Spontaneous Acrosome Reaction Research Articles

Relocalization of Izumo and beta1-integrin during spontaneous acrosome reaction

Relocalization of Izumo and beta1-integrin during spontaneous acrosome reaction

Role of complement regulatory proteins CD46, CD55 and CD59 in reproduction

In humans, CD46 has been detected on the acrosomal membrane in sperm, in contrast to widespread surface expression on somatic cells where it plays a key role in the protection from complement attack. In rodents, CD46 is expressed solely on the acrosomal membrane of mature sperm and their immediate precursors, spermatids. A monoclonal antibody against the short consensus repeat (SCR1) ectodomain of CD46 blocks binding of human sperm to zona-free oocytes in vitro. However, CD46-knockout mice are fertile and have an accelerated spontaneous acrosome reaction. Wild-caught field mice (Apodemus) also exhibit a rapid acrosome reaction and CD46 is not expressed in Apodemus sperm. CD46 may, therefore, play a role in stabilization of the acrosomal membrane. Two other complement regulatory proteins, CD55 and CD59, are localized on the plasma membrane of mammalian sperm. In human sperm, CD55 and CD59 are expressed also on the inner acrosomal membrane. It remains to be clarified what is the role of CD46, CD55 and CD59 during fertilization and what are the advantages of not expressing CD46 in field mice sperm.

Expression of Tas1 taste receptors in mammalian spermatozoa: functional role of Tas1r1 in regulating basal Ca²⁺ and cAMP concentrations in spermatozoa.

BackgroundDuring their transit through the female genital tract, sperm have to recognize and discriminate numerous chemical compounds. However, our current knowledge of the molecular identity of appropriate chemosensory receptor proteins in sperm is still rudimentary. Considering that members of the Tas1r family of taste receptors are able to discriminate between a broad diversity of hydrophilic chemosensory substances, the expression of taste receptors in mammalian spermatozoa was examined.Methodology/Principal FindingsThe present manuscript documents that Tas1r1 and Tas1r3, which form the functional receptor for monosodium glutamate (umami) in taste buds on the tongue, are expressed in murine and human spermatozoa, where their localization is restricted to distinct segments of the flagellum and the acrosomal cap of the sperm head. Employing a Tas1r1-deficient mCherry reporter mouse strain, we found that Tas1r1 gene deletion resulted in spermatogenic abnormalities. In addition, a significant increase in spontaneous acrosomal reaction was observed in Tas1r1 null mutant sperm whereas acrosomal secretion triggered by isolated zona pellucida or the Ca2+ ionophore A23187 was not different from wild-type spermatozoa. Remarkably, cytosolic Ca2+ levels in freshly isolated Tas1r1-deficient sperm were significantly higher compared to wild-type cells. Moreover, a significantly higher basal cAMP concentration was detected in freshly isolated Tas1r1-deficient epididymal spermatozoa, whereas upon inhibition of phosphodiesterase or sperm capacitation, the amount of cAMP was not different between both genotypes.Conclusions/SignificanceSince Ca2+ and cAMP control fundamental processes during the sequential process of fertilization, we propose that the identified taste receptors and coupled signaling cascades keep sperm in a chronically quiescent state until they arrive in the vicinity of the egg - either by constitutive receptor activity and/or by tonic receptor activation by gradients of diverse chemical compounds in different compartments of the female reproductive tract.

SPINK3 modulates mouse sperm physiology through the reduction of nitric oxide level independently of its trypsin inhibitory activity

Serine protease inhibitor Kazal-type (SPINK3)/P12/PSTI-II is a small secretory protein from mouse seminal vesicle which contains a KAZAL domain and shows calcium (Ca(2+))-transport inhibitory (caltrin) activity. This molecule was obtained as a recombinant protein and its effect on capacitated sperm cells was examined. SPINK3 inhibited trypsin activity in vitro while the fusion protein GST-SPINK3 had no effect on this enzyme activity. The inactive GST-SPINK3 significantly reduced the percentage of spermatozoa positively stained for nitric oxide (NO) with the specific probe DAF-FM DA and NO concentration measured by Griess method in capacitated mouse sperm; the same effect was observed when sperm were capacitated under low Ca(2+) concentration, using either intracellular (BAPTA-AM) or extracellular Ca(2+) (EDTA) chelators. The percentage of sperm showing spontaneous and progesterone-induced acrosomal reaction was significantly lower in the presence of GST-SPINK3 compared to untreated capacitated spermatozoa. Interestingly, this decrease was overcome by the exogenous addition of the NO donors, sodium nitroprusside (SNP), and S-nitrosoglutathione (GSNO). Phosphorylation of sperm proteins in tyrosine residues was partially affected by GST-SPINK3, however, only GSNO was able to reverse this effect. Sperm progressive motility was not significantly diminished by GST-SPINK3 or BAPTA-AM but enhanced by the addition of SNP. This is the first report that demonstrates that SPINK3 modulates sperm physiology through a downstream reduction of endogenous NO concentration and independently of SPINK3 trypsin inhibitory activity.

