Sperm Protease Research Articles

Progesterone action on the human sperm surface is potentiated by an egg-associated acrosin activator

The progesterone-induced Ca 2+ influx and acrosomal exocytosis in human sperm are recently described examples of non-genomic steroid action on the cell surface. These progesterone effects are known to be inhibited by synthetic protease inhibitors. In this study we tested a hypothesis that a physiological activator of the sperm protease acrosin modulates the sperm response to progesterone. It was found that the activator augments the amplitude of the progesterone-induced Ca 2+ transient and accelerates the progesterone-induced acrosomal exocytosis. These observations suggest a physiological significance of the modulation of nongenomic steroid effects by protease regulators.

Acrosin activity in human spermatozoa in relation to semen quality and in-vitro fertilization

Acrosin, a sperm proteinase released during acrosomal exocytosis, facilitates penetration through the oocyte vestments. The purpose of this investigation was to determine if a correlation exists between the acrosin activity of ejaculated human spermatozoa, before motility enrichment techniques, and in-vitro fertilization (IVF) success using selected (glass wool or swim-up) spermatozoa. Since all the oocytes were retrieved from women receiving exogenous hormonal stimulation and a mixed population of mature and immature oocytes were encountered, only cases with > or = 50% mature oocytes were analysed. Under these conditions, the acrosin activity was significantly greater (P < 0.01) in the ejaculates in which spermatozoa ultimately fertilized > or = 70% of the mature oocytes, than in the ejaculates in which spermatozoa ultimately fertilized < 70% of the mature oocytes. Furthermore, a strong correlation (r = 0.962, P = 0.0001) was detected between pre-IVF acrosin activity and subsequent high (> or = 70%) IVF success. Acrosin activity from normozoospermic and oligo-asthenozoospermic men was also compared and was significantly (P < 0.01) higher for the normozoospermic group. These data suggest that measurement of acrosin activity may be a valuable clinical laboratory assay for assessing the sperm fertilizing potential and that low acrosin activity is associated with abnormal semen characteristics.

Mouse gamete adhesion molecules.

Complementary adhesion molecules are located on the surface of mouse eggs and sperm. These molecules support species-specific interactions between sperm and eggs that lead to gamete fusion (fertilization). Modification of these molecules shortly after gamete fusion assists in prevention of polyspermic fertilization. mZP3, an 83,000-Mr glycoprotein located in the egg extracellular coat, or zona pellucida, serves as primary sperm receptor. Gamete adhesion in mice is carbohydrate-mediated, since sperm recognize and bind to certain mZP3 serine/threonine- (O-) linked oligosaccharides. As a consequence of binding to mZP3, sperm undergo the acrosome reaction, which enables them to penetrate the zona pellucida and fertilize the egg. A 56,000-Mr protein called sp56, which is located in plasma membrane surrounding acrosome-intact mouse sperm heads, is a putative primary egg-binding protein. It is suggested that sp56 recognizes and binds to certain mZP3 O-linked oligosaccharides. Acrosome-reacted sperm remain bound to eggs by interacting with mZP2, a 120,000-Mr zona pellicida glycoprotein. Thus, mZP2 serves as secondary sperm receptor. Perhaps a sperm protease associated with inner acrosomal membrane, possibly (pro)acrosin, serves as secondary egg-binding protein. These and, perhaps, other egg and sperm surface molecules regulate fertilization in mice. Homologous molecules apparently regulate fertilization in other mammals.

Decrease in calmodulin concentrations during heparin-induced capacitation in bovine spermatozoa.

Concentrations of the intracellular Ca(2+)-mediator calmodulin (CaM), were measured by radioimmunoassay during heparin-induced capacitation of bull spermatozoa. Heparin reduced sperm CaM concentrations in a dose-dependent manner corresponding with an increase in in-vitro fertilization rates. Such reductions were observed after heparin treatment for 4-6 h, which is in agreement with the length of the capacitation period in bulls and was concomitant with an increase in CaM concentration in the incubation medium, suggesting translocation of CaM from the spermatozoa to the surrounding milieu. This CaM translocation was inhibited partly by the protease inhibitor benzamidine, suggesting a role for the sperm protease in this process.

