Fertilization with Acrosome-Reacted Mouse Sperm: Implications for the Site of Exocytosis

COMMENTARY

Fertilization with acrosome-reacted mouse sperm: Implications for the site of exocytosis

Matteo A. Avella and Jurrien Dean1 Laboratory of Cellular and Developmental Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892

or successful mammalian fertil- A B ization, sperm penetrate the outer Acrosome-intact investments of female gametes in IAM F OAM the ampulla of the oviduct and 1. Before contact with the zona pellucida Sperm fuse with the egg plasma membrane. The A Nucleus c r o s “fertilizing” sperm traverse the cumulus om e mass surrounding ovulated eggs, bind Z on Plasma p a 2. Binding to the Membrane temporarily, and then penetrate through er P Sperm iv A i e zona pellucida c Nucleus te ro the zona pellucida, a structured extracel- l ll s o lin u m e e c lular glycomatrix, to enter into the peri- s i Fusion of PM and OAM p d a a A c Acrosome exocytosis vitelline space (Fig. 1 ). Underlying e Sperm the anterior plasma membrane of the Egg Nucleus 3. Initial penetration ial Segment mouse sperm head is the acrosome, of the zona matrix IAM Equator a Golgi-derived subcellular organelle. Acrosome-reacted Sperm that approach ovulated eggs in the female reproductive tract have an intact Fig. 1. Sperm acrosome reaction. (A) The fertilizing sperm, capacitated by passage through the female acrosome. Before fertilization, the outer reproductive tract, must undergo acrosome exocytosis before fusion with the egg in the perivitelline acrosomal membrane fuses with the sperm space. The site of this acrosome reaction has been variously described as occurring (i) while sperm pass plasma membrane resulting in exocytosis through the cumulus oophorus (gray background) surrounding ovulated eggs; (ii) upon sperm binding of acrosomal contents (Fig. 1B). Only to the surface of the extracellular zona pellucida; and (iii) during initial penetration of the zona matrix. (B) The acrosome is a Golgi-derived subcellular organelle that underlies the anterior plasma membrane acrosome-reacted sperm are observed in of the sperm head (Upper Right). Upon mobilization of internal stores of calcium, the plasma membrane the perivitelline space between the egg (PM) fuses with the outer acrosomal membrane (OAM), and acrosomal contents are released through plasma membrane and the inner aspect of resultant fenestrations (Center). Acrosome-reacted sperm within the perivitelline space have an exposed the zona pellucida and only acrosome- inner acrosomal membrane (IAM) and fuse to the egg via the residual equatorial segment of the plasma reacted sperm fuse with the egg. Thus, at membrane (Lower Left). some point during sperm penetration through the investments of ovulated eggs, role (7). Alternatively, a recent investi- fuse with normal sperm (16). In each case, acrosome exocytosis must occur. But gation that used live imaging of Acr-EGFP sperm accumulate in the perivitelline A where (Fig. 1 )? This is the issue that sperm to fertilize eggs in cumulus reports space and are acrosome-reacted. Using concerns Inoue et al. (1) in their study that, with few exceptions, the fertilizing these sperm, the investigators report in PNAS. sperm is already acrosome-reacted when it that acrosome-reacted sperm are able to Ovulated eggs in the ampulla of the encounters the zona pellucida (8). fertilize normal eggs in cumulus. In a oviduct are enmeshed in the cumulus Earlier reports differ remarkably on the first experiment, normal female mice were oophorus composed of glycosamino- −/− acrosome status of sperm within the cu- mated to Izumo I male mice. Eight glycans interspersed with a gradient of mulus mass, with some investigators ob- residual cumulus cells from the ovarian hours after coitus, eggs were collected serving acrosome-intact (9, 10) and others follicle. Until recently, the prevailing from the oviduct and sperm were me- acrosome-reacted (8, 11) sperm. Further model for mice was that acrosome-intact chanically released from the perivitelline clouding the issue is the observation that Izumo sperm pass through the cumulus oophorus space. The -null sperm were in- some noneutherian mammals do not form cubated with normal eggs in cumulus, and and exocytosis is triggered by binding to fi the zona pellucida. Only acrosome-intact a cumulus mass, and the ef cacy of fertil- sperm were observed in the perivitelline sperm are observed by EM on the surface izing cumulus-free compared with cumu- space of five of 74 normal eggs (7%) 12 h of the zona pellucida and only acrosome- lus-intact eutherian eggs is variously after reinsemination. – reacted sperm are observed in the peri- reported as decreased or unchanged (12 These observations corroborate those vitelline space (2). The presence of vesic- 14). With these disparate reports, the site from earlier studies in the rabbit and ulated acrosomal shrouds on the zona for induction of acrosome exocytosis con- guinea pig. Normally, rabbit eggs accu- surface (3–5) and the reported ability of tinues to intrigue and remains a worthy mulate multiple sperm in their perivitel- solubilized zonae pellucidae or isolated focus of investigation. line space because of a porous post- mouse ZP3 mouse to induce acrosome Inoue et al. (1) now provide evidence fertilization block to zona penetration. exocytosis (6) are consistent with this that acrosome-reacted mouse sperm can Presumed to be acrosome-reacted, these model. However, the persistence of acro- penetrate the zona pellucida and fertilize sperm fertilize after reinsemination, albeit some-intact Acr-EGFP sperm on the sur- mouse eggs that develop to birth. These face of the mouse zona pellucida for elegant studies take advantage of two ge- several hours is seemingly at odds with netically modified mouse lines. The first Author contributions: M.A.A. and J.D. wrote the paper. −/− gamete binding transducing a signal that (Izumo I ) lacks a sperm surface protein The authors declare no conflict of interest. triggers acrosome exocytosis, and raises and is unable to fuse with normal eggs See companion article on page 20008. Cd9−/− the possibility that initial penetration into (15); the second ( ) lacks an egg 1To whom correspondence should be addressed. E-mail: the zona matrix plays the more important plasma membrane protein and is unable to [email protected].