Effect of the addition of beta-mercaptoethanol to a thawing solution supplemented with caffeine on the function of frozen-thawed boar sperm and on the fertility of sows after artificial insemination

We have reported that artificial insemination (AI) with frozen-thawed boar semen supplemented with caffeine increased the number of uterine sperm by inhibiting the migration of polymorphonuclear leukocytes (PMNs) into the uterine lumen, thereby improving the fertility of gilts and sows. The objective of the present study was to examine the effects of the addition of the antioxidant beta-mercaptoethanol (bME) and caffeine to the thawing solution on the function of frozen-thawed sperm, on the phagocytic activity of PMNs for sperm, and on the fertility of sows after AI. When frozen-thawed sperm were cultured in the presence of 25 or 50 μm bME, sperm capacitation and spontaneous acrosome reactions were inhibited (P < 0.01). There was no effect of bME on phagocytic activity of PMNs for sperm in vitro. When hormonally treated (400 IU of equine chorionic gonadotropin + 200 IU of human chorionic gonadotropin) weaned sows experienced a single intrauterine insemination with frozen-thawed sperm (25 × 108 sperm per 50 ml dose) 40 h after subsequent hCG administration, pregnancy and farrowing rates were unaffected by the addition of 50 μm bME (pregnancy rate, 20 vs 21% in controls; farrowing rate, 20 vs 21%; n = 15 and 14, respectively). However, litter size tended to be higher than in the presence of 50 μm bME compared to its absence (10.0 ± 1.0 vs 5.7 ± 1.5, respectively; P < 0.07). Thus, the addition of bME to the thawing solution containing caffeine could be of benefit for improving the function of frozen-thawed sperm without influencing the phagocytic activity of PMNs for sperm. Although there were no statistically significant effects of bME on pregnancy or farrowing rates, the litter size tended to be higher in the sows subjected to a fixed-time single AI treatment with synchronized ovulation.

Replacement of calcium for strontium in hamster sperm incubation media: effect on sperm function

Calcium (Ca(2+)) is an absolute requirement for a decisive sperm function event: the acrosome reaction (AR). Physiologically, sperm capacitation is a prerequisite for this specialized exocytosis and both events are intimately related. In an effort to separate capacitation from AR, we have been using a modified sperm incubation medium where Ca(2+) is replaced by Strontium (Sr(2+)). The aim of this report is to analyze with more detail the difference between sperm incubated with Ca(2+) or Sr(2+) in several events. We found that sperm undergo the capacitation-related changes in the chlortetracycline (CTC) pattern and tyrosine phosphorylation, and also bind to the zona pellucida (ZP) when using Sr(2+)-instead of Ca(2+)-containing media. However, the spontaneous AR typical of hamster sperm does not take place in Sr(2+)-medium, even if sperm are previously capacitated with Ca(2+). Nevertheless, Sr(2+) was able to sustain AR when cells were treated with thapsigargin or depolarized with K(+) in Na(+)-depleted medium. Considering that the absence of Na(+) increased spontaneous AR in Sr(2+)-medium, we tested whether Na(+)-transport systems could be involved in the inability of Sr(2+)-incubated sperm to undergo AR. We found that when sperm incubated in Sr(2+)-medium are treated with amiloride to inhibit epithelial Na(+) channel (ENaC), they are able to undergo spontaneous AR. The same result was obtained when analyzing AR on the ZP. On the contrary, addition of ouabain (a Na(+)/K(+)-ATPase inhibitor) or DIDS (a Na(+)/HCO3(-) co-transporter inhibitor) showed no effect. These results suggest that, differing from what happens in Ca(2+)-incubated sperm, cells incubated in Sr(2+)-modified medium would have an active ENaC.

Exogenous glucose improves long-standing human sperm motility, viability, and mitochondrial function

A relevant fraction of human sperm can be maintained in simple phosphate-buffered saline (PBS) for more than 1 week at room temperature, a time that could be doubled if glucose was available. This does not seem to be directly due to sperm mitochondrial activity.

