香京julia种子在线播放

Sexual reproduction requires the fusion of two gametes in a critical multistep process termed fertilization. To ensure the success of fertilization, each step needs to proceed in a very precise manner. One of the key steps that ensures successful fertilization is acrosome reaction (AR). Sperm-egg fusion is a carbohydrate-dependent event that takes place via interaction between several glycan-binding molecules (receptors) present on the sperm plasma membrane with their corresponding glycans (ligands) localized to the zona pellucida (oocyte) (Yanagimachi, 1994; Tulsiani et al., 1997; Shur, 1998; Töpfer-Petersen, 1999; Wassarman, 1999). This irreversible interaction of the gametes leads to a calcium-mediated signal transduction cascade of events that results in the release of the acrosomal contents via exocytosis, which is termed AR. Several hydrolytic and proteolytic acrosomal enzymes are released in order to facilitate sperm fusion with the oocyte (Ikawa et al., 2010). Any structural or functional acrosomal abnormality could impair sperm fusion, and ultimately result in infertility. Moreover, studies have shown that intra-cytoplasmic insemination with sperm containing acrosomal abnormalities did not lead to successful fertilization, even in the absence of fertilization barriers, because the oocyte was unable to be efficiently activated (Nasr-Esfahani et al., 2008, 2010a,b). Only intracytoplasmic sperm injection (ICSI) followed by assisted oocyte activation with calcium ionophore was found to achieve high live birth-rates (Shang et al., 2019). Thus, the acrosome is indispensable for fertilization.

Acrosome biogenesis in mammals is accompanied with spermatid differentiation during spermiogenesis, which is characterized by the transformation of a spermatid into a spermatozoon. The process continues throughout the reproductive lifespan of the male. Although the acrosome morphology varies from species to species, two basic parts make up the acrosome in all mammals: (1) a large anterior part that varies in shape (paddle, hook, and spatula-like) and in size (Bedford, 2014) and (2) an equatorial segment (ES), which is the smaller and thinner part of the acrosome found in the middle of the sperm head. The acrosomal contents are enclosed in a single membrane that is generally divided into an outer acrosomal membrane (OAM) and an inner acrosomal membrane (IAM). The OAM lies immediately beneath the plasma membrane of the spermatid and both of these membranes fuse at the time of AR (Yanagimachi, 2011). The IAM lies above the nuclear envelope as a cap and does not fuse during AR. The luminal contents are heterogeneous and are usually categorized as soluble and particulate material. The soluble material is comprised of hydrolytic enzymes that take part in AR and help to disperse the oocyte coverings. The particulate material is the acrosomal matrix that facilitates the sperm-oocyte interaction during fertilization by providing a stable protein scaffold (Buffone et al., 2004; Suryavathi et al., 2015).

Researchers have characterized the process of spermiogenesis in mice, and they have identified 16 steps, with acrosome biogenesis being a key event. Acrosome biogenesis is classically divided into four major phases: Golgi (1–3 steps), cap (4–7 steps), acrosome (elongation) (8–12 steps), and maturation (final) phases (13–16 steps) (Figure 1). This four-phase division of acrosome biogenesis was proposed more than half-a-century ago, based mainly on light microscopic analysis of testicular sections stained with periodic acid–Schiff (PAS) (Clermont and Leblond, 1955).

