Exposure of Sperm Head Equatorin after Acrosome Reaction and Its Fate after Fertilization in Mice1

Abstract

Equatorin is a sperm head equatorial protein, possibly involved in sperm-oocyte fusion (Toshimori et al., Biol Reprod 1998; 59:22-29). In the present work, we have shown that equatorin contained in the posterior acrosome is detectable only after spontaneous or induced acrosome reactions following fixation and permeabilization, but not in intact spermatozoa. The presence of protease inhibitors during sonication or ionophore treatments does not inhibit the exposure of the antigenic epitope. The zona-penetrated spermatozoa lying in the perivitelline space display equatorin, similar to those of the acrosome-reacted ones. After sperm-egg fusion during in vitro fertilization (IVF), the equatorin dissociates from the sperm head equatorial region and remains at the vicinity of the decondensing male pronuclei. During pronuclear apposition stage, it is pushed away from the pronuclei, possibly by the perinuclear microtubules. After first cleavage, equatorin is inherited by one of the proembryonic cells. The residual equatorin disappears after the second cleavage. Microinjected whole spermatozoa or sperm heads into the MII stage oocytes display equatorin similar to those of the perivitelline sperm. After activation, it dissociates from the sperm nuclei in a similar manner as during IVF. The mode of equatorin degeneration during fertilization is similar to those of the sperm tail components or mitochondria, but different from those of the membrane associated proteins.

A model of the acrosome reaction progression via the acrosomal membrane-anchored protein equatorin

“…Unfortunately, however, most of these molecules besides equatorin cannot be retained on the sperm surface that can reach the perivitelline space; equatorin can be retained on sperm that have reached the perivitelline space where sperm-egg fusion initiates (C Ito, K Yamatoya, K Yoshida & K Toshimori, 2007K Toshimori, -2009, unpublished data, which will be published separately). This line of evidence indicates that re-locatable molecules such as G11 antigen (Allen & Green 1995), equatorin (this study and Manandhar & Toshimori 2001), and Izumo (Inoue et al 2005, Buffone et al 2009a are possibly involved in sperm-egg fusion or the oolemma-binding process, which leads to egg activation.…”

“…To understand the details of how equatorin is translocated over the sperm surface of the equatorial segment plasma membrane during the acrosome reaction, as we previously suggested (Manandhar & Toshimori 2001), we used MN9-immunogold electron microscopy (IEM) to analyze the behavior (localization and interaction with surroundings) of equatorin; sperm analyzed were recovered from early ( Fig. 6A) and advanced stages ( Fig.…”

“…Initiation of the acrosome reaction is characterized by the start of membrane fusion between the plasma membrane and the underlying outer acrosomal membrane (OAM) over the anterior acrosome. During the acrosome reaction, many acrosomal enzymes are released from the acrosome, while some acrosomal molecules are reported to move to the cell surface (Allen & Green 1995, Manandhar & Toshimori 2001, Inoue et al 2005, Miranda et al 2009). This evidence implies that the released acrosomal molecules may interact with the pre-existing molecules on the cell surface; this event is thought to be among processes that prime sperm for playing some role(s) during sperm-egg interaction that leads to gamete membrane fusion (reviewed by Yanagimachi (1994) and Toshimori (2009)).…”

Discovery of Predictive Biomarkers for Litter Size in Boar Spermatozoa*

“…Furthermore, EQTN is preserved in the equatorial segment and exposed after acrosome reaction to facilitate sperm-oocyte interaction and fertilization (30 -32). Recent studies have demonstrated that Eqtn knockout mouse (Eqtn Ϫ/Ϫ ) spermatozoa have a deficiency in acrosome reaction and fertilization, but they have normal motility and morphology (33). These results indicate that EQTN may play a key role in acrosome reaction and sperm-oocyte interaction.…”

Fluorometric Study of Rabbit Sperm Head Membrane Phospholipid Asymmetry During Capacitation and Acrosome Reaction Using Annexin-v Fitc

“…Allen=Green [1] found that after A23187-induced acrosome reaction of guinea pig sperm, a group of equatorial segment epitopes are exposed that may play some role in the fusion of sperm and oocyte. Equatorin is a sperm head equatorial Fluorometric Study of Rabbit Sperm Head Membraneprotein possibly involved in sperm-oocyte fusion, which is detectable only after spontaneous or induced acrosome reactions [21].…”