Group X secreted phospholipase A2 specifically decreases sperm motility in mice

Different mammalian secreted phospholipases A(2) (sPLA(2) s) are expressed in male reproductive organs and/or in sperm cells but their cellular functions are still not fully characterized. Because several reports indicate a link between cellular lipids and sperm motility, we have investigated the effect of mouse group IIA, IID, IIE, V, and X sPLA(2) s on sperm motility. Among these enzymes, only mouse group X sPLA(2) (mGX sPLA(2) ) acts as a potent inhibitor of sperm motility that decreases track speed (VCL) and lateral displacement of the head (ALH) of both noncapacitated and capacitated sperm. The inhibitory effect of mGX sPLA(2) is dependent on its enzymatic activity because (i) both the proenzyme form of mGX sPLA(2) (pro-mGX) and the H48Q mutant of mGX sPLA(2) have very weak enzymatic activity and are unable to modulate sperm motility and (ii) LY329722, a specific inhibitor of sPLA(2) s, blocks the inhibitory effect of mGX sPLA(2) . Moreover, mGX sPLA(2) exerts a gradual potency on sperm subpopulations with different velocities, an effect which may be linked to the heterogeneity of lipid composition in these sperm subpopulations. Finally, we found that endogenous mGX sPLA(2) released during spontaneous acrosome reaction modulates sperm motility of capacitated sperm. Together, our results suggest a new role of sPLA(2) in sperm physiology where the sPLA2 selects a sperm subpopulation for fertilization based on its effect on sperm motility.

Sperm acrosome reaction detection in intrauterine insemination

To investigate the relationship between sperm acrosome reaction (AR) and the clinical pregnancy rate of intrauterine insemination (IUI). We detected the sperm spontaneous AR rate and Ca2+ ionophore A23187-induced AR rate in 128 patients who accepted IUI treatment, collected their clinical data and analysed the relationship between sperm AR rate and clinical pregnancy rate of IUI. There was no statistical difference between the spontaneous AR rates in the pregnant group and the non-pregnant group (7.7% vs. 7.0%, P>0.05), but there was statistical difference between the induced AR rates(51.9 % vs. 43.5%, P<0.05). There was statistical difference in the clinical pregnancy rate among the 3 IUI groups divided by induced AR rate (≤20.0%, 20.1%-49.9%, and ≥50.0%; 4.8% vs. 12.5% vs. 18.6%, P<0.05). The spontaneous AR rate has nothing to do with the clinical pregnancy rate of IUI, but the induced AR rate is associated with the clinical pregnancy rate of IUI.

EFEITO DOS MEIOS DILUÍDORES MMC (MÍNIMA CONTAMINAÇÃO) E LGM (LACTOSE-GEMA MODIFICADO) NA VIABILIDADE DO SÊMEN DE CÃES CONSERVADO POR 24 HORAS EM EQUITAINER®.

The aim of this study was to compare the viability of canine semen after dilution in Minimum Contamination Medium (MMC) and Modified Lactose-Egg yolk extender (LGM) and incubation at 4 oC for 12 and 24 hours. Thirteen ejaculates were collected from 5 adult dogs by digital manipulation. Macroscopic and microscopic characteristics were assessed right after collection. Semen was divided to be diluted (1:3) in MMC and LGM and subsequently chilled in Equitainer®. Seminal parameters of motility and spermatic morphology, membrane integrity (hyposmotic test), spermatic viability and spontaneous acrosome reaction (Trypan-blue Giemsa stain) were evaluated in fresh and chilled semen after 12 and 24 hours of incubation at 4 oC. No difference between extenders was identified in semen conservation after 12 hours. After 24 hours just spermatic motility was different (p>0.05). But, after both periods of conservation, semen diluted in LGM maintained most of the characteristics verified in the fresh semen. The mean of true and false acrosome reaction did not exceed 2% in both semen extenders, which demonstrates no influence of media component and incubation period on these phenomena at 4 oC. In conclusion, these results indicate that both extenders can be used in canine semen conservation at 4 oC in conteiner during 12 hours without significant changes in semen characteristics. KEYWORDS: Dogs, sperm, cooling.

