Sperm-Egg Interaction

A crucial step of fertilization is the sperm-egg interaction that allows the two gametes to fuse and create the zygote. In the mouse, CD9 on the egg and IZUMO1 on the sperm stand out as critical players, as Cd9−/− and Izumo1−/− mice are healthy but infertile or severely subfertile due to defective sperm-egg interaction. Moreover, work on several nonmammalian organisms has identified some of the most intriguing candidates implicated in sperm-egg interaction. Understanding of gamete membrane interactions is advancing through characterization of in vivo and in vitro fertilization phenotypes, including insights from less robust phenotypes that highlight potential supporting (albeit not absolutely essential) players. An emerging theme is that there are varied roles for gamete molecules that participate in sperm-egg interactions. Such roles include not only functioning as fusogens, or as adhesion molecules for the opposite gamete, but also functioning through interactions in cis with other proteins to regulate membrane order and functionality. 477 A nn u. R ev . P hy si ol . 2 01 2. 74 :4 77 -5 02 . D ow nl oa de d fr om w w w . a nn ua lr ev ie w s. or g by U ni ve rs ity o f Sy dn ey o n 04 /2 8/ 13 . F or p er so na l u se o nl y. PH74CH21-Evans ARI 9 January 2012 11:26 Sperm-egg fusion: the bona fide membrane fusion event; results in cytoplasmic continuity between the gametes (Figure 1d ) Sperm-egg binding (or sperm-egg adhesion): cell-cell interaction that precedes membrane fusion and that is defined experimentally as maintained after a specific series of washes (distinguishing adhesion from mere sperm attachment) Sperm-egg interaction (or sperm-egg membrane interaction): the interaction of the sperm membrane with the egg membrane, including sperm-egg adhesion and membrane fusion OVERVIEW, DEFINITIONS, AND SETTING THE STAGE Fertilization, the union of the gametes (sperm and egg in heterogametic species), is a critical component of sexual reproduction. This process brings together two terminally differentiated cells that carry the genomes of two different individuals and creates a totipotent cell, the zygote, and a new, genetically distinct individual. Fertilization occurs through a series of coordinated steps, culminating in the merger of the two cells. This merger is achieved through gamete interactions, specifically cell adhesion and then membrane fusion of the gamete plasma membranes. The term sperm-egg fusion has been used to refer collectively to the entire process of gamete membrane interactions that culminate in sperm incorporation into the egg. Wherever possible, this review strives for specificity in describing cellular events, using the terms sperm-egg binding and sperm-egg fusion as distinct steps and the term sperm-egg membrane interaction more generically where appropriate. Indeed, the two proteins most strongly implicated in mouse sperm-egg interaction, IZUMO1 on the sperm and CD9 on the egg, are often described as having roles in so-called sperm-egg fusion, although these proteins lack features associated with a fusogenic molecule, and these proteins may have other functions (addressed below). Distinct events lead to the fusion of two membranes. For cell-cell fusion (including sperm-egg fusion), cell adhesion is a precursor step, progressing from interaction of the membranes to the close apposition of the membranes. This membrane interaction ultimately leads to the actual fusion event, with mixing of the lipid bilayers, formation of fusion pores, and establishment of cytoplasmic continuity (Figure 1d ). Analogies exist in other systems: Intracellular vesicles dock or tether prior to fusion, myoblasts undergo a process of adhesion and alignment that leads to fusion, and membrane-coated virus particles bind to host cells and then progress to fusion (1–4). Sperm binding to the egg is a step-wise process as well, starting with loose attachment and progressing to firm adhesion that can lead to membrane fusion. In in vitro fertilization (IVF) assays, sperm readily attach to the zona pellucida (ZP) or to the egg membrane of ZP-free eggs (Figure 1c). Washes of inseminated eggs are required to remove loosely attached sperm, allowing identification of the subpopulation of sperm that are truly bound (5, 6) (see also sidebar entitled “Technical Aspects of Assessing Sperm-Egg Membrane Interactions”). TECHNICAL ASPECTS OF ASSESSING SPERM-EGG MEMBRANE INTERACTIONS Just as it is important to appreciate the distinct steps of sperm-egg interaction (see text and Figure 1d ), it is similarly valuable to understand the assays used to assess these interactions. Sperm-egg fusion is relatively easy to assess through observation of the characteristic morphological changes in the sperm head that occur when sperm DNA is exposed to the egg cytoplasm or through use of eggs that are loaded with a dye such as Hoechst to observe dye transfer to the sperm DNA (7, 8). Sperm-egg adhesion needs to be distinguished from the mere attachment of sperm. As is done in other cell adhesion assays, which define an adherent cell as one that stays bound after a defined series of washes (9), washes are required to remove loosely attached sperm to distinguish the sperm that are truly bound (5, 6). A key variable in IVF assays is sperm concentration (or sperm-egg ratio) used in the insemination, which can impact the extent of binding and fusion. Although a dramatic reduction in gamete interaction observed with eggs challenged with high numbers of sperm is an exciting result, there are also cases in which a difference can be detected with experimental conditions that less heavily favor fertilization [i.e., lower sperm-egg ratios (10, 11)]. Such assay modifications detect more subtle abnormalities in gamete interaction, but as noted in the main text, both robust and subtle phenotypes likely have implications for human reproductive health. 478 Evans A nn u. R ev . P hy si ol . 2 01 2. 74 :4 77 -5 02 . D ow nl oa de d fr om w w w . a nn ua lr ev ie w s. or g by U ni ve rs ity o f Sy dn ey o n 04 /2 8/ 13 . F or p er so na l u se o nl y. PH74CH21-Evans ARI 9 January 2012 11:26 Nucleus Cumulus layer Zona pellucida First polar body Metaphase II spindle