The 1st phase is called the Golgi phase because the Golgi apparatus is an essential organelle that supports early spermiogenesis (Leblond and Clermont, 1952; Hess, 1990; Russell et al., 1993). During the 1st phase, the Golgi apparatus is very active in producing several glycoproteins, and the trans-Golgi network gives rise to several small proacrosomal vesicles that are required for the formation of a mature acrosome. These proacrosomal granules fuse to form a large solitary acrosomal granule near the concave region of the nuclear surface. The central part of the acrosomal granule is bound to the nuclear envelope while the peripheral part is associated with the perinuclear theca (BOX 1; Figure 2). In the 2nd (Cap) phase, the acrosomal granule becomes enlarged with glycoprotein-rich contents. Moreover, it begins to flatten upon touching the nuclear envelope, and spreads over the nucleus to form a cap. At the same time, the Golgi complex moves to the nascent neck region located in the distal end. The acrosomal granule gradually covers 1/3 of the nuclear surface and spreads, transforming into a very thin layer (Figure 1). Near the distal end of the developing acrosome, the “acroplaxome” can be found, which is an important structure that supports spermiogenesis. It consists of a marginal ring, which consists of an acrosomal plate that is made up of keratin and F-actin (BOX 1; Figure 2). At the time of elongation of the spermatid head, the marginal ring is associated with the growing edge of the acrosome and the nuclear surface (nuclear plate). Thus, acroplaxome not only binds the acrosome with the nucleus but also ensures the developing acrosomal cap remains anchored to the nuclear envelope undergoing elongation (Kierszenbaum et al., 2003). In the 3rd (acrosomal) phase, the acrosomal system begins to migrate over the ventral surface of the elongating spermatid nucleus and this migration ends in step 14 spermatid (BOX 1; Figure 1; Hess and de Franca, 2008). At this stage, the acrosome undergoes condensation and attaches itself to the IAM, while chromatin also undergoes intense condensation. Several cytoskeletal proteins including calmodulin (Camatini et al., 1992), actin (Talbot and Kleve, 1978), and α-spectrin-like antigens (Virtanen et al., 1984), play very important roles in the acrosomal organization. The elongating spermatids show initiation of manchette microtubule formation near the nuclear ring region (perinuclear ring), thinning of the cytoplasm and a gradual orientation of the acrosome toward the overlying plasma membrane (BOX 1; Figure 2). The 4th (maturation) phase involves a few changes in nuclear morphology and acrosomal migration. Condensation of the nucleus continues and acrosomal granule spreads over the entire acrosomal membrane and the acrosome differentiates into anterior and posterior regions. The anterior portion becomes the acrosome apex, while the rest of the acrosome covers nearly all the nuclear surface, except the part attached to the sperm tail (Russell et al., 1993). Moreover, excess cytoplasm, cytoplasmic components (lipids) and multiple unwanted organelles including mitochondria, vesicles, and ribosomes are disposed of in the form of cytoplasmic droplets prior to spermiation (Figure 1; Hess et al., 1993; Russell et al., 1993; De Franca et al., 1995). The resulting residual bodies released from spermatids are taken up and digested by Sertoli cells. Although, the morphogenetic process of the acrosome is well studied, the precise molecular mechanism of sperm acrosome biogenesis is not yet fully understood. Here, we describe the most recent progress that has been made in understanding the molecular mechanism underlying acrosome biogenesis.

Several ER and Golgi-associated proteins actively participate in acrosome biogenesis. The endoplasmic reticulum (ER) is the main site of protein synthesis and folding (Vitale et al., 1993), while the Golgi apparatus directs glycosylation, processing, and sorting of newly synthesized proteins by the ER. The biosynthesis of some acrosome-specific proteins, such as acrosin, starts during the meiotic pachytene stage and continues as round spermatids enter the elongation stage (Anakwe and Gerton, 1990; Kashiwabara et al., 1990; Escalier et al., 1991). These proteins then enter the exocytic route and are transported to their respective targeted area in the form of proacrosomal granules that originate from the Golgi apparatus. The presence of these proacrosomal granules in the pachytene stage has been confirmed by numerous studies (Nicander and Ploen, 1969; Fawcett, 1975; Anakwe and Gerton, 1990; Suarezquian et al., 1991; Ramalho-Santos et al., 2002). Moreover, the presence of several ER-associated proteins such as protein O-mannosyltransferase 1 (POMT1) (Prados et al., 2007), POMT2 (Willer et al., 2002) and Calreticulin (Nakamura et al., 1993), have also been detected in the acrosome. Another ER-associated protein that has been detected in mouse testes is HSP90B1 (gp96/Grp94; glucose-related protein 94) (Asquith et al., 2005; Yang and Li, 2005). Germ cell-specific Hsp90b1 knockout resulted in spermatozoa characterized by globular/abnormal heads, similar to those in globozoospermia syndrome (Audouard and Christians, 2011). Therefore, HSP90B1 has been suggested to be a testis-specific chaperone and to be required for the proper folding of acrosomal proteins. The impairment of protein folding during HSP90B1 deficiency suggests an important role of ER protein folding in acrosome biogenesis. β-Glucosidase 2 (GBA2) is another ER-associated protein, and is involved in the metabolism of bile acid–glucose conjugates (Matern et al., 1997). GBA2 disruption results in the formation of abnormal spermatozoa characterized by enlarged heads and the absence of acrosome (Yildiz et al., 2006). In short, these proteins are indispensable for acrosome biogenesis.

In addition, the highly dynamic trafficking of the Golgi-derived vesicles is also involved in acrosome biogenesis. For instance, several Golgi proteins, including Golgin-95/GM130, Golgin-97, Giantin, and β-COP have been found in acrosomal associated membranes (Moreno and Schatten, 2000; Moreno et al., 2000; Ramalho-Santos et al., 2001; Hermo et al., 2010). Among these proteins, β-COP and Clathrin have been described to participate in anterograde and retrograde transport of vesicles during acrosome formation (Martínez-Menárguez et al., 1996b; Moreno et al., 2000; Ramalho-Santos et al., 2001). Golgi-derived vesicle trafficking during acrosome biogenesis can generally be divided into three steps: vesicle formation, trafficking, and fusion. Some of the important proteins that contribute to these three events are described below.

