F-actin in guinea pig spermatozoa: its role in calmodulin translocation during acrosome reaction

A loss-of-function variant in SSFA2 causes male infertility with globozoospermia and failed oocyte activation

Globozoospermia (OMIM: 102530) is a rare type of teratozoospermia (< 0.1%). The etiology of globozoospermia is complicated and has not been fully revealed. Here, we report an infertile patient with globozoospermia. Variational analysis revealed a homozygous missense variant in the SSFA2 gene (NM_001130445.3: c.3671G > A; p.R1224Q) in the patient. This variant significantly reduced the protein expression of SSFA2. Immunofluorescence staining showed positive SSFA2 expression in the acrosome of human sperm. Liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) and Coimmunoprecipitation (Co-IP) analyses identified that GSTM3 and Actin interact with SSFA2. Further investigation revealed that for the patient, regular intracytoplasmic sperm injection (ICSI) treatment had a poor prognosis. However, Artificial oocyte activation (AOA) by a calcium ionophore (A23187) after ICSI successfully rescued the oocyte activation failure for the patient with the SSFA2 variant, and the couple achieved a live birth. This study revealed that SSFA2 plays an important role in acrosome formation, and the homozygous c.3671G > A loss-of-function variant in SSFA2 caused globozoospermia. SSFA2 may represent a new gene in the genetic diagnosis of globozoospermia, especially the successful outcome of AOA-ICSI treatment for couples, which has potential value for clinicians in their treatment regimen selections.

Actin cytoskeleton and sperm function

For the acquisition of the ability to fertilize the egg, mammalian spermatozoa should undergo a series of biochemical transformations in the female reproductive tract, collectively called capacitation. The capacitated sperm can undergo the acrosomal exocytosis process near or on the oocyte, which allows the spermatozoon to penetrate and fertilize it. One of the main processes in capacitation involves dynamic cytoskeletal remodeling particularly of actin. Actin polymerization occurs during sperm capacitation and the produced F-actin should be depolymerized prior to the acrosomal exocytosis. In the present review, we describe the mechanisms that regulate F-actin formation during sperm capacitation and the F-actin dispersion prior to the acrosomal exocytosis. During sperm capacitation, the actin severing proteins gelsolin and cofilin are inactive and they undergo activation prior to the acrosomal exocytosis.

During capacitation in bull spermatozoa, actin and PLC-ζ undergo dynamic interactions

The migration pattern of sperm-specific phospholipase C-ζ (PLC-ζ) was followed and the role of this migration in actin cytoskeleton dynamics was determined. We investigated whether PLC-ζ exits sperm, opening the possibility that PLC-ζ is the 'spermatozoidal activator factor' (SOAF). As capacitation progresses, the highly dynamic actin cytoskeleton bound different proteins to regulate their location and activity. PLC-ζ participation at the start of fertilization was established. In non-capacitated spermatozoa, PLC-ζ is in the perinuclear theca (PT) and in the flagellum, therefore it was decided to determine whether bovine sperm actin interacts with PLC-ζ to direct its relocation as it progresses from non-capacitated (NC) to capacitated (C) and to acrosome-reacted (AR) spermatozoa. PLC-ζ interacted with actin in NC spermatozoa (100%), PLC-ζ levels decreased in C spermatozoa to 32% and in AR spermatozoa to 57% (P < 0.001). The level of actin/PLC-ζ interaction was twice as high in G-actin (P < 0.001) that reflected an increase in affinity. Upon reaching the AR spermatozoa, PLC-ζ was partially released from the cell. It was concluded that actin cytoskeleton dynamics control the migration of PLC-ζ during capacitation and leads to its partial release at AR spermatozoa. It is suggested that liberated PLC-ζ could reach the egg and favour fertilization.

Role of Wasp and the small GTPases RhoA, RhoB, and Cdc42 during capacitation and acrosome reaction in spermatozoa of English guinea pigs

Cytoskeleton remodeling is necessary for capacitation and the acrosome reaction in spermatozoa. F-actin is located in the acrosome and equatorial region during capacitation, but is relocated in the post-acrosomal region during the acrosome reaction in spermatozoa from bull, rat, mice, and guinea pig. Actin polymerization and relocalization are generally regulated by small GTPases that activate Wasp protein, which coordinates with Arp2/3, profilin I, and profilin II to complete cytoskeletal remodeling. This sequence of events is not completely described in spermatozoa, though. Therefore, the aim of this study was to determine if Wasp interacts with small GTPases (RhoA, RhoB, and Cdc42) and proteins (Arp2/3, profilin I, and profilin II) that co-localize with F-actin during capacitation and the acrosome reaction in English guinea pig spermatozoa obtained from the vas deferens. The spermatozoa were capacitated in calcium-free medium, incubated with an activator or an inhibitor of GTPases, and then induced to acrosome react using calcium. The distribution patterns of F-actin were compared to the patterns of Wasp and its putative interaction partners: Wasp and RhoB, but not RhoA or Cdc42, localization overlap with F-actin during capacitation and the acrosome reaction. Activation of small GTPases localized RhoB to the post-acrosomal region whereas their inhibition prevented acrosome exocytosis. Arp2/3 and profilin II appear to interact with Wasp in the post-acrosomal region and flagellum, while profilin I and Wasp could be found in the equatorial region. Thus, Wasp and F-actin distribution overlap during capacitation and acrosome reaction, and small GTPases play an important role in cytoskeleton remodeling during these processes in spermatozoa. Mol. Reprod. Dev. 83: 927-937, 2016 © 2016 Wiley Periodicals, Inc.