The &amp;lt;i&amp;gt;in vitro&amp;lt;/i&amp;gt; anti-fertility effects of &amp;lt;i&amp;gt;Acacia nilotica&amp;lt;/i&amp;gt; on human spermatozoa function

BACKGROUND: Acacia nilotica pods have been used since ancient times as a male contraceptive even though no scientific evidence exists about its efficacy. METHODS: Crude aqueous and ethanol extracts of Acacia nilotica pods powder were administered to human spermatozoa in vitro at different concentrations and incubation periods. RESULTS: The aqueous extract of Acacia nilotica significantly decreased sperm progressive motility in a dose and time dependent manner, but no differences were observed in cell viability. The aqueous extract also caused an increase in spontaneous acrosome reaction of the treated cells. No significant differences in all the sperm functions assessed were observed when the sperm were treated with the ethanol extract. CONCLUSION: These data have demonstrated that the aqueous extract of Acacia nilotica has contraceptive properties by interrupting sperm transportation as evidenced by the reduction in progressive motility. Furthermore, the contraceptive properties have been demonstrated in vitro by inducing premature spontaneous acrosome reaction which could affect sperm-egg interaction. In vivo studies would confirm the contraceptive properties of Acacia nilotica.

In vitro assessment of the adverse effects of antiretroviral drugs on the human male gamete

This study was designed to investigate the in vitro effects of didanosine, zidovudine, saquinavir and indinavir, commonly used in highly active antiretroviral therapy, on human sperm fertility parameters. Thirty semen samples from healthy men were collected and prepared by gradient density method. Aliquots of 90% fractions with >80% motile spermatozoa were incubated for 1, 3, and 6h with different concentrations of the antiretroviral drugs (20, 40, and 80μg/ml). Sperm motility was evaluated by computer assisted sperm analysis system. Sperm mitochondrial potential was evaluated using 3,3′-dihexyloxacarbocyanine iodide (DIOC6) and the acrosome reaction was examined using pisum sativum agglutinin method. A dose-dependent decrease in sperm motility was observed with saquinavir. Saquinavir also induced a significant time and dose-dependent decrease in mitochondrial potential and an increase in spontaneous acrosome reaction. These findings indicate that, in vitro, higher doses of saquinavir have adverse effects on sperm motility, mitochondrial potential and acrosome reaction.

Capacitation suppression by mouse seminal vesicle autoantigen involves a decrease in plasma membrane Ca2+‐ATPase (PMCA)‐mediated intracellular calcium

Successful fertilization is tightly regulated by capacitation and decapacitation processes. Without appropriate decapacitation regulation, sperm would undergo a spontaneous acrosome reaction which leads to loss of fertilization ability. Seminal plasma is known to negatively regulate sperm capacitation. However, the suppressive mechanisms still remain unclear. In this study, we demonstrate the decapacitation mechanism of mouse seminal vesicle autoantigen (SVA) might target membrane sphingomyelin (SPM) and regulate plasma membrane Ca(2+)-ATPase (PMCA) activity. The SVA was shown to suppress sperm capacitation induced by a broad panel of capacitation factors (bovine serum albumin (BSA), PAF, and cyclodextrin (CD)). Furthermore, SVA significantly decreased [Ca(2+)](i) and NaHCO(3)-induced [cAMP](i). Cyclic AMP agonists bypassed the SVA's suppressive ability. Importantly, the SVA may regulate PMCA activity which was evidenced by the fact that the SVA decreased the [Ca(2+)](i) and intracellular pH (pH(i)) of sperm; meanwhile, a PMCA inhibitor (carboxyeosin) could reverse SVA's suppression of [Ca(2+)](i). The potential target of the SVA on membrane SPM/lipid rafts was highlighted by the high binding affinity of SPM-SVA (with a K(d) of ~3 µM) which was close to the IC(50) of SVA's suppressive activity. Additionally, treatment of mink lung epithelial cells with the SVA enhanced plasminogen activator inhibitor (PAI)-1 expression stimulated by tumor growth factor (TGF)-β and CD. These observations supported the membrane lipid-raft targeting of SVA. In summary, in this paper, we demonstrate that the decapacitation mechanism of the SVA might target membrane sphingolipid SPM and regulate PMCA activity to lower [Ca(2+)](i), thereby decreasing the [cAMP](i) level and preventing sperm pre-capacitation.

