spermatozoon (sperm)" develop, acquire their functions, meet and fuse with each other, to initiate embryonic and zygotic development. Here, it will be briefly reviewed how oocyte cytoplasmic components are orchestrated to undergo hormone-induced oocyte maturation and sperm-induced activation of development. I then review how sperm-egg membrane interaction/fusion and activation of development in the fertilized egg are accomplished and regulated through egg coat- or egg plasma membrane-associated components, highlighting recent findings and future directions in the studies using Xenopus laevis as a model experimental animal.

Egg activation in physiological polyspermy

Fertilization is indispensable not only for restoring diploid genomes but also for the initiation of early embryonic cell cycles in sexual reproduction. While most animals exhibit monospermy, which is ensured by polyspermy blocks to prevent the entry of extra sperm into the egg at fertilization, several animals exhibit physiological polyspermy, in which the entry of several sperm is permitted but only one sperm nucleus participates in the formation of a zygote nucleus. Polyspermy requires that the sperm transmit the egg activation signal more slowly, thus allowing the egg to accept several sperm. An increase in intracellular Ca(2+) concentration induced by the fertilizing sperm is both necessary and sufficient for egg activation in polyspermy. Multiple small Ca(2+) waves induced by several fertilizing sperm result in a long-lasting Ca(2+) rise, which is a characteristic of polyspermic amphibian eggs. We introduced a novel soluble sperm factor for egg activation, sperm-specific citrate synthase, into polyspermic newt eggs to cause Ca(2+) waves. Citrate synthase may perform dual functions: as an enzyme in mitochondria and as a Ca(2+)-inducing factor in egg cytoplasm. We also discuss the close relationship between the mode of fertilization and the Ca(2+) rise at egg activation and consider changes in this process through evolution in vertebrates.

Sperm acrosin is responsible for the sperm binding to the egg envelope during fertilization in Japanese quail (Coturnix japonica)

An antibody library against quail sperm plasma membrane components was established and a mAb, which strongly inhibits sperm perforations of the perivitelline membrane (PVM) was obtained from the library. The antigen molecule of the mAb showed an apparent molecular weight of 45 kDa, and was distributed both on the surface and in the acrosomal matrix of the sperm head. Periodate oxidation revealed that the epitope of the antigen includes a sugar moiety. Tandem mass spectrometry analysis of the antigen revealed that the mAb recognizes sperm acrosin. When sodium dodecyl sulfate-solubilized PVM immobilized on a polyvinylidene difluoride membrane was incubated with sperm plasma membrane lysates, the sperm acrosin was detected on the PVM immobilized on the membrane, indicating that the sperm acrosin interacts with the components of PVM. Indeed, the mAb effectively inhibited the binding of acrosome-intact sperm to the PVM. These results indicate that the 45 kDa sperm acrosin is involved in the binding of sperm to the PVM in fertilization of Japanese quail.

Gamete membrane microdomains and their associated molecules in fertilization signaling

Fertilization is the fundamental system of biological reproduction in many organisms, including animals, plants, and algae. A growing body of knowledge has emerged to explain how fertilization and activation of development are accomplished. Studies on the molecular mechanisms of fertilization are in progress for a wide variety of multicellular organisms. In this review, we summarize recent findings and debates about the long-standing questions concerning fertilization: how egg and sperm become competent for their interaction with each other, how the binding and fusion of these gamete cells are made possible, and how the fertilized eggs initiate development to a newborn. We will focus on the structure and function of the membrane microdomains (MDs) of egg and sperm that may serve as a platform or signaling center for the aforementioned cellular functions. In particular, we provide evidence that MDs of eggs from the African clawed frog, Xenopus laevis, play a pivotal role in receiving extracellular signals from fertilizing sperm and then transmitting them to the egg cytoplasm, where the tyrosine kinase Src is present and responsible for the subsequent signaling events collectively called egg activation. The presence of a new signaling axis involving uroplakin III, an MD-associated transmembrane protein, and Src in this system will be highlighted and discussed.