Vesicle formation and trafficking are key events of acrosome biogenesis. Stromal membrane-associated protein 2 (SMAP2) regulates the production of clathrin-coated vesicles from the trans-Golgi network (TGN) by interacting with CALM (Clathrin and the Clathrin assembly protein) and Syntaxin 2 (a component of SNARE complex that helps in membrane fusion), contributing to acrosome biogenesis (Funaki et al., 2013). GOPC (Golgi-associated PDZ- and coiled-coil motif-containing protein) was identified as a frizzled-interacting protein that is involved in vesicular trafficking from the Golgi apparatus (Yao et al., 2001). GOPC is predominantly localized in the trans-Golgi region in round spermatids and plays an important role in the transport and fusion of the proacrosomal vesicles with the growing acrosome (Yao et al., 2002). Golgin subfamily A member 3 (GOLGA3) is another Golgi-associated protein that is highly expressed during the round spermatid stage and is believed to contribute to acrosome biogenesis by interacting with GOPC (Banu et al., 2002; Hicks and Machamer, 2005; Bentson et al., 2013). Another important protein that is involved in protein transport is protein interacting with C kinase 1 (PICK1), which is primarily localized around the Golgi apparatus in spermatids (Arvan and Castle, 2013). PICK1 regulates vesicle trafficking from the Golgi apparatus to the developing acrosome by interacting with GOPC (Xiao et al., 2009). Hence, Golgi-associated proteins are of great significance in vesicle formation and trafficking during acrosome biogenesis. Autophagic machinery also participates in acrosome biogenesis by regulating vesicular trafficking. Autophagy refers to the intracellular catabolic pathway which is responsible for the degradation and recycling of organelles and cytosolic proteins via autophagosomes (double-membrane vesicle) (Yang and Klionsky, 2009; Mizushima and Levine, 2010). Microtubule-associated protein 1A/1B-light chain 3 (LC3) activated by autophagy related protein 7 (ATG7) is delivered to Golgi-derived vacuoles either directly or indirectly (via phagophores) where the activated protein either facilitates the fusion of vesicles or guides them toward the nucleus (Wang et al., 2014). These investigations suggest that the autophagic pathway might be involved in acrosome biogenesis.

Proacrosomal granules undergo fusion with each other to form a single large acrosomal granule at the nuclear surface. Vesicle fusion requires SNARE complexes that help in the fusion of opposing membranes (Rizo and Südhof, 2002; Ungermann and Langosch, 2005; Zhao and Brunger, 2016). Moreover, the disruption of fatty acid desaturase 2 (FADS2) results in Syntaxin 2 scattering, which eventually impairs acrosome formation (Roqueta-Rivera et al., 2011). TATA element Modulatory Factor (TMF/ARA160) is another Golgi-associated protein required for the fusion of vesicles to the targeted membrane (Bel et al., 2012; Miller et al., 2013), and interruption of its expression leads to a complete absence of the acrosomes, suggesting that TMF/ARA160 probably supports the transport and docking of proacrosomal vesicles to the nucleus (Lerer-Goldshtein et al., 2010). Human Rev-binding (HRB) is another critical protein required for the docking of Golgi-derived proacrosomal vesicles; it binds to the cytosolic side of proacrosomal vesicles and links the Golgi apparatus and the nuclear surface (Kang-Decker et al., 2001). Polypeptide N-acetylgalactosaminyltransferase 3 (GALNT3) is located in the cis-medial region of the Golgi, and its disruption leads to failure of proacrosomal vesicles fusion and transport to nuclear surface suggesting the significance of protein O-glycosylation in acrosome biogenesis (Miyazaki et al., 2013). Besides, many other proteins important for vesicle formation or trafficking such as SMAP2 and GOPC also contribute to the fusion of proacrosomal vesicles (Yao et al., 2002; Funaki et al., 2013).

