To acquire the capacity to fertilize the oocyte, mammalian spermatozoa must undergo a series of biochemical reactions in the female reproductive tract, which are collectively called capacitation. The capacitated spermatozoa subsequently interact with the oocyte zona-pellucida and undergo the acrosome reaction, which enables the penetration of the oocyte and subsequent fertilization. However, the spontaneous acrosome reaction (sAR) can occur prematurely in the sperm before reaching the oocyte cumulus oophorus, thereby jeopardizing fertilization. One of the main processes in capacitation involves actin polymerization, and the resulting F-actin is subsequently dispersed prior to the acrosome reaction. Several biochemical reactions that occur during sperm capacitation, including actin polymerization, protect sperm from sAR.
1. Introduction
Prior to penetrating the oocyte, mammalian spermatozoa should undergo a highly regulated process called the acrosome reaction (AR). The physiological AR is a precise regulated Ca
2+-dependent exocytotic process induced by the sperm–oocyte contact, causing a rapid increase in intracellular Ca
2+ concentrations, thereby initiating the AR
[1][2]. It is generally accepted that the physiological AR occurs as a result of the interaction of intact sperm with the oocyte zona-pellucida (ZP). Florman and Storey suggested that the ZP is the site of the AR in mice
[3], though it was also suggested that mouse sperm begin to undergo the AR in the upper isthmus of the oviduct
[4]. During IVF in mice, acrosome-intact sperm remain attached to the ZP for a longer time than reacted sperm, thereby facilitating fertilization
[5][6]. However, it has been suggested that mouse sperm that undergo the AR before contact with the oocyte ZP can still fertilize the oocyte
[7]. Pre-treatment of bovine sperm with ZP-glycoproteins causes an increase in the AR and significantly inhibits the subsequent penetration of these sperm into the oocyte, suggesting that the AR occurs after the initial interaction between the sperm and the oocyte, at least in cows
[8]. ZP isolated from various species are able to induce the AR in mice, hamsters, guinea pigs, rabbits, cows, monkeys and humans
[9].
Before initial contact with the oocyte and in order to undergo the AR, mammalian sperm must first undergo several biochemical processes in the female reproductive tract, which are collectively called capacitation (rev. in
[10]). Actin polymerization occurs during sperm capacitation and that the F-actin is then dispersed prior to the AR
[11]. Inhibition of F-actin formation during sperm capacitation results in the spontaneous acrosome reaction (sAR)
[12]. The sAR is a premature form of the AR that does not lead to productive fertilization. It is defined as an AR that occurs in sperm incubated under capacitation conditions but without any AR-inducer, while the physiological AR is defined as an AR that occurs in capacitated sperm after induction by ZP or by other known inducers such as Ca
2+-ionophores or progesterone. Morphologically, the sAR appears similar to the induced AR. However, sperm samples with a high proportion of cells that have undergone the sAR result in poor success in human IVF
[13]. In varicocele patients, the autoimmune antisperm reaction is accompanied by the presence of the sAR and a lack of induced reactions and an increase in intracellular reactive oxygen species (ROS) concentration and DNA fragmentation
[14]. Sperm in obese men show a low fertility rate and elevated sAR levels, which are associated with altered circulating levels of estradiol (E2) and sperm cholesterol content
[15]. A similar increase in the sAR was seen in spermatozoa from mice fed a high-fat diet
[16]. These results suggest that a decrease in E2 and fatty acid levels may influence spermatogenesis
[17] and may affect some steps of acrosome biogenesis that will have consequences for fertilization. The molecule 2-arachidonoylglycerol (2AG) affects the in vitro functionality of human sperm by reducing motility, inhibiting capacitation and triggering the sAR
[18]. It was shown in human sperm that 2AG inhibits the Ca
2+-channel CatSper and accumulates in the cell when the progesterone-dependent lipid hydrolase ABHD2 is blocked
[19].
The degree of the sAR in human sperm may have clinical importance in predicting the results of IVF, as it is negatively correlated with the achievement via IVF of high-quality embryos and pregnancy rate
[20]. Loading sterols into chicken spermatozoa before cryopreservation enhances their quality by inhibiting early apoptotic changes and the sAR
[21]. However, sperm from polyzoospermic men demonstrate a low sAR rate as well as low levels of Ca
2+-ionophore (A23187)-induced AR
[22][23][24]. Nevertheless, in boar sperm, the percentage of the sAR was not significantly different in fertile (4.5%) versus subfertile boars (4.75%)
[25]. Thus, there are differences among various species regarding the correlation between the sAR and fertilization rate.
In mice, the sAR renders spermatozoa fertilization incompetent
[3]. Moreover, an intact acrosome is required for the chemotaxis of mouse spermatozoa towards the oocyte
[26], indicating that spermatozoa that undergo the sAR before reaching the oocyte cumulus oophorus are unlikely to respond to the oocyte chemotactic signals. Suarez showed that 98% of rabbit sperm collected from the oviduct ampulla at the beginning of fertilization were acrosome-intact
[27], suggesting that acrosome-reacted sperm are unable to penetrate the oviduct. Thus, to achieve fertilization, the sperm must prevent the AR from occurring before contact with the oocyte.