Prostasomes: inhibitors of capacitation and modulators of cellular signalling in human sperm

Seminal fluid inhibits sperm capacitation mainly because of its high cholesterol content. Prostasomes are the main source of cholesterol in seminal fluid. They are known to have numerous protective properties and are able to transfer proteins and lipids to spermatozoa, but their impact on capacitation and acrosome reaction (AR) is not yet well understood. The aim of this study was to determine the effects of prostasomes on human sperm capacitation and AR. After 80% Percoll selection, freshly ejaculated human spermatozoa were incubated for 3 h under capacitating conditions with prostasomes, phosphodiesterase inhibitor 3-iso-butyl-methylxantine (IBMX), or a combination of prostasomes and IBMX. Physiological concentration of prostasomes significantly decreased tyrosine phosphorylation levels of human sperm capacitation markers P110 and P80 (p < 0.01), and the proportions of capacitated (p < 0.05) and acrosome-reacted spermatozoa (p < 0.05). Prostasomes significantly increased the proportion of spermatozoa that did not incorporate propidium iodide and significantly attenuated the effect of IBMX on P110 tyrosine phosphorylation. Prostasomes had no effect on the pH(i) increase associated with capacitation. They significantly increased intracellular cAMP concentration ([cAMP](i)) and, when prostasomes and IBMX were present together, [cAMP](i) was further increased. To our knowledge, this is the first study to show clearly that prostasomes inhibit capacitation and spontaneous AR.

Sperm treatment affects capacitation parameters and penetration ability of ejaculated and epididymal boar spermatozoa

This work was designed to study how this ability is affected by different sperm treatments routinely used for in vitro fertilization (IVF) assay. In this study, boar sperm samples from epididymal or ejaculated origin were processed by three different methods: left unwashed (NW group), washed in Dulbecco's phosphate-buffered saline supplemented with 0.1% BSA (BSA group), and washed on a Percoll ® gradient (PERCOLL group). After preparation of semen samples, changes in motility patterns were studied by CASA, calcium uptake by spectrofluorimetry, and ROS generation, spontaneous acrosome reaction, and lipid disorder by means of flow cytometry. Finally IVF assays were also performed with the different semen samples and penetrability results evaluated at 2 and 4 h post insemination (hpi). Independently of the sperm treatment, epididymal spermatozoa showed higher values of progressive motility, percentage of live cells with low lipid disorder, and penetration ability at 4 hpi than the corresponding ejaculated spermatozoa. Ejaculated spermatozoa showed higher levels of calcium uptake, ROS generation and percentage of spontaneous acrosome reaction than epididymal sperm. Regarding sperm treatments, PERCOLL group showed the highest values for some motility parameters (linearity of the curvilinear trajectory, straightness, and average path velocity/curvilinear velocity), ROS generation and penetration ability at 2 and 4 hpi; however this same group showed the lowest values for sperm curvilinear velocity and lateral head displacement. From all experimental groups, ejaculated-PERCOLL -treated spermatozoa showed the highest fertilization ability after 2 hpi. Results suggest that capacitation pathways can be regulated by suitable treatments making the ejaculated sperm able to reach capacitation and fertilize oocytes in similar levels than epididymal spermatozoa, although most of the studied capacitation-associated changes do not correlate with this ability.

Expression of a truncated form of KIT tyrosine kinase in human spermatozoa correlates with sperm DNA integrity

TR-KIT, a truncated form of KIT (the KITL receptor), corresponding to the c-terminal half of the intracellular split tyrosine kinase domain, is expressed during the haploid stages of mouse spermatogenesis, and is one of the candidate sperm factors possibly involved in egg activation at fertilization. Immunocytochemistry of adult human testis, and studies of human semen samples from volunteer donors through immunofluorescence, confocal microscopy, flow cytometry, western blot and RT-PCR analyses were performed. We show that the TR-KIT is expressed during spermiogenesis in the human testis, and that it is maintained in human ejaculated spermatozoa. TR-KIT is localized both in the equatorial segment and in the sub-acrosomal region of the human sperm head. The equatorial localization of the TR-KIT persists after the spontaneous acrosome reaction. Cytometric analysis of several sperm samples from volunteer donors, showed variable degrees of the TR-KIT-specific immunolabeling, and a significant inverse correlation (Pearson's coefficient, r = -0.76, P < 0.0001, n = 23) of the TR-KIT positivity with markers of sperm damage, i.e. DNA fragmentation, as revealed by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-nick end labeling (TUNEL) analysis and the intense clusterin positivity. We also found less significant inverse correlation with altered head morphology (r = -0.47, P < 0.05, n = 23) and direct correlation with sperm forward motility parameters (r = 0.59, P < 0.01, n = 23). The TR-KIT is present in the equatorial region of human spermatozoa, which is the first sperm component entering into the oocyte cytoplasm after fusion with the egg. This localization is consistent with the function previously proposed for this protein in mice. In addition, the TR-KIT represents a potential predictive parameter of human sperm quality.