Role of Actin Cytoskeleton During Mammalian Sperm Acrosomal Exocytosis

Mammalian sperm require to undergo an exocytotic process called acrosomal exocytosis in order to be able to fuse with the oocyte. This ability is acquired during the course of sperm capacitation. This review is focused on one aspect related to this acquisition: the role of the actin cytoskeleton. Evidence from different laboratories indicates that actin polymerization occurs during capacitation, and the detection of several actin-related proteins suggests that the cytoskeleton is involved in important sperm functions. In other mammalian cells, the cortical actin network acts as a dominant negative clamp which blocks constitutive exocytosis but, at the same time, is necessary to prepare the cell to undergo regulated exocytosis. Thus, F-actin stabilizes structures generated by exocytosis and supports the physiological progression of this process. Is this also the case in mammalian sperm? This review summarizes what is currently known about actin and its related proteins in the male gamete, with particular emphasis on their role in acrosomal exocytosis.

Cytoskeletal proteins F-actin and β-dystrobrevin are altered by the cryopreservation process in bull sperm

The cryopreservation process has an important impact on sperm structure and physiology. The negative effects have been mainly observed on the plasma membrane, which is directly stabilized by the cytoskeleton. Since cytoskeleton proteins are osmosensitive and thermosensitive, the aim of this study was to evaluate the damage caused to the bull sperm cytoskeleton by cryopreservation (freezing-thawing). Fresh and frozen-thawed bull semen samples were exposed to a treatment with the neutral detergent Brij 36-T. Electron microscopy evidenced important damages at the sperm perinuclear theca after the protein extraction protocol; the perinuclear theca was partially solubilized, the perinuclear theca substructure disappeared in the cryopreserved samples. Furthermore, the sperm head's shape was significantly altered on the cryopreserved samples. Fluorescence analysis showed a decrease of the intensity of actin and dystrobrevin on the frozen-thawed samples. Western blot assays revealed a stronger signal for actin and β-dystrobrevin in the frozen-thawed sperm samples than in the fresh ones. Our results suggest that the cryopreservation process highly alters the sperm cytoskeleton stability, causing its proteins to become more fragile and therefore more susceptible to be extracted.

Calpain modulates capacitation and acrosome reaction through cleavage of the spectrin cytoskeleton

Research on fertilization in mammalian species has revealed that Ca(2+) is an important player in biochemical and physiological events enabling the sperm to penetrate the oocyte. Ca(2+) is a signal transducer that particularly mediates capacitation and acrosome reaction (AR). Before becoming fertilization competent, sperm must experience several molecular, biochemical, and physiological changes where Ca(2+) plays a pivotal role. Calpain-1 and calpain-2 are Ca(2+)-dependent proteases widely studied in mammalian sperm; they have been involved in capacitation and AR but little is known about their mechanism. In this work, we establish the association of calpastatin with calpain-1 and the changes undergone by this complex during capacitation in guinea pig sperm. We found that calpain-1 is relocated and translocated from cytoplasm to plasma membrane (PM) during capacitation, where it could cleave spectrin, one of the proteins of the PM-associated cytoskeleton, and facilitates AR. The aforementioned results were dependent on the calpastatin phosphorylation and the presence of extracellular Ca(2+). Our findings underline the contribution of the sperm cytoskeleton in the regulation of both capacitation and AR. In addition, our findings also reveal one of the mechanisms by which calpain and calcium exert its function in sperm.

Impact of marine drugs on cytoskeleton-mediated reproductive events

Marine organisms represent an important source of novel bioactive compounds, often showing unique modes of action. Such drugs may be useful tools to study complex processes such as reproduction; which is characterized by many crucial steps that start at gamete maturation and activation and virtually end at the first developmental stages. During these processes cytoskeletal elements such as microfilaments and microtubules play a key-role. In this review we describe: (i) the involvement of such structures in both cellular and in vitro processes; (ii) the toxins that target the cytoskeletal elements and dynamics; (iii) the main steps of reproduction and the marine drugs that interfere with these cytoskeleton-mediated processes. We show that marine drugs, acting on microfilaments and microtubules, exert a wide range of impacts on reproductive events including sperm maturation and motility, oocyte maturation, fertilization, and early embryo development.

Possible participation of calmodulin in the decondensation of nuclei isolated from guinea pig spermatozoa

The guinea pig spermatozoid nucleus contains actin, myosin, spectrin and cytokeratin. Also, it has been reported that phalloidin and/or 2,3-butanedione monoxime retard the sperm nuclear decondensation caused by heparin, suggesting a role for F-actin and myosin in nuclear stability. The presence of an F-actin/myosin dynamic system in these nuclei led us to search for proteins usually related to this system. In guinea pig sperm nuclei we detected calmodulin, F-actin, the myosin light chain and an actin-myosin complex. To define whether calmodulin participates in nuclear-dynamics, the effect of the calmodulin antagonists W5, W7 and calmidazolium was tested on the decondensation of nuclei promoted by either heparin or by a Xenopus laevis egg extract. All antagonists inhibited both the heparin- and the X. laevis egg extract-mediated nuclear decondensation. Heparin-mediated decondensation was faster and led to loss of nuclei. The X. laevis egg extract-promoted decondensation was slower and did not result in loss of the decondensed nuclei. It is suggested that in guinea pig sperm calmodulin participates in the nuclear decondensation process.

In guinea pig sperm, aldolase A forms a complex with actin, WAS, and Arp2/3 that plays a role in actin polymerization

Glycolytic enzymes have, in addition to their role in energy production, other functions in the regulation of cellular processes. Aldolase A has been reported to be present in sperm, playing a key role in glycolysis; however, despite its reported interactions with actin and WAS, little is known about a non-glycolytic role of aldolase A in sperm. Here, we show that in guinea pig spermatozoa, aldolase A is tightly associated to cytoskeletal structures where it interacts with actin, WAS, and Arp2/3. We show that aldolase A spermatozoa treatment increases their polymerized actin levels. In addition, we show that there is a direct correlation between the levels of polymerized actin and the levels of aldolase A-actin interaction. Our results suggest that aldolase A functions as a bridge between filaments of actin and the actin-polymerizing machinery.