2. Role of Reactive Oxygen Species (ROS) and Mitochondrial Activity in the sAR
Oxidative stress is currently considered to be a main cause of male infertility (rev. by
[28]). Although presence of a basal level of ROS is essential for the onset of sperm-activating processes such as capacitation
[29], its increased levels disturb sperm functions, thereby leading to male infertility by mechanisms such as lipid peroxidation and DNA damage
[30]. The levels of ROS are therefore precisely regulated in sperm, mainly by superoxide dismutase (SOD), which coverts superoxide anions to H
2O
2 [31], and by catalase
[32], which decomposes H
2O
2. Reactive oxygen species (ROS) are formed during sperm capacitation, which is important for the activation of CaMKII
[12] and PLD
[33]. Treatment of bovine sperm with 50 µM H
2O
2 causes a significant increase in CaMKII phosphorylation/activation, a state that is completely reversed by 100 µM H
2O
2 [34]. In human sperm, the addition of SOD causes a decrease in the sAR
[35]. In bovine sperm, hydrogen peroxide promotes capacitation, mimicking the role of bicarbonate in activating the soluble adenylate cyclase to activate the cAMP/PKA
[36]. Also, ROS have been implicated in protein tyrosine phosphorylation, which mediates capacitation in several species
[37]. Nevertheless, in boar sperm, ROS do not promote capacitation but stimulate the sAR
[38].
The knockout of several genes, including β-Defensin, the Lipocalin family LCN8
[39] or Aldehyde-dehydrogenase ALDH4A1, a key enzyme in mitochondrial prolin metabolism
[40], results in an increase in mouse sperm sAR levels. However, the upregulation of cytochrome C in pig sperm promotes the sAR, indicating that mitochondrial activity stimulates the sAR
[41]. The regulation of the mitochondrial electron transport chain controls the production of ROS and protects the sperm from the sAR. Thus, FER, as an important regulator of mitochondrial activity is responsible for providing ATP for various sperm functions, leading to proper fertilization
[42].
Interestingly, to prevent spermatozoa from potential oxidative stress damage, and probably from the sAR, the fatty-acid composition of rodent sperm membranes is altered by increasing the percentage of peroxidation-resistant fatty acids under competitive conditions
[43].
Paraoxonase 1 (PON1) is a high-density lipoprotein-associated enzyme that acts as an antioxidant
[44]. PON1 protects human sperm from the sAR
[45]. Endogenous semen PON1 activity is negatively associated with the sAR, suggesting that PON1 protects against the sAR by reducing ROS levels
[45]. It was also shown that a reduction in PON1 levels in semen is associated with infertility
[46].
3. Role of Energy Metabolism in the sAR
It is well known that sperm ATP is produced by glycolysis and mitochondrial respiration. The inhibition of either glycolysis or oxidative phosphorylation in bovine sperm does not affect capacitation or sAR levels; however, when both systems are inhibited, no capacitation occurs, and there is a significant increase in sAR levels
[47]. Under such ATP starvation, the increase in the sAR is triggered by Ca
2+ influx into the sperm via the CatSper cation channel. There is no change in PKA activity when glycolysis or mitochondrial respiration is inhibited, while a complete reduction in PKA activity was observed when both systems were inhibited
[47]. Protein tyrosine phosphorylation (PTP), also known to increase during sperm capacitation, was partially reduced by the inhibition of one metabolic system and completely blocked when the two metabolic systems were inhibited
[47]. These studies show that ATP, PKA and PTP are involved in the mechanisms protecting sperm from the sAR.
In pig sperm, the levels of the β-subunit of H
+-ATPase, the ATP-producing enzyme in the mitochondria, isocitrate-dehydrogenase (IDH) and pyruvate-dehydrogenase are enhanced during capacitation, while the level of enolase, a critical enzyme in anaerobic glycolysis, is decreased
[41]. IDH is the main regulatory enzyme of the Krebs cycle, and its increase during capacitation indicates the involvement of the malate–aspartate shuttle required to maintain the levels of reduced NADP necessary for capacitation. Thus, mitochondrial and glycolytic activities are involved in the mechanism of sperm capacitation and protect sperm from the sAR.
4. Role of Protein Acetylation in the sAR
Protein hyperacetylation protects bovine sperm from the sAR through an exchange protein directly activated by cAMP (EPAC) and via CaMKII-dependent and PKA-independent mechanisms
[48]. Protein acetylation, including tubulin acetylation, is involved in sperm energy metabolism and motility
[49]. Recently, several studies have described changes in the levels of acetylated proteins during human sperm capacitation
[50][51]. Different protein acetylation profiles were observed in sperm during capacitation versus fertilization, suggesting that protein acetylation is involved in the fertilization process
[51]. Changes in protein acetylation are also seen during axonemal microtubule construction
[52][53], suggesting that poor sperm motility and male infertility may be associated with perturbed tubulin acetylation
[53]. Moreover, hyperacetylation in non-capacitated mouse sperm induces capacitation-associated molecular events, including the activation of PKA and of the sperm-specific Ca
2+-channel CatSper, hyperpolarization of the plasma membrane, hyperactivated motility and an increase in the AR
[54]. Incubation of bovine sperm under non-capacitated conditions revealed a significant increase in the sAR that was reduced in the presence of deacetylase inhibitors, which caused protein hyperacetylation
[48].
This entry is adapted from the peer-reviewed paper 10.3390/ijms242317005