www.pnas.org/cgi/doi/10.1073/pnas.1118234109 PNAS Early Edition | 1of2 Downloaded by guest on September 28, 2021 with only 24% efficiency (17). Likewise, ers, and two pups, one of which was can- Late-arriving sperm might encounter a acrosome-reacted guinea pig sperm are nibalized, were born. This confirms that dissipating cumulus mass (or no cumulus able to penetrate the zona pellucida and fertilization with acrosome-reacted sperm mass in some species) and therefore rely fertilize eggs in vitro (100%) as long as on interactions with the zona pellucida to they maintain hyperactive motility (18). Inoue et al. confirm induce acrosome exocytosis. If correct, These results indicate that acrosome-re- this formulation poses a number of in- acted sperm can penetrate the zona pel- that acrosome-reacted triguing questions. If the acrosome re- lucida and fertilize eggs, but the efficiency action occurs within the cumulus before in rabbits and mice is surprisingly low. If the fertilizing sperm reaches the surface of fl sperm can penetrate these observations accurately re ect in the zona matrix, what molecular mecha- vivo fertilization (i.e., sperm acrosome the zona matrix and nisms account for observed taxon speci- exocytosis before encountering the zona ficity in the induction of acrosome exo- pellucida), a higher rate of fertilization fertilize eggs. cytosis (5)? If both acrosome-intact and fl might be anticipated, but could re ect the acrosome-reacted sperm can bind to effect of decreased hyperactive motility, as the surface of the zona pellucida (19, 20), noted with guinea pig sperm (18). Alter- results in live births and is consistent with the recent report that images Acr-EGFP are the same or different sperm macro- natively, there may be added complexity to molecules involved in binding, or is there gamete recognition and induction of ac- sperm as they fertilize eggs in cumulus (8), redistribution of a common sperm re- rosome exocytosis not captured under the and concludes that the fertilizing sperm ceptor? Although the results of Inoue reported experimental conditions. undergo acrosome exocytosis within the et al. (1) confirm that acrosome-reacted It was not clear from the studies in cumulus before encountering the sperm can penetrate the zona matrix and rabbit and guinea pig (17, 18) if fertiliza- zona pellucida. tion or formation of two-cell embryos (the Collectively, these diverse observations fertilize eggs, our incomplete under- endpoint of these experiments) reflected raise the possibility that induction of ac- standing of the site of acrosome exocytosis a physiological outcome. Therefore, to rosome exocytosis might be triggered dif- and its triggering mechanism(s), as well as determine the developmental potential of ferently depending on the conditions the function of acrosomal contents, make eggs inseminated with acrosome-reacted encountered by the fertilizing sperm. the acrosome reaction a rich and compelling sperm, Inoue et al. (1) recover sperm from Sperm that reach the egg relatively soon environment for future investigations. the perivitelline space of Cd9-null females after ovulation would pass through the cumulus mass, which could trigger acro- ACKNOWLEDGMENTS. This work was supported mated in vivo with normal males. After by the Intramural Research Program of the Na- reinsemination of normal eggs in vitro, some exocytosis, thus preparing sperm tional Institutes of Health/National Institute of embryos were transferred to foster moth- for passage through the zona matrix (8). Diabetes and Digestive and Kidney Diseases.

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