Characterizing mouse male germ cell-specific actin capping protein alpha3 (CPalpha3): dynamic patterns of expression in testicular and epididymal sperm

AIM

To characterize mouse capping protein alpha3 (CPalpha3) during spermatogenesis and sperm maturation.

METHODS

We produced rat anti-CPalpha3 antiserum and examined the expression of CPalpha3 in various mouse tissues using Western blot analysis and the localization of CPalpha3 in testicular and epididymal sperm using immunohistochemical analyses. We also examined how the localization of CPalpha3 and beta-actin (ACTB) in sperm changed after the acrosomal reaction by performing immunohistochemical analyses using anti-CPalpha3 antiserum and anti-actin antibody.

RESULTS

Western blot analysis using specific antiserum revealed that CPalpha3 was expressed specifically in testes. Interestingly, the molecular weight of CPalpha3 changed during sperm maturation in the epididymis. Furthermore, the subcellular localization of CPalpha3 in sperm changed dynamically from the flagellum to the post-acrosomal region of the head during epididymal maturation. The distribution of ACTB was in the post-acrosomal region of the head and the flagellum. After inducing the acrosomal reaction, the CPalpha3 and ACTB localization was virtually identical to the localization before the acrosomal reaction.

CONCLUSION

CPalpha3 might play an important role in sperm morphogenesis and/or sperm function.

In spermatozoa, Stat1 is activated during capacitation and the acrosomal reaction

A role for sperm-specific proteins during the early embryonic development has been suggested by a number of recent studies. However, little is known about the participation of transcription factors in that stage. Here, we show that the signal transducer and activator of transcription 1 (Stat1), but not Stat4, was phosphorylated in response to capacitation and the acrosomal reaction (AR). Moreover, Stat1 phosphorylation correlated with changes in its localization: during capacitation, Stat1 moved from the cytoplasm to the theca/flagellum fraction. During AR, Stat1 phosphorylation increased again. In addition, blocking protein kinase A (PKA) and PKC during capacitation abolished both phosphorylation and migration of Stat1. Our results show tight spatio-temporal rearrangements of Stat1, suggesting that after fertilization Stat1 participates in the first rounds of transcription within the male pronucleus.

Identification of cellular isoform of oviduct-specific glycoprotein: role in oviduct tissue remodeling?

The oviduct is known to secrete mucins (MUC1 and MUC9) under the influence of ovarian steroids. The secreted form of MUC1 binds gametes in the oviduct, whereas the cellular form seen in breast cancers has been implicated in cell adhesion and morphogenesis. The secreted MUC9 or oviduct-specific glycoprotein (OGP), in addition to being a mucin, belongs to family 18 glycosylhydrolases and is known to bind gametes and embryos in the oviduct. Studies in our laboratory have identified non-muscle myosin IIA (involved in cell shape, polarity, and morphogenesis) as the protein partner to OGP in gametes. In view of the crucial role of the cortical cytoskeleton in the selective internalization of tight junctions (TJs) /adherent junctions (AJs) or apical junctional complex (AJC) in simple epithelial cells during tissue remodeling, the present study has been undertaken to evaluate the existence of a cellular form of OGP in oviductal tissue, which itself undergoes cyclic tissue remodeling. In silico analysis of the deduced amino-acid sequence of OGP has revealed the presence of several conserved motifs; these imply that OGP is a component of multi-protein complexes such as TJs. Corroborative immunoelectron-microscopic analysis in peri-ovulatory oviduct epithelia in the bonnet monkey has revealed the presence of OGP at the TJ. Co-localization studies of OGP and cadherin demonstrate that, whereas OGP is localized at the tonofilaments of the TJs, cadherin is localized at the intercellular space of the AJ. The possible role of OGP in oviductal tissue remodeling is discussed in light of the present findings and those reported in the literature.

F-actin involvement in guinea pig sperm motility

Sperm motility is a must for natural fertilization to occur. During their travel through the epididymis, mammalian spermatozoa gradually acquire the ability to move. This is accomplished through a sliding movement of the outer doublet microtubules of the axoneme which is energized by the dynein ATPase. Within its complex structure, the mammalian sperm flagellum contains F-actin and thus, we decided to test in the guinea pig sperm flagellum the role of F-actin in motility. During maturation, capacitation, and the acrosome reaction, a gradual decrease of the relative concentration of F-actin was observed. Motility increased as spermatozoa became able to fertilize. Gelsolin, phalloidin, and KI inhibited sperm motility. Gelsolin canceled sperm motility within 20 min of treatment while 0.6 M KI had immediate effects. Phalloidin diminished hyperactive sperm motility slightly. All three compounds significantly increased the relative concentration of F-actin. Latrunculins are conventional drugs that destabilize the F-actin cytoskeleton. Latrunculin A (LAT A) did not affect sperm motility; but significantly increased F-actin relative concentration. The results suggested that in guinea pig spermatozoa, randomly severing F-actin filaments inhibits flagellar motility; while end filament alteration does not. Thus, specific filament regions seem to be important for sperm motility.