Acrosin, the peculiar sperm-specific serine protease

The sperm enzyme acrosin has long been known as one of the key enzymes in the mammalian fertilization process. Elucidation of primary structures of preproacrosin from various species have allowed a deeper insight into the structural organization and the complex evolution of the sperm proteinase acrosin. In addition to the typical elements of serine proteases, the acrosin molecule possesses one novel domain that might convey DNA-binding properties.

Guinea pig proacrosin is synthesized principally by round spermatids and contains O‐linked as well as N‐linked oligosaccharide side chains

Proacrosin is the zymogen precursor of acrosin, a sperm protease believed to play an essential role in fertilization. In this study, we used primary cultures of guinea pig spermatogenic cells to examine the temporal appearance and mechanisms of synthesis and processing of proacrosin during acrosome development. Following [35S]methionine incorporation and immunoprecipitation, cultured spermatogenic cells were found to synthesize two forms of proacrosin (Mr 54,000 and 57,000). Proacrosin was synthesized mainly by round spermatids. By immunoblotting, proacrosin became very prominent in round spermatids and persisted throughout spermiogenesis. Pulse-chase experiments demonstrated that the Mr 54,000 form of proacrosin was converted to the Mr 57,000 form, presumably reflecting posttranslational processing of carbohydrate side chains. When spermatogenic cells were cultured in the presence of tunicamycin, the synthesized proacrosin had an Mr of 54,000. However, in vitro translation of mRNA extracted from guinea pig testis followed by immunoprecipitation indicated that the core polypeptide of proacrosin has an Mr of 44,000. Guinea pig spermatogenic cells incorporated glucosamine and fucose into the oligosaccharides of proacrosin. Treatment of guinea pig testis proacrosin with N-glycosidase or O-glycosidase reduced the Mr by 3-7%. These results indicate that proacrosin is synthesized by postmeiotic cells and the enzyme contains N- and O-linked oligosaccharides.

Activation of Ca2+channels during the acrosome reaction of sea urchin sperm is inhibited by inhibitors of chymotrypsin‐like proteases

Probable participation of sperm protease in the acrosome reaction was investigated using several inhibitors and substrates. Among those examined, L-1-tosylamide-2-phenylethyl chloromethyl ketone (TPCK) and chymostatin, chymotrypsin inhibitors, p-nitrophenyl-p'-guanidinobenzoate (NPGB), a serine protease inhibitor, and N-benzoyl-L-tyrosine ethyl ester (BTEE), a chymotrypsin substrate, inhibited the egg jelly-induced acrosome reaction of Strongylocentrotus intermedius. TPCK and BTEE, however, did not inhibit the reaction caused by ionophores, A23187, or nigericin. To know the mechanism of inhibition by chymotrypsin inhibitors and substrates of the egg jelly-induced acrosome reaction, intracellular Ca2+ concentration [( Ca2+]i) and pH (pHi) were measured with fura-2 and 2',7'-bis (carboxy-ethyl)carboxyfluorescein (BCECF), respectively. Egg jelly caused increase of [Ca2+]i, which was depressed by BTEE. Egg jelly also caused a transient rise of pHi, which was not depressed by BTEE. In the presence of verapamil, the acrosome reaction by egg jelly was significantly inhibited concomitant with depressed increase of [Ca2+]i. The rise of pHi was not depressed by verapamil. Thus, modes of action of BTEE and of verapamil are similar to each other. Bringing these findings together, the authors present a view that a chymotrypsin-like protease of sea urchin sperm activates verapamil-sensitive Ca2+ channels, which take part in the acrosome reaction.