Advances in Swine In Vitro Embryo Production Technologies

Recent advances in new technologies to produce cloned and genetically modified pigs involve manipulating oocytes and/or embryos in vitro. Although a great deal of progress has been made, the current IVM-IVF systems still result in major problems: a high rate of polyspermy; and a low development rate and low quality of blastocysts for in vitro compared with the in vivo-produced embryos. This study summarizes recent advancements in IVM-IVF-IVC porcine systems. Recent methods to select monospermic embryos are also discussed. Finally, achievements in vitrification and in somatic cell nuclear transfer are discussed.

Use of Cholesterol in Sperm Cryopreservation: Present Moment and Perspectives to Future

Sperm cryosurvival rates are not optimal for most species. Therefore, new cryopreservation strategies are needed with the objective of increasing the number of surviving sperm and the quality of those sperm after thawing. Cholesterol plays important roles in many sperm functions, including effects on membrane properties. One of these effects is to stabilize membranes at low temperatures. Thus, species that produce sperm which possess high membrane cholesterol : phospholipid ratios are more resistant to cold shock than sperm with low cholesterol : phospholipid ratios. Therefore, increasing the cholesterol content of sperm membranes may be a strategy that can improve sperm quality after freeze-thawing. In this review, information is presented related to using cyclodextrins pre-loaded with cholesterol for cryopreserving sperm from different species. The topics discussed include both in vitro and in vivo assessments of sperm quality after cryopreservation, as well as how increasing sperm cholesterol content affects other sperm functions.

Insulin and IR‐β in pig spermatozoa: a role of the hormone in the acquisition of fertilizing ability

Recent studies have revealed that insulin, the main regulator of the glucose homeostasis in somatic cells, is expressed in human spermatozoa which are also able to secrete it. This study investigated the expression of insulin and insulin receptor beta in pig spermatozoa, at immunohistochemical protein and mRNA level. The immunofluorescence assay revealed that insulin and its receptor were co-localized in the sperm midpiece, while insulin was also detected in the acrosomal region. Western blot evidenced a 36 kDa band for insulin and a 95 kDa band for insulin receptor, such as reported in somatic cells. In addition, both insulin and insulin receptor transcripts were detected in pig spermatozoa. Interestingly, a possible biological role of the hormone was evidenced during pig sperm capacitation and acrosome reaction. In fact, the results showed that insulin (0.01 and 0.1 nm) can induce both the activities. A possible autocrine short loop of insulin in pig spermatozoa was suggested by the evaluation of the hormone secretion in both uncapacitated and capacitated spermatozoa. Furthermore, spontaneous sperm capacitation and acrosome reaction were stimulated by glucose and inhibited by the blockage of insulin release (nifedipine). In conclusion, this work has firstly demonstrated the expression of insulin and of its receptor, as well as the insulin secretion by pig spermatozoa, thereby suggesting an unexpected significance of the hormone in the acquisition of the male gamete fertilizing ability.

Group X phospholipase A2 is released during sperm acrosome reaction and controls fertility outcome in mice

Ejaculated mammalian sperm must undergo a maturation process called capacitation before they are able to fertilize an egg. Several studies have suggested a role for members of the secreted phospholipase A2 (sPLA2) family in capacitation, acrosome reaction (AR), and fertilization, but the molecular nature of these enzymes and their specific roles have remained elusive. Here, we have demonstrated that mouse group X sPLA2 (mGX) is the major enzyme present in the acrosome of spermatozoa and that it is released in an active form during capacitation through spontaneous AR. mGX-deficient male mice produced smaller litters than wild-type male siblings when crossed with mGX-deficient females. Further analysis revealed that spermatozoa from mGX-deficient mice exhibited lower rates of spontaneous AR and that this was associated with decreased in vitro fertilization (IVF) efficiency due to a drop in the fertilization potential of the sperm and an increased rate of aborted embryos. Treatment of sperm with sPLA2 inhibitors and antibodies specific for mGX blocked spontaneous AR of wild-type sperm and reduced IVF success. Addition of lysophosphatidylcholine, a catalytic product of mGX, overcame these deficiencies. Finally, recombinant mGX triggered AR and improved IVF outcome. Taken together, our results highlight a paracrine role for mGX during capacitation in which the enzyme primes sperm for efficient fertilization and boosts premature AR of a likely phospholipid-damaged sperm subpopulation to eliminate suboptimal sperm from the pool available for fertilization.