Identification and characterization of oviductal glycoprotein-binding protein partner on gametes: epitopic similarity to non-muscle myosin IIA, MYH 9

The mammalian estrogen induced oviductal glycoprotein (OGP) has been known to associate with capacitated sperm, oocytes and developing embryos. This study aimed to identify the putative binding partner of OGP on gametes using N-terminal peptide of bonnet monkey (Macaca radiata) OGP, Nmon, as bait. A protein(s) of molecular size approximately 54 kDa was detected by far-western blot analysis of detergent solubilized human sperm proteins. MALDI-TOF mass spectra analysis of approximately 54 kDa tryptic peptides gave a significant hit to non-muscle myosin heavy chain. Biochemical characterization of approximately 54 kDa was done with antibodies specific to non-muscle myosin IIA, MYH9. The approximately 54 kDa protein, possible breakdown product of MYH9, immunoreacted with MYH9 antibody in western blot analysis. OGP binding to approximately 54 kDa could also be demonstrated in far-western blot analysis of detergent solubilized human sperm proteins and nuclear matrix intermediate filament (NM-IF) preparations from human sperm and mouse oocytes. Far-western blot analysis of MYH9 enriched by immunoprecipitation identified the native approximately 220 kDa protein as OGP-binding partner. The identical and characteristic immunogold localization pattern of Nmon and MYH9 on sperm NM-IF preparation substantiated these findings. The results suggest that OGP binds to both gametes through its interaction with MYH9 through the non-glycosylated N-terminal conserved region of OGP, spanning the residues 11-137.

In guinea pig spermatozoa, the procaine-promoted synchronous acrosome reaction results in highly fertile cells exhibiting normal F-actin distribution

In guinea pig spermatozoa, procaine induces Ca(2+) independent hyperactivated motility suggestive of sperm capacitation. Nonetheless, in the presence of high extracellular Ca(2+), procaine increases cytoplasmic Ca(2+). We analyze the procaine effect on the acrosome reaction (AR) processes in guinea pig spermatozoa. Results indicated that: (i) in spermatozoa pre-incubated 5-30 min in MCM-PLG medium, procaine produced synchronous AR, (ii) the acrosome-reacted sperm number increased with the capacitation period before procaine treatment and with procaine concentration, (iii) acrosome reaction was blocked when Ca(2+) was omitted, (iv) plasma membrane-outer acrosomal membrane fusion started within 2 min after procaine treatment, (v) in acrosome-reacted spermatozoa, actin polymerization occurred and F-actin was located in the equatorial and post-acrosomal regions and (vi) procaine treatment resulted in highly fertile acrosome-reacted spermatozoa. This is the first report indicating that procaine promotes synchronic AR in mammalian spermatozoa. If procaine promotes premature AR of spermatozoa in vivo, it might be a factor for infertility in patients exposed to this local anesthetic.

Cytoskeleton localization in the sperm head prior to fertilization

Three major cytoskeletal proteins, actin, tubulin and spectrin, are present in the head of mammalian spermatozoa. Although cytoskeletal proteins are implicated in the regulation of capacitation and the acrosome reaction (AR), their exact role remains poorly understood. The aim of this study was to compare the distribution of the sperm head cytoskeleton before and after the AR in spermatozoa representing a range of acrosome size and shape. Spermatozoa from the human and three rodents (rat, hamster and grey squirrel) were fixed before and after the AR in appropriate medium in vitro. Indirect immunofluorescent localization of cytoskeletal proteins was undertaken with antibodies recognizing actin, spectrin and alpha-tubulin. Preparations were counterstained with propidium iodide and examined by epifluorescent and confocal microscopy. Our results clearly demonstrated changes in localization of cytoskeleton during the AR, mainly in the apical acrosome with further changes to the equatorial segment and post-acrosomal regions. The pattern of cytoskeletal proteins in the sperm head of all the species was similar in respect to various sub-compartments. These observations indicated that the sperm head cortical cytoskeleton exhibits significant changes during the AR and, therefore, support the image of cytoskeletal proteins as highly dynamic structures participating actively in processes prior to fertilization.

Actin, myosin, cytokeratins and spectrin are components of the guinea pig sperm nuclear matrix

The nuclear matrix (NM) of somatic cells is an internal nuclear framework structure, with a structural function and participation in DNA replication and transcription. The NM has been described in mouse, hamster and human spermatozoa. In this study, an NM structural component of the guinea pig sperm nucleus was obtained by removing nuclear proteins and DNA from DTT-CTAB nuclei. Removal was achieved with high ionic strength salt and microccocal nuclease treatments including a heparin treatment to cause a slight swelling of the nucleus and facilitate material extraction. Actin, myosin, cytokeratins and spectrin were detected associated to NM by indirect immunofluorescence, immunogold staining and Western blotting analysis using specific antibodies. The presence of NM in guinea pig sperm nucleus is shown for the first time and some of its components are identified. This is also the first report on cytokeratins and myosin presence in guinea pig sperm. A retarding effect of nuclear decondensation caused by heparin is induced after phalloidin and/or diacetyl-monoxime (a myosin ATPase activity inhibitor) treatment, suggesting a role for F-actin and myosin in the maintenance of nuclear stability in sperm. The actin role was supported by the decondensing effect that citochalasin D and gelsolin had on sperm nuclei.

Actin polymerization in the equatorial and postacrosomal regions of guinea pig spermatozoa during the acrosome reaction is regulated by G proteins

The acrosome reaction (AR) is an exocytotic process of spermatozoa, and an absolute requirement for fertilization. During AR, actin polymerization is necessary in the equatorial and postacrosomal regions of guinea pig sperm for spermatozoa incorporation deep into the egg cytoplasm, but not for plasma membrane (PM) fusion nor the early steps of egg activation. To identify the mechanisms involved in this sperm actin polymerization, we searched for the protein members, known to be involved in a highly conserved model, that may apply to any cellular process in which de novo actin polymerization occurs from G protein activation. WASP, Arp 2/3, profilins I and II, and Cdc42, RhoA and RhoB GTPases were localized by indirect immunofluorescence (IIF) in guinea pig spermatozoa and their presence corroborated by Western blotting. WASP and profilin II were translocated to the postacrosomal region (Arp2/3 already were there) in long-term capacitated and acrosome-reacted spermatozoa, at the same time as actin polymerization occurred. These events were inhibited by GDP-beta-S and promoted by lysophosphatidic acid (LPA) and GTP-gamma-S, a small GTPase inhibitor and two activators, respectively. By immunoprecipitation, Cdc42-WASp association was identified in capacitated but not in noncapacitated gametes. Polymerized actin in the postacrosomal region is apparently anchored both to the postacrosomal perinuclear theca region and the overlying PM. Results suggest that GTPases are involved in sperm actin polymerization, in the postacrosomal region and the mechanism for polymerization might fit a previously proposed model (Mullins, 2000: Curr Opin Cell Biol 12:91-96).

Role of actin cytoskeleton in mammalian sperm capacitation and the acrosome reaction

In order to fertilize, the mammalian spermatozoa should reside in the female reproductive tract for several hours, during which they undergo a series of biochemical modifications collectively called capacitation. Only capacitated sperm can undergo the acrosome reaction after binding to the egg zona pellucida, a process which enables sperm to penetrate into the egg and fertilize it. Polymerization of globular (G)-actin to filamentous (F)-actin occurs during capacitation, depending on protein kinase A activation, protein tyrosine phosphorylation, and phospholipase D activation. F-actin formation is important for the translocation of phospholipase C from the cytosol to the sperm plasma membrane during capacitation. Prior to the occurrence of the acrosome reaction, the F-actin should undergo depolymerization, a necessary process which enables the outer acrosomal membrane and the overlying plasma membrane to come into close proximity and fuse. The binding of the capacitated sperm to the zona pellucida induces a fast increase in sperm intracellular calcium, activation of actin severing proteins which break down the actin fibers, and allows the acrosome reaction to take place.

The role of F-actin cytoskeleton-associated gelsolin in the guinea pig capacitation and acrosome reaction

The acrosomal reaction (AR) is a regulated sperm exocytotic process that involves fusion of the plasma membrane (PM) with the outer acrosomal membrane (OAM). Our group has described F-actin cytoskeletons associated to these membranes. It has been proposed that in regulated exocytosis, a cortical cytoskeleton acts as a barrier that obstructs membrane fusion, and must be disassembled for exocytosis to occur. Actin-severing proteins from the gelsolin family have been considered to break this barrier. The present study attempted to determine if gelsolin has a function in guinea pig sperm capacitation and AR. By indirect immunofluorescence (IIF), gelsolin was detected in the apical and postacrosomal regions of the head and in the flagellum in both capacitated and non-capacitated guinea pig spermatozoa. By Western blotting, gelsolin was detected in isolated PM and OAM of non-capacitated spermatozoa. Gelsolin and actin were detected in a mixture of PM-OAM obtained by sonication, and both proteins were absent in membranes of capacitated spermatozoa. Inhibition of three different pathways of PIP2 hydrolysis during capacitation did not cancel gelsolin loss from membranes. Gelsolin was detected by Western blotting associated to membrane cytoskeletons obtained after phalloidin F-actin stabilization and Triton-X treatment; additionally, by immunoprecipitation, it was shown that gelsolin is associated with actin. By electron microscopy we observed that skeletons disassemble during capacitation, but phalloidin prevents disassembly. A three-dimensional skeleton was observed that apparently joins PM with OAM. Exogenous gelsolin stimulates AR assayed in a permeabilized spermatozoa model. Results suggest that gelsolin disassembles F-actin cytoskeletons during capacitation, promoting AR.

Remodeling of the actin cytoskeleton during mammalian sperm capacitation and acrosome reaction

The sperm acrosome reaction and penetration of the egg follow zona pellucida binding only if the sperm has previously undergone the poorly understood maturation process known as capacitation. We demonstrate here that in vitro capacitation of bull, ram, mouse, and human sperm was accompanied by a time-dependent increase in actin polymerization. Induction of the acrosome reaction in capacitated cells initiated fast F-actin breakdown. Incubation of sperm in media lacking BSA or methyl-beta-cyclodextrin, Ca(2+), or NaHCO(3), components that are all required for capacitation, prevented actin polymerization as well as capacitation, as assessed by the ability of the cells to undergo the acrosome reaction. Inhibition of F-actin formation by cytochalasin D blocked sperm capacitation and reduced the in vitro fertilization rate of metaphase II-arrested mouse eggs. It has been suggested that protein tyrosine phosphorylation may represent an important regulatory pathway that is associated with sperm capacitation. We show here that factors known to stimulate sperm protein tyrosine phosphorylation (i.e., NaHCO(3), cAMP, epidermal growth factor, H(2)O(2), and sodium vanadate) were able to enhance actin polymerization, whereas inhibition of tyrosine kinases prevented F-actin formation. These data suggest that actin polymerization may represent an important regulatory pathway in with sperm capacitation, whereas F-actin breakdown occurs before the acrosome reaction.

Cytochalasin-D retards sperm incorporation deep into the egg cytoplasm but not membrane fusion with the egg plasma membrane

The fertilization process is impaired when spermatozoa are previously incubated with Cytochalasin-D (Cyt-D). Although this fact reveals the participation of polymerized actin in fertilization, the specific event obstructed by Cyt-D treatment has not been determined. To identify this event, we capacitated guinea pig spermatozoa in minimal capacitating medium with pyruvate and lactate (MCM-PL) with Cyt-D, to inseminate hamster zona pellucida (ZP)-free eggs. Cyt-D (70 microM) decreased F-actin relative concentration in capacitated spermatozoa to a larger extent than in spermatozoa incubated under control conditions. Cyt-D also cancelled the F-actin increase normally observed in acrosome-reacted cells, and decreased the number of these cells with normal F-actin localization at the equatorial zone. Insemination of eggs with Cyt-D treated spermatozoa did not change early fertilization events such as the egg cortical reaction (CR), membranes fusion, and egg F-actin new localization, but clearly retarded, by 16 hr, spermatozoa incorporation deep into the egg cytoplasm, and decondensation of egg metaphase II chromosomes. These results show that actin polymerization is necessary for spermatozoa incorporation deep into the egg cytoplasm, but not for plasma membrane fusion nor egg activation early steps.

Receptors and channels regulating acrosome reactions

Prospective clinical studies informed by cloning and sequencing of sperm surface receptors and metal ion channels have elucidated critical early steps in the acrosome reaction that explain aspects of metal ion-related male infertility. Induction of the acrosome reaction is proposed to include non-nuclear progesterone receptor activation of Shaker-related sperm head voltage-gated potassium ion channels (VGKC). Men express VGKC isoforms with differing sensitivities to lead (Pb(2+)) inhibition, thus explaining interindividual variabilities in Pb(2+)-related male infertility. VGKC opening induces calcium (Ca(2+)) transients, and a signalling cascade induced by zona receptor aggregation requires an actin cytoskeleton created by the VGKC-induced Ca(2+) transients. Actin polymerization and stabilization, favoured by zinc (Zn(2+)) and depolymerized by cadmium (Cd(2+)), may mediate low Zn(2+) and high Cd(2+) infertile states. Zona receptor aggregation induces phosphotyrosine signals at sites, including sperm voltage-dependent Ca(2+) channels (VDCC), intermediate in electrophysiology between T- and L-type channels. Sperm surface VDCC localize at the sperm equatorial segment, the terminus of zona receptor translocation. Opening of VDCC admits a second Ca(2+) wave that activates phospholipase C phosphorylated in the zona receptor cascade. Phospholipase C induces fusogenic lipids and activates actin-severing proteins, depolymerizing the actin cytoskeleton and permitting apposition and fusion of acrosomal and plasma membranes.

Comparative distribution of short dystrophin superfamily products in various guinea pig spermatozoa domains

In this study, the presence and cellular distribution of dystrophin family products (i.e. Dp71d, Dp71f-like protein and dystrobrevin) was examined by indirect immunofluorescence and Western blotting in guinea pig spermatozoa. Two dystrophin-associated proteins, beta-dystroglycan and alpha-syntrophin, and nNOS a protein frequently associated with alpha-syntrophin, were determined. In spermatozoa lacking plasma membrane and acrosome, Dp71f-like protein was found in the postacrosomal perinuclear theca and also in the middle piece of the flagellum. In the flagellum, Dp71f-like protein is localized together with alpha-syntrophin and nNOS. Dp71d was present in the plasma membrane of the middle piece with beta-dystroglycan, alpha-syntrophin and nNOS. Dp71d was also present in plasma membrane of the post acrosomal region, but only with nNOS. Finally, dystrobrevin was located all along skeletal flagellum structures and in the subacrosomal hemisphere of the perinuclear theca. This distinct and complementary distribution in various domains of spermatozoa may reveal a specific function for each short dystrophin family product, in the stabilization of the domains where they are located.

Actin polymerisation during morphogenesis of the acrosome as spermatozoa undergo epididymal maturation in the tammar wallaby (Macropus eugenii)

In the tammar wallaby (Macropus eugenii), post-testicular acrosomal shaping involves a complex infolding and fusion of the anterior and lateral projections of the scoop-shaped acrosome into a compact button-like structure occupying the depression on the anterior end of the sperm nucleus. The present study has generated cytochemical and histological evidence to demonstrate that the occurrence of actin filaments (F-actin, labelled by Phalloidin-FITC) in the acrosome of tammar wallaby spermatozoa is temporally and spatially associated with the process of acrosomal shaping in the epididymis, through a pool of monomeric actin (G-actin, labelled by Rh-DNase I) present in the acrosome throughout all stages of epididymal maturation. F-actin was not detected in the acrosome of testicular spermatozoa, but was found in the infolding and condensing acrosome of caput and corpus epididymal spermatozoa. When the spermatozoa completed acrosome shaping in the cauda epididymidis, F-actin disappeared from the acrosomal area. The strong correlation between the occurrence of F-actin and the events of acrosomal shaping suggested that the post-testicular shaping of the acrosome might depend on a precise succession of assembly and disassembly of F-actin within the acrosome as the spermatozoa transit the epididymis. Thus, actin filaments might play a significant role in the acrosomal transformation, as they are commonly involved in morphological changes in somatic cells.

Involvement of an F-actin skeleton on the acrosome reaction in guinea pig spermatozoa

The acrosome reaction (AR) is a regulated exocytotic process. In several cell types, an actin network situated under the plasma membrane (PM) acts as a physical barrier to prevent this exocytosis. In seeking a function for a cortical skeleton in guinea pig spermatozoa, the PM and the outer acrosomal membrane (OAM) were investigated for the presence of F-actin and spectrin, proteins generally found in cell cortical skeletons. Both membrane types were visualized in whole-mount preparations by electron microscopy. PM proteins gave positive reaction to the Na(+),K(+)-ATPase antibody and the OAM proteins did not react to the antibody. Furthermore, a Triton X-100-resistant skeleton was obtained from both membrane types. Using gold immunoelectron microscopy, F-actin was visualized in the PM and in the OAM skeletons, while spectrin was only detected in the PM skeleton. The presence of an F-actin cortical skeleton in the sperm PM suggests that F-actin may be involved in the AR. The significantly higher number of AR elicited by cytochalasin D (Cyt-D) treatment(P<0.005) and data showing a significant (P>0.03) decrease in F-actin relative concentration in capacitating spermatozoa, agree with this suggestion. Furthermore, the proposal is strengthened by the fact that stabilization of F-actin by phalloidin (Ph) significantly (P>0.01) diminished AR induced by Ca(2+) in a streptolysin O (SLO)-permeabilized sperm model.

A perinuclear theca substructure is formed during epididymal guinea pig sperm maturation and disappears in acrosome reacted cells

The perinuclear theca (PT) is a unique cytoskeletal mammalian sperm structure that surrounds the nucleus. Using negatively stained whole-mount preparations, we detected a PT substructure on the apical region of the postacrosomal theca layer of guinea pig spermatozoa. The PT substructure consists of projections resembling eyelashes, circling the sperm head. The PT substructure was absent in caput but appeared in corpus epidydimal spermatozoa. The same finding was observed in sheep and rabbit spermatozoa. The PT substructure persisted in capacitating spermatozoa, but was absent in acrosome reacted gametes. No labeling of the PT substructure was observed by the immunogold technique using antibodies against calmodulin, spectrin, myosin, and vimentin. A 34-kDa band appeared as a possible PT substructure protein. The PT was positive to the antibodies and the presence of the above-mentioned proteins was confirmed by Western blot. F-actin gold label was observed in mature spermatozoa on the PT substructure base zone. Results using cytochalasin D and phalloidin point to a role of F-actin in the PT substructure formation/disassembly processes. Ca(2+), bicarbonate, and proteases might be involved in the mechanism of the substructure disassembly. Novel PT morphological changes occurring during sperm epidydimal maturation and at acrosome reaction, respectively, are discussed in relation to the PT stability and function.

Calcium-dependent actin filament-severing protein scinderin levels and localization in bovine testis, epididymis, and spermatozoa

We assessed the levels and localization of the actin filament-severing protein scinderin, in fetal and adult bovine testes, and in spermatozoa during and following the epididymal transit. We performed immunoblots on seminiferous tubules and interstitial cells isolated by enzymatic digestion, and on bovine chromaffin cells, spermatozoa, aorta, and vena cava. Immunoperoxidase labeling was done on Bouin's perfusion-fixed testes and epididymis tissue sections, and on spermatozoa. In addition, immunofluorescence labeling was done on spermatozoa. Immunoblots showed one 80-kDa band in chromaffin cells, fetal and adult tubules, interstitial cells, spermatozoa, aorta, and vena cava. Scinderin levels were higher in fetal than in adult seminiferous tubules but showed no difference between fetal and adult interstitial cells. Scinderin levels were higher in epididymal than in ejaculated spermatozoa. Scinderin was detected in a region corresponding with the subacrosomal space in the round spermatids and with the acrosome in the elongated spermatids. In epididymal spermatozoa, scinderin was localized to the anterior acrosome and the equatorial segment, but in ejaculated spermatozoa, the protein appeared in the acrosome and the post-equatorial segment of the head. In Sertoli cells, scinderin was detected near the cell surface and within the cytoplasm, where it accumulated near the base in a stage-specific manner. In the epididymis, scinderin was localized next to the surface of the cells; in the tail, it collected near the base of the principal cells. In Sertoli cells and epididymal cells, scinderin may contribute to the regulation of tight junctional permeability and to the release of the elongated spermatids by controlling the state of perijunctional actin. In germ cells, scinderin may assist in the shaping of the developing acrosome and influence the fertility of the spermatozoa.

A potential role for cadmium in the etiology of varicocele-associated infertility

OBJECTIVE

To determine whether mannose ligand receptor and acrosome reaction deficits in sperm from men with varicocele are related to the transition metal content of their semen.

DESIGN

Cadmium and zinc in semen and blood plasma were assayed for fertile males, men without varicocele who required intracytoplasmic sperm injection to achieve fertilization, and men evaluated for potential varicocele-associated infertility. The relationship between actin cytoskeletal distributions and acrosome status was determined for fertile donor sperm in the presence and absence of exogenous cadmium.

SETTING

University hospital-based molecular biology research laboratory.

PATIENT(S)

Patients from two university hospital-based IVF-assisted reproductive technology programs and two male urology private practices.

INTERVENTION(S)

Fertile donor sperm were exposed to exogenous cadmium during capacitating incubations followed by culture at temperatures up to 41 degrees C.

MAIN OUTCOME MEASURE(S)

Metal ion levels in semen and blood plasma were determined by graphite furnace atomic absorption spectroscopy. Motile sperm were examined for mannose ligand binding and the ability to undergo spontaneous and induced acrosome reactions. Unfixed, Triton-permeabilized sperm were probed with antiactin and antimyosin antibodies.

RESULT(S)

Cadmium was elevated and zinc was decreased in the seminal plasma of men with varicocele. The content of these metals in semen and blood was not correlated. Cadmium exposure in vitro reduced mannose receptor expression, acrosome exocytosis, and cytoskeletal formation by fertile donor sperm.

CONCLUSION(S)

Defects in transition metal regulation or excessive cadmium exposure are involved in varicocele-associated infertility.

Actin localization in ram spermatozoa: effect of freezing/thawing, capacitation and calcium ionophore-induced acrosomal exocytosis

We have analyzed, by immunofluorescence, the localization of actin in ram spermatozoa, its colocalization with the actin-binding protein, gelsolin, and the effect of freeze/thawing, in vitro capacitation, and induced acrosomal exocytosis on its distribution. The monoclonal anti-actin and anti-gelsolin antibodies used recognized single bands at 43,000 and 90,000 kDa, respectively. In all spermatozoa, intense actin staining was observed in the whole length of the flagellum and, depending on the protocol used, in the neck and postacrosomal region of the head. Comparison of three staining methods, together with the use of NBD-phallacidin, allowed us to characterize ram sperm actin as a monomeric, intracellular, membrane-associated protein. Gelsolin was also present in ram spermatozoa and precisely colocalized with actin. Processes involving alterations in membrane structure such as freezing/thawing, in vitro capacitation, and calcium ionophore-induced acrosomal exocytosis provoked changes in the exposure of actin to the antibody. This strongly suggests a physical association of this protein to the plasma membrane, most likely by its intracellular side. The possible role of actin in sperm function is discussed.

Centrifugation on Percoll gradient enhances fluorescent lectin binding on human sperm: a flow cytometric analysis

Centrifugation on a Percoll gradient is commonly used to enrich sperm preparations in mobile forms prior to in vitro fertilization attempts. It has been suggested that this method also induces the capacitation of sperm, a step preceding the acrosomal reaction allowing egg fertilization. The modifications of the acrosomal membrane involved in these physiological events likely include alterations of glycosylation patterns. This hypothesis was investigated by comparing the membrane binding of 15 fluorescein-conjugated lectins on 60 samples of sperm, before and after Percoll centrifugation. The numbers of labeled sperm and their mean fluorescence intensity, recorded in flow cytometry, significantly increased for 10 and 14 of the 15 lectins tested. Microscopic examination of the labeled sperm showed that the acrosome and equatorial plates were more often labeled after Percoll centrifugation, confirming the hypothesis that this method modifies the glycosylation pattern of structures important for egg fertilization.

Sperm protein (sp50) binds to acrosome and plasma membranes in a Ca(2+)-dependent manner: possible role in acrosome reaction

Annexins are a family of Ca(2+)-binding proteins involved in the exocytotic process. The presence and the role of annexins in mammalian spermatozoa have not been well established. Two annexin-like proteins were obtained from guinea pig testis, a doublet of Mr 31-33 kD (p31/33) and a protein of Mr 50 kD (p50). Both proteins were able to bind to erythrocyte ghosts in a Ca(2+)-dependent fashion. Polyclonal antibodies against p31/33 reacted with two major proteins, Mrs 50 kD (sp50) and 42 kD (sp42), from mature and immature guinea pig spermatozoa. p50 and sp50 are likely the native proteins from testis and spermatozoa, respectively, and they are seemingly related. By immunofluorescence, sp50 was only found in the acrosome region of immature and capacitated and noncapacitated spermatozoa, and its location was intracellular. In spermatozoa undergoing acrosome reaction, sp50 was detected in the whole acrosome, while in spermatozoa that had undergone acrosome reaction sp50 was not detected. However, in the protein pattern of acrosome reaction vesicles, anti-p31/33 antibody revealed diffuse bands of Mr 35-38 kD. sp50 was able to bind to plasma membrane fragments and acrosome outer membrane from demembranated sperm in a Ca(2+)-dependent fashion. The presence of sp50 in the acrosome region, its distribution throughout the acrosome membrane just before the acrosome reaction, and its ability to bind both plasma and outer acrosome membranes in a Ca(2+)-dependent manner suggest that sp50 may participate in the acrosome reaction mechanism in guinea pig spermatozoa.

Calmodulin binding proteins in the membrane vesicles released during the acrosome reaction and in the perinuclear material in isolated acrosome reacted sperm heads

Calmodulin has been suggested as the Ca(2+)-mediator in diverse cellular functions via its interaction with a number of proteins in a calcium-dependent manner. Its participation in the acrosome reaction has been suggested based on its localization in the acrosome region, on the effects produced by calmodulin antagonists, and by the changes in calmodulin compartmentation observed to occur throughout guinea pig acrosome reaction. To define the role of calmodulin in the membrane fusion events that occur during the acrosome reaction, the identification of calmodulin-binding proteins, by the overlay technique with biotinylated or unmodified calmodulin, was made in the following sperm fractions: in the membrane vesicles released during the acrosome reaction, in the remaining perinuclear material of acrosome reacted sperm heads and in a total membrane fraction from intact spermatozoa. The membrane vesicles released after the acrosome reaction showed four major calmodulin-binding proteins, M(r)s 66, 95, 97 and 110 kDa. The perinuclear material showed a 31-34, 43 and 97 kDa calmodulin-binding polypeptides. The membrane fraction from intact sperm showed eleven calmodulin-binding proteins, M(r)s between 14-110 kDa. Most of the binding proteins detected by this method corresponded to the class of calcium-independent calmodulin-binding proteins but proteins which only interacted with calmodulin in a calcium-inhibited mode were also observed. No calcium-dependent calmodulin-binding proteins were detected in any of the fractions studied. A possible role of these binding proteins in calmodulin compartmentation is discussed. The potential role of these binding proteins in membrane fusion and in membrane receptor localization in the postacrosomal region remain to be defined.

ADP-ribosylation of Rho proteins inhibits sperm motility

The highly homologous Rho proteins RhoA, RhoB and RhoC are low-molecular-mass GTP-binding proteins. They are selectively ADP-ribosylated by Clostridium botulinum ADP-ribosyltransferase C3 (C3 exoenzyme). The biological function of the Rho proteins is still unclear; there is evidence that they are involved in the regulation of the filamental network of cells. Here we report that C3 exoenzyme-like toxins ADP-ribosylate small GTP-binding proteins in bovine spermatozoa and inhibit sperm motility. These findings indicate that Rho proteins which reportedly regulate the microfilament system are basically involved in sperm motility.