Ammonium ions enhance proteolytic activation of adenylate cyclase and decrease its senszsitivity to inhibition by “P”-site agonists

A detergent-dispersed adenylate cyclase from rat brain was used to study the effects of ammonium salts and polyamines on the proteolytic activation of the enzyme by a sperm protease and on the sensitivity of adenylate cyclase to inhibition via its “P”-site. A purified preparation of a trypsin-like, serine protease from bovine sperm was used to activate solubilized adenylate cyclase in the presence of guanosine 5′- O-(3-thiotriphosphate (GTPγS). The proteolytically activated form of adenylate cyclase was found to be particularly sensitive to further activation by ammonium bicarbonate. The activation by NH 4HCO 3 was found to be due to the NH 4 + cation and was characterized by an increased V max and by a decreased sensitivity of adenylate cyclase to inactivation by elevated concentrations of the sperm protease or by trypsin. NH 4Cl and (NH 4) 2SO 4 also caused biphasic effects on adenylate cyclase, which mimicked but were less effective than those caused by NH 4HCO 3. Consistent with observations of others, adenylate cyclase activity was enhanced by ammonium ions whether in the presence of reversible (Mn 2+) or irreversible (GTPγS) activators. Mn 2+- and GTPγS-stimulated activities were similarly optimally enhanced by 30 m m (NH 4) 2SO 4 and by 30 to 150 m m NH 4Cl or NH 4HCO 3. Ammonium ions did not increase the activity of the purified catalytic unit. Moreover, the effect of ammonium ions was not accompanied by an increased rate of activation by GTPγS, suggesting that the activation of G s (guanine nucleotide-dependent stimulatory component) may not be the primary cause of stimulation by ammonium salts. Several polyamines at millimolar concentrations blocked the stimulatory effect of NH 4 +. This was observed when adenylate cyclase was activated by Mn 2+, but not when it was activated by GTPγS or by the sperm protease + GTPγS. The inhibitory effect of polyamines was not due to the formation of a complex with ATP. Both the increase in V max of the Mn 2+-stimulated enzyme by NH 4 + and the decrease in V max caused by spermine were accompanied by an increase in the enzyme's K m MnATPapp. Spermine increased the IC 50 for inhibition of Mn 2+-activated adenylate cyclase by 2′,5′-dideoxyadenosine (2′,5′-ddAdo) from 0.75 to 4.6 μ m, consistent with the idea that increased sensitivity of P-site-mediated inhibition is associated with increased enzyme activity. In contrast, activation of Mn 2+-stimulated adenylate cyclase by 30 m m (NH 4) 2SO 4 also reduced sensitivity to inhibition by 2′,5′-ddAdo(IC 50 1.1 μ m). This decreased sensitivity to inhibition by 2′,5′-ddAdo induced by ammonium ion was also observed on GTPγS-activated or proteolytically activated adenylate cyclase. The data support the idea that activation of adenylate cyclase by ammonium salts may be due to an effect on the conformation and/or stability of the active form of the enzyme, leading to increased enzyme activity as well as to decreased proteolytic inactivation.

Isolation of the boar sperm acrosin peptide released during the conversion of alpha-form into beta-form.

The sperm proteinase acrosin occurs in several enzymatically active forms which differ from each other in molecular mass. The high-molecular-mass alpha-form (53 kDa) is converted into the low-molecular-mass beta-form (38 kDa) by auto-proteolysis. As these two forms possess identical N-termini and identical A-chains (light chains) the difference must reside in the C-terminal parts of their B-chains (heavy chains). It could be demonstrated by gel electrophoresis that on incubation of alpha-acrosin, in addition to beta-acrosin, a main degradation product of approx. 18 kDa was formed. This fragment was isolated by gel filtration chromatography. The amino-acid composition of the fragment corresponded to the difference between that of alpha-acrosin and of beta-acrosin, and showed a strikingly high proportion of proline. It is suggested that this hydrophobic segment from the C-terminal region of alpha-acrosin accounts for the special membrane-associating property of the enzyme.

Wheat germ agglutinin treatment of follicle cell-free mouse oocytes inhibits fertilization.

Controversy exists whether treatment of follicle cell-free oocytes with wheat germ agglutinin (WGA) prevents fertilization. Lack of inhibition in one case has led to the suggestion that acrosin may not be a zona lysin. To re-examine the effect of the WGA, the zona pellucida of follicle cell-free mouse oocytes was made more resistant to proteinase digestion by treatment with 10 or 50 micrograms/ml WGA. Such WGA-treated oocytes showed decreased fertilizability when washed to remove excess WGA and incubated with capacitated spermatozoa. Oocyte cleavage was used as an end point, because a large number of spermatozoa adhered to the eggs after WGA treatment, making observation of sperm penetration and pronucleus formation unreliable. Resistance to proteinase digestion increased, and the fertilizability decreased with the higher amount of WGA. The action of WGA was most likely not mediated by a direct effect on sperm motility, sperm acrosin activity, sperm binding to the zona pellucida, or oocyte cleavage. WGA did not affect the acrosome reaction of guinea pig spermatozoa. These data show that WGA treatment of follicle cell-free mouse oocytes results in decreased fertilizability, possibly by rendering the zona pellucida more resistant to sperm proteinase digestion.

Proacrosin/acrosin during guinea pig spermatogenesis

Enriched populations of guinea pig spermatogenic cells were isolated by sedimentation velocity at unit gravity. Each cell population was analyzed for the presence of members of the proacrosin/acrosin family by enzymography, immunoblotting, and immunofluorescence. Following sodium dodecyl sulfate-polyacrylamide gel electrophoresis in gels containing 0.1% gelatin, protease activities with molecular weights of 55,000 (major) and 50,000 (minor) were detected in round spermatid extracts. Condensing spermatid extracts contained protease activities with molecular weights between 55,000 and 50,000. These major protease activities had molecular weights similar to antigens detected by immunoblotting with a monospecific rabbit antiserum directed against purified boar acrosin. Extracts of guinea pig sperm and the soluble acrosomal components released following the acrosome reaction induced with ionophore A23187 contained three major protease activities ( M r 32,000, 34,000, 47,000) but only the 47,000 M r protease cross-reacted with the antibody. The spermatid and sperm protease activities were inhibited and activated by classical effectors of acrosin activity from other species. Immunofluorescence demonstrated that proacrosin/acrosin was present as early as the Golgi phase of spermiogenesis. In addition, immunoreactivity was confined to the acrosomes in a manner characteristic of each spermatid stage. These results demonstrate that proacrosin/acrosin can be detected in the earliest spermiogenic stages by electrophoretic and immunological techniques and suggest that changes in the molecular weights of proacrosin/acrosin occur as spermatids mature.

Acrosin shows zona and fucose binding, novel properties for a serine proteinase

The major fucose-binding protein of 53 kDa from boar spermatozoa was isolated to apparent homogeneity using a two-step procedure including high-performance gel filtration and reversed-phase chromatography. The N-terminal sequence of the protein revealed that it is identical with the sperm proteinase acrosin. By means of a solid-phase zona-binding assay based on the avidin-biotin system it was demonstrated that acrosin also interacts strongly with porcine zona pellucida. Thus, the acrosin molecule combines specific proteolytic activity with zona- and carbohydrate-affinity properties, i.e. previously unrecognized properties of a serine proteinase. It seems likely that this special affinity of acrosin directs the proteolytic activity to its structural target in the in vivo situation.

Partial purification and characterization of human sperminogen.

A recently recognized non-proacrosin zymogen referred to as sperminogen has been purified from human spermatozoa, and several of its properties have been determined. The purification procedure included acid extraction of washed ejaculated sperm at pH 3.0, followed by gel filtration of the solubilized extract over a Sephadex G-75 superfine column. The sperminogen eluted from the column in a single band that was completely separated from the proacrosin band. This separation was confirmed by a gelatin-sodium dodecyl sulfate-polyacrylamide gel electrophoresis (gelatin-SDS-PAGE) zymograph. This zymograph also demonstrated that the final sperminogen preparation contained four forms of zymogen, with molecular weights between 32,000 and 36,000. At neutral pH, the sperminogen was converted into spermin, its enzymatically active form, yielding a sigmoidal curve typical of zymogen autoactivation. The effects of several factors on the rate of this autoconversion indicate specific differences between sperminogen and proacrosin. Spermin hydrolyzed N-alpha-benzoyl-L-arginine ethyl ester (BzArgOEt), and was inhibited by lima bean trypsin inhibitor, pancreatic trypsin inhibitor, N-acetyl-L-leucyl-L-leucyl-L-argininal (leupeptin), and tosyl-L-lysine chloromethyl ketone, indicating that the enzyme has a trypsin-like specificity and probably belongs to the class of trypsin-like enzymes. Since acrosin is generally believed to be the only trypsin-like enzyme in mammalian sperm, the demonstration of human sperminogen and spermin necessitates further inquiry into the functions and the relationships between sperm proteinase systems.

Changes in composition of the porcine zona pellucida during development of the oocyte to the 2-to 4-cell embryo

Our objective was to identify any changes that occur in the composition of the porcine zona pellucida during development of the 2- to 4-cell embryo from the oocyte. Oocytes, unfertilized eggs and single and 2- to 4-cell embryos have been recovered surgically and their zonae pellucidae 125I-labelled and analysed individually by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). The zonae from ovulated eggs possessed two major glycoproteins Mr250 000 and 90 000 which were absent from follicular oocytes but present in the fluid from the oestrus, but not luteal, oviduct. The glycoproteins remained on the zona pellucida of 2- to 4-cell embryos whose analysis showed the presence of additional polypeptides of Mr 150 000, 57 000, 50 000 and 25 000. It is concluded (i) that shortly after ovulation, and in spite of the presence around the egg of cumulus oophorus and corona radiata cells, significant amounts of oviducal glycoproteins are able to bind firmly to the zona pellucida, and (ii) that after contact with spermatozoa there is evidence of a limited hydrolysis of the structure by the sperm protease acrosin.

Timing of action of sperm proteases in ascidian fertilization

The timing of action of three sperm proteases, acrosin, spermosin, and a chymotrypsin-like enzyme, in the fertilization of the ascidian,Halocynthia roretzi, was examined by adding specific protease inhibitors at various times after insemination. The results indicate that the last two enzymes both function at the early stage of the process of sperm penetration through the egg investment, while acrosin functions at the late stage.

The action of acrosin on the zona pellucida.

The presence of hydrolytic enzymes in and associated with the sperm head has long argued for their functioning in fertilization. Several observations led investigators to propose that the acrosomal trypsin-like enzyme, acrosin in mammals, functioned in fertilization in aiding the sperm to penetrate the zona pellucida. While many have raised significant objections to this role, the action of acrosin on its presumed physiological substrate has not been characterized in a biochemical fashion. The intent of this study was to examine the effect of sperm proteases on the innermost egg envelopes in a parallel study, with the pig, Sous scrofa and the South African clawed frog, Xenopus laevis. With the pig, a great deal of information exists concerning the boar enzyme, acrosin, but little is known about the chemical structure of the zona pellucida. The opposite situation exists in X. laevis where the vitelline envelope is well characterized chemically, but little is known about the putative sperm lysins.

The Membrane-Bound Spermatozoal Adenylyl Cyclase System Does not Share Coupling Characteristics with Somatic Cell Adenylyl Cyclases*

Membrane-bound adenylyl cyclases from ram, dog, and human sperm are unresponsive to fluoride and guanylylimidodiphosphate [GMP-P(NH)P], two agents that stimulate the adenylyl cyclases of somatic cells by an action on the stimulatory guanine nucleotide-binding regulatory (Ns) component of adenylyl cyclase. We have investigated whether this is because the sperm cell catalytic unit is functionally uncoupled from Ns but, nevertheless, capable of interacting with it, or because the sperm cell adenylyl cyclase system is unique and regulated differently from that of somatic cells. Sperm cells were found to be deficient in Ns, as evidenced by the inability of detergent extracts from sperm cell membranes and fractions to reconstitute Ns-mediated regulation of the adenylyl cyclase of cyc- S49 cells. In addition, attempts to label Ns in sperm cell membranes by [32P]ADP ribosylation with cholera toxin revealed that, if present, Ns is less than 1% of that found in human erythrocyte membranes. This, however, was not the only reason for the unresponsiveness of sperm cell adenylyl cyclase, since fluoride stimulation of the sperm cell enzyme could not be induced by reconstituting it with Ns purified from human erythrocytes (hRBC). When intact hRBC membranes were added to sperm cell fractions in the presence of fluoride, the activities that resulted were greater than the sum of the individual activities. This apparent reconstitution of fluoride regulation of sperm cell adenylyl cyclase could be blocked by lima bean trypsin inhibitor and appears to have resulted from proteolytic activation of the hRBC adenylyl cyclase by sperm proteases. Sperm cell membranes also appear to lack a functional inhibitory regulatory protein of the adenylyl cyclase system (Ni), since they did not contain an ADP-ribosylatable substrate for pertussis toxin action. These results suggest that the sperm cell adenylyl cyclase system is unique and different from that of somatic cells. Sperm cells appear to neither contain Ns or Ni nor possess the ability of their adenylyl cyclase system to interact with Ns from an exogenous source.

A 53,000-Da esterase in Strongylocentrotus purpuratus semen is derived from phagocytic cells, not sperm

Semen from the sea urchin Strongylocentrotus purpuratus contains sperm and a small volume (1%) of phagocytes, which often contain degraded sperm. A 53,000-Da esterase in the semen is inhibited by diisopropyl fluorophosphate, but not by soybean trypsin inhibitor (STI). Differential centrifugation experiments now reveal that 70% of the esterase activity (formerly described as a sperm protease precursor; Levine and Walsh, 1980) is associated with the phagocytes, which sediment more rapidly than the sperm. The 53K esterase is also present in spawned ovaries and testes. However, as previously reported, the sperm do contain an STI-inhibitable protease as shown by the digestion of [ 14C]lysozyme. Intact sperm exhibit STI-inhibitable hydrolytic activity toward N-α-[ 3H]benzoyl- l-arginine ethyl ester [ 3H]BAEE), but crude homogenates do not until they are extracted at pH 2.5 and fractionated by ion exchange chromatography. Although not obtained in pure form, the protease activity appears to migrate with a molecuar weight of 20,000 (gel filtration). The protease and the esterase differ markedly in acid stability. After preincubation at pH 2.5 the protease still hydrolyzes [ 3H]BAEE, while the esterase is irreversibly inactivated. This last observation may explain an earlier interpretation ( A. E. Levine and K. A. Walsh, 1980, J. Biol. Chem. 255, 4814–4820) that the 53K enzyme dissociated at pH 2.5 into two unequal subunits, one of which was the active protease. Since it has been shown that the contaminating phagocytes contribute most of the esterase activity of the semen, the occurrence of even a small number of nonsperm cells cannot be ignored in future investigations of sperm enzymes.

Evidence for the participation of two sperm proteases, spermosin and acrosin, in fertilization of the ascidian, Halocynthia roretzi: inhibitory effects of leupeptin analogs on enzyme activities and fertilization.

Ten kinds of argininal-containing compounds were examined for their inhibitory effects on the fertilization of the solitary ascidian and on the activities of acrosin and spermosin, trypsin-like proteases isolated from spermatozoa of this animal. Benzyloxycarbonyl-Val-Pro-argininal (I) and benzyloxycarbonyl-Phe-Leu-argininal (II) showed the strongest inhibition on the fertilization. Leupeptin (acetyl-Leu-Leu-argininal, III) was ranked next (I, II greater than III). The activity of ascidian acrosin was susceptible to most of the compounds, among which II was the best inhibitor and followed with I and III (II greater than I, III). Spermosin suffered significant inhibition only with I and II (I greater than II). These results suggest that not only acrosin but also spermosin is involved in fertilization of the ascidian.

Effects of protease inhibitors on sperm-related events in sea urchin fertilization

Evidence for sperm-borne proteolytic enzymes exposed during the acrosome reaction in sea urchin sperm has been accumulating. To investigate the possible role(s) such enzymes have in fertilization, we studied the effects of several protease inhibitors on sperm-related events. Soybean trypsin inhibitor, Nα-p-tosyl- l-lysine, chloromethyl ketone, phenylmethylsulfonyl fluoride, and chymostatin neither reduced the number of acrosome reactions nor interfered with gamete binding. p-Nitrophenyl-p′-guanidinobenzoate caused sperm to fuse into irregular clumps, rendering them unable to fertilize eggs. However, l-1-tosylamide-2-phenylethyl chloromethyl ketone (TPCK), an inhibitor of chymotrypsin, prevented the acrosome reaction in Strongylocentrotus purpuratus, S. droