To fuse with oocytes, spermatozoa of eutherian mammals must pass through extracellular coats, the cumulus cell layer, and the zona pellucida (ZP). It is generally believed that the acrosome reaction (AR) of spermatozoa, essential for zona penetration and fusion with oocytes, is triggered by sperm contact with the zona pellucida. Therefore, in most previous studies of sperm-oocyte interactions in the mouse, the cumulus has been removed before insemination to facilitate the examination of sperm-zona interactions. We used transgenic mouse spermatozoa, which enabled us to detect the onset of the acrosome reaction using fluorescence microscopy. We found that the spermatozoa that began the acrosome reaction before reaching the zona were able to penetrate the zona and fused with the oocyte's plasma membrane. In fact, most fertilizing spermatozoa underwent the acrosome reaction before reaching the zona pellucida of cumulus-enclosed oocytes, at least under the experimental conditions we used. The incidence of in vitro fertilization of cumulus-free oocytes was increased by coincubating oocytes with cumulus cells, suggesting an important role for cumulus cells and their matrix in natural fertilization.
The acrosome reaction is a complex, calcium-dependent reaction that results in an exocytotic event required for successful fertilization of the egg. It has long been thought that the acrosome reaction occurs upon sperm binding to the zona pellucida, a viscoelastic layer surrounding the oocyte. Recent studies have suggested that the reaction may even occur before the sperm encounters the zona, perhaps mediated by progesterone or some other agonist. It has been particularly difficult to understand differences between progesterone-induced and zona-induced reactions experimentally and whether one substance is the more biologically relevant trigger. Until this present work, there has been little effort to mathematically model the acrosome reaction in sperm as a whole. Instead, attention has been paid to modeling portions of the pathways involved in other cell types. Here we present a base model for the acrosome reaction which characterizes the known biochemical reactions and behaviors of the system. Our model allows us to analyze several pathways that may act as a stabilizing mechanism for avoiding sustained oscillatory calcium responses often observed in other cell types. Such an oscillatory regime might otherwise prevent acrosomal exocytosis and therefore inhibit fertilization. Results indicate that the acrosome reaction may rely upon multiple redundant mechanisms to avoid entering an oscillatory state and instead maintain a high resting level of calcium, known to be required for successful acrosomal exocytosis and, ultimately, fertilization of the oocyte.
Lead (Pb) exposure at high concentrations is associated with poor sperm quality, acrosome alterations, and low fertilization rate. Sperm capacitation and the acrosome reaction (AR) are required for successful fertilization. Actin polymerization is crucial for correct capacitation, and small GTPases, such as RhoA, Rac1, and Cdc42, are involved. This study aimed to evaluate the effects of Pb on sperm fertilization ability, capacitation, AR, and the mechanisms involved in mice exposed to low Pb concentrations. CD1 mice were exposed to 0.01% Pb for 45 days through their drinking water and their spermatozoa were collected from the cauda epididymis-vas deferens to evaluate the following: AR (oAR: initial, sAR: spontaneous, and iAR: induced) using the PNA-FITC assay, sperm capacitation (P-Tyr levels), actin polymerization (phalloidin-TRITC), MDA production (stress oxidative marker), the RhoA, Rac1, and Cdc42 protein levels, and the fertilization (IVF). After the treatment, the blood Pb (PbB) concentration was 9.4 ± 1.6 μg/dL. Abnormal sperm morphology and the oAR increased (8 and 19%, respectively), whereas the iAR decreased (15%) after a calcium ionophore challenge, and the actin polymerization decreased in the sperm heads (59%) and tails (42%). Rac1 was the only Rho protein to significantly decrease (33%). Spermatozoa from the Pb-treated mice showed a significant reduction in the fertilization rate (19%). Our data suggest that Pb exposure at environmental concentrations (PbB < 10 μg/dL) decreases the acrosome function and affects the sperm fertilization ability; this is probably a consequence of the low Rac1 levels, which did not allow adequate actin polymerization to occur.
The mammalian oviduct synthesizes and secretes a major glycoprotein known as oviductin (OVGP1), which has been shown to interact with gametes and early embryos. Here we report the use of recombinant DNA technology to produce, for the first time, the secretory form of human OVGP1 in HEK293 cells. HEK293 colonies stably expressing recombinant human OVGP1 (rHuOVGP1) were established by transfecting cells with an expression vector pCMV6-Entry constructed with OVGP1 cDNA. Large quantities of rHuOVGP1 were obtained from the stably transfected cells using the CELLSPIN cell cultivation system. A two-step purification system was carried out to yield rHuOVGP1 with a purity of >95%. Upon gel electrophoresis, purified rHuOVGP1 showed a single band corresponding to the 120-150 kDa size range of human OVGP1. Mass spectrometric analysis of the purified rHuOVGP1 revealed its identity as human oviductin. Immunofluorescence showed the binding of rHuOVGP1 to different regions of human sperm cell surfaces in various degrees of intensity. Prior treatment of sperm with 1% Triton X-100 altered the immunostaining pattern of rHuOVGP1 with an intense immunostaining over the equatorial segment and post-acrosomal region as well as along the length of the tail. Addition of rHuOVGP1 in the capacitating medium further enhanced tyrosine phosphorylation of sperm proteins in a time-dependent manner. After 4-h incubation in the presence of rHuOVGP1, the number of acrosome-reacted sperm induced by calcium ionophore significantly increased. The successful production of rHuOVGP1 can now facilitate the study of the role of human OVGP1 in fertilization and early embryo development.
Despite knowledge that glucose metabolism is essential for the regulation of signaling cascades in the sperm that are pre-assembled into specific areas and function at multistage for fertilization, the physiological roles of glucose in avian sperm are poorly understood. Accumulated results of studies conducted in our laboratory and others indicate that sperm possess membrane microdomains, or membrane rafts, which play important roles in several processes, including the induction of acrosome reaction (AR). When characterizing proteomes associated with chicken sperm rafts, we observed marked enrichment of glucose transporter 3 (GLUT3). Here we show that glucose uptake is mediated by membrane rafts and stimulates AR induction by activating AMP-activated protein kinase (AMPK). Using a specific antibody, we observed that GLUT3 is localized to the entire flagellum and acrosome region and highly associated with membrane rafts. The addition of glucose stimulated AR in a dose-dependent manner without affecting sperm motility. AR and glucose uptake assays were performed using both inhibitors and activators, and demonstrated that glucose-dependent AR results from the activity of a glucose transporter located in membrane rafts and associated with AMPK. To better understand the mechanism of AMPK activation by glucose, we evaluated localization and phosphorylation status of AMPK alpha, showing that glucose uptake stimulates AMPK alpha phosphorylation, leading to its complete activation. Together, these results lead us to propose a novel mechanism by which glucose uptake stimulates the AMPK signaling pathway via membrane rafts, resulting in maximal acrosomal responsiveness in avian sperm as migrating upward to a fertilization site. Glucose uptake is mediated by membrane rafts, which results in stimulation of AR induction via AMP-activated protein kinase (AMPK) activation.
ABSTRACT STUDY QUESTION In addition to sperm motility, which major biological characteristics of sperm fertility potential are associated with mitochondrial functionality? SUMMARY ANSWER Sperm fertilization capacities, including acrosin activity, acrosome reaction (AR) capability and chromatin integrity, are related to the mitochondria functionality as evaluated by the mitochondrial membrane potential (MMP). WHAT IS KNOWN ALREADY Correlative studies suggest a potential role of sperm MMP in predicting sperm fertilization ability and ensuring sperm motility. However, researches characterizing other determinants of sperm fertility potential according to MMP are lacking. STUDY DESIGN, SIZE, DURATION The sperm MMP was examined in 627 young college students in the Male Reproductive Health in Chongqing College Students (MARHCS) cohort study in 2014. Among these participants, acrosin activity and chromatin integrity were measured in 378 and 604 subjects, respectively. These two determinants of sperm fertility potential were first compared among high-, moderate- and low-MMP groups in the college population. The effects of MMP collapse caused by carbonyl cyanide 3-chlorophenylhydrazone (CCCP) on acrosin activity, AR, DNA fragmentation, reactive oxygen species (ROS) production, and ATP content in human spermatozoa were evaluated in vitro. PARTICIPANTS/MATERIALS, SETTING, METHODS The sperm MMP was evaluated by using JC-1 staining, acrosin activity was measured using a N-α-benzoyl-dl-arginine-para-nitroanilide HCl (BAPNA) substrate method, the integrity of chromatin represented by DNA fragmentation index (DFI) was measured by sperm chromatin structure assay (SCSA), AR was evaluated with chlortetracycline staining, and intracellular ROS production was evaluated with dihydroethidium. ATP concentration was determined with luciferase. Measurements were performed by spectrophotometry or flow cytometry. MAIN RESULTS AND THE ROLE OF CHANCE Nonparametric analysis revealed significantly higher acrosin activity and a lower DFI in subjects with moderate or high MMP compared to those with low MMP. After adjustment for potential confounders, increases of 7.9 and 44.4% in sperm acrosin activity and deceases of 12.0 and 25.2% in the sperm DFI were found in the moderate- and high-MMP groups, respectively. The MMP dissipation induced by CCCP caused significant declines in acrosin activity and AR capacity and increased DFI in human spermatozoa. Moreover, sperm MMP dissipation induced ROS overproduction and decreased ATP content. LIMITATIONS, REASONS FOR CAUTION We cannot exclude a contribution of leukocytes to ROS production and no size gating was used to exclude these cells from the FACS measurements. No simultaneous live-dead staining was done and a contribution of dead sperm to the MMP and acrosome assays cannot be excluded. WIDER IMPLICATIONS OF THE FINDINGS Mitochondrial functionality might be necessary to maintain sperm acrosin activity, AR and chromatin integrity. Tests of mitochondrial functionality should be developed and used independently of or in addition to conventional semen parameters in infertility diagnosis or risk-assessment processes. STUDY FUNDING/COMPETING INTEREST(S) This study was supported by the Key Program of the National Natural Science Foundation of China (No. 81630087) and the National Natural Science Foundation of China (No. 81703254). None of the authors have any competing interests to declare.
Mammalian sperm must undergo a series of biochemical and physiological modifications, collectively called capacitation, in the female reproductive tract prior to the acrosome reaction （AR）. The mechanisms of these modifications are not well characterized though protein kinases were shown to be involved in the regulation of intracellular Ca2＋ during both capacitation and the AR. In the present review, we summarize some of the signaling events that are involved in capacitation. During the capacitation process, phosphatidyl-inositol-3-kinase （PI3K） is phosphorylated/activated via a protein kinase A （PKA）-dependent cascade, and downregulated by protein kinase C a （PKCa）. PKCa is active at the beginning of capacitation, resulting in PI3K inactivation. During capacitation, PKCa as well as PP172 is degraded by a PKA-dependent mechanism, allowing the activation of PI3K. The activation of PKA during capacitation depends mainly on cyclic adenosine monophosphate （cAMP） produced by the bicarbonate-dependent soluble adenylyl cyclase. This activation of PKA leads to an increase in actin polymerization, an essential process for the development of hyperactivated motility, which is necessary for successful fertilization. Actin polymerization is mediated by PIP2 in two ways： first, PIP2 acts as a cofactor for phospholipase D （PLD） activation, and second, as a molecule that binds and inhibits actin-severing proteins such as gelsolin. Tyrosine phosphorylation of gelsolin during capacitation by Src family kinase （SFK） is also important for its inactivation. Prior to the AR, gelsolin is released from PIP2 and undergoes dephosphorylation/activation, resulting in fast F-actin depolymerization, leading to the AR.
Abstract Both transcriptionally and translationally inactive sperm need preassembled pathways into specific cellular compartments to function. Although initiation of the acrosome reaction (AR) involves several signaling pathways including protein kinase A (PKA) activation, how these are regulated remains poorly understood in avian sperm. Membrane rafts are specific membrane regions enriched in sterols and functional proteins and play important roles in diverse cellular processes, including signal transduction. Our recent studies on chicken sperm demonstrated that membrane rafts exist and play a role in multistage fertilization. These, combined with the functional importance of membrane rafts in mammalian sperm AR, prompted us to investigate the roles of membrane rafts in signaling pathways leading to AR in chicken sperm. Using 2-hydroxypropyl-β-cyclodextrin (2-OHCD), we found that the disruption of membrane rafts inhibits PKA activity and AR without affecting protein tyrosine phosphorylation; however, these inhibitions were abolished in the presence of a cyclic 3,5-adenosine monophosphate (cAMP) analog. In addition, biochemical experiments showed a decrease in cAMP content in 2-OHCD-treated sperm, suggesting the involvement of soluble adenylyl cyclase (sAC) and transmembrane adenylyl cyclase (tmAC). Pharmacological experiments, combined with transcriptome analysis, showed that sAC and tmAC are present and involved in AR induction in chicken sperm. Furthermore, stimulation of both isoforms reversed the inhibition of PKA activity and AR in 2-OHCD-treated sperm. In conclusion, our results demonstrated that membrane rafts play an important role in AR induction by regulating the cAMP-dependent pathway and that they provide a mechanistic insight into membrane regulation of AR and sperm function in birds. Membrane rafts regulate cAMP-dependent pathway via potentiating sAC and tmAC activities in chicken sperm, which enable them to enhance acrosomal responsiveness.
The sperm acrosome reaction (AR), an essential event for mammalian fertilization, involves Ca2+ permeability changes leading to exocytosis of the acrosomal vesicle. The acrosome, an intracellular Ca2+ store whose luminal pH is acidic, contains hydrolytic enzymes. It is known that acrosomal pH (pHacr) increases during capacitation and this correlates with spontaneous AR. Some AR inducers increase intracellular Ca2+ concentration ([Ca2+]i) through Ca2+ release from internal stores, mainly the acrosome. Catsper, a sperm specific Ca2+ channel, has been suggested to participate in the AR. Curiously, Mibefradil and NNC55‐0396, two CatSper blockers, themselves elevate [Ca2+]i by unknown mechanisms. Here we show that these compounds, as other weak bases, can elevate pHacr, trigger Ca2+ release from the acrosome, and induce the AR in both mouse and human sperm. To our surprise, μM concentrations of NNC55‐0396 induced AR even in nominally Ca2+ free media. Our findings suggest that alkalization of the acrosome is critical step for Ca2+ release from the acrosome that leads to the acrosome reaction. We show evidence that an elevation of acrosomal pH triggers Ca2+ release from the acrosome and induce the acrosomal reaction in both, mouse and human spermatozoa.