In addition to the transportation to the concave region of the nuclear surface and fusion to form a single large acrosomal granule, the attachment and spreading of the acrosome over the nucleus is also very important to its function. Sperm acrosome-associated 1 (SPACA1), an acrosomal membrane protein, participates in the process of acrosome attachment to the nucleus and the disruption of SPACA1 leads to the detachment of the acrosome from the nucleus (Fujihara et al., 2012). Zona pellucida-binding protein 1 (ZPBP1) is another important protein localized on the periphery of the acrosomal membrane (Lin et al., 2007; Yu et al., 2009), its absence results in the compaction failure of the acrosome and subsequently leads to acrosome fragmentation (Lin et al., 2007). Fer Testis (FerT) is a member of Fes/Fps (nonreceptor tyrosine kinases family), and it regulates cytoskeletal reorganization, cell adhesion, and vesicular transport (Greer, 2002) by attaching itself to the cytosolic surface of OAM and coexists with phosphorylated cortactin (an F-actin regulator protein) in the acroplaxome (Kierszenbaum, 2006; Kierszenbaum et al., 2008). Developmental pluripotency-associated 19-like 2 (DPY19L2) acts as a bridge between the nucleus and the acroplaxome, and its deficiency leads to a loss of the acrosome due to disruption of the nuclear/acroplaxome junction (Pierre et al., 2012). Similarly, disruption of Lamin A/C (a structural component of the nuclear envelope) (Dittmer and Misteli, 2011) leads to acrosome fragmentation and malformed acrosome (Shen et al., 2014). A vast array of specific proteins is involved in acrosome biogenesis and any defect may result in malformation of the acrosome and eventually lead to infertility (Table 1; Figure 3).

Although most acrosomal substances are transported to the developing acrosome via the ER-Golgi route (Fawcett and Hollenberg, 1963; Clermont and Tang, 1985), numerous other routes are also suspected to exist for the transfer of acrosomal components to the developing acrosome. Toshimori (1998) has reported the existence of an extra-Golgi tract and a Golgi tract, which includes a Golgi-acrosomal granule tract and a Golgi-head cap tract (Toshimori, 1998).

Previously, immunocytochemical investigations of glycoprotein synthesis in the Golgi established the acrosome as a direct Golgi derivate (Friend and Fawcett, 1974; Tang et al., 1982; Aguas and da Silva, 1985; Anakwe and Gerton, 1989; Moreno et al., 2000). Later, the acrosome was proposed to be a specialized lysosome based on an acidic pH, protease activities and the presence of hyaluronidase (Hartree and Srivastava, 1965; Allison and Hartree, 1970). A non-lysosomal origin of the acrosome has also been proposed (Martínez-Menárguez et al., 1996a). Martínez-Menárguez et al. (1996a) reported the absence of two well known lysosomal markers; lysosomal membrane glycoprotein (Igp) 120 and mouse Lamp-1 in acrosomal membranes. Moreover, acrosomal and proacrosomal vesicles both lacked two important endosomal markers, cation-dependent and -independent mannose 6-phosphate receptors suggesting some lysosomal features are absent in the acrosomes (Martínez-Menárguez et al., 1996a). In addition, small GTPases were found to be associated with acrosome development (Ramalho-Santos et al., 2001), leading researchers to postulate that the acrosome is a unique cellular organelle, and could be considered a secretory granule (Moreno, 2003). More recently, the acrosome has been suggested as a novel lysosome-related organelle (LRO) (Berruti et al., 2010; Berruti and Paiardi, 2011, 2015). The details of our understanding about acrosome biogenesis are given below:

Although the origin and the first appearance of acrosomes are yet to be determined, it could have originated from the simplest eukaryotes, such as yeast. A prerequisite for yeast mating is the formation of shmoo tips/ projections (Merlini et al., 2013), which show several features similar to the typical acrosome, such as the presence of degradative enzymes and the trafficking of some vesicles. Therefore, the primary form of the acrosome could have developed very early in the evolutionary history. Nevertheless, the existence of a true acrosome can be traced back to the evolution of heterogamy, heterozygosity, and cross-fertilization. Initially, all organisms were homogametic, and most were self-fertilizing. Later, some organisms evolved cross-fertilization. To better adapt the ever-changing circumstances of the earth (Shields, 1982; Schmidt-Rhaesa, 2007; Stearns, 2013), protective coverings around eggs developed to ensure gamete integrity, and sperms needed to arm with a powerful weapon in its arsenal, the acrosome, to evade these protective gamete vestments (Schmidt-Rhaesa, 2007). This review provides a brief historical overview and highlights new break-through on acrosome biogenesis. The acrosome might be generated from a combination of many membrane trafficking systems during gametes fusion, and it might have evolved during the arms race between sperm and ovum. A collective effort to uncover unidentified components, their interactions, and their regulatory mechanism(s) is urgently needed to elucidate a more complete picture of this highly complicated secretory vesicle. Our current era definitely will be a time of deep understanding of acrosome biogenesis.

MK collected the data, drew the figures, and wrote the manuscript. HG revised the figures and the manuscript. WL proposed the idea and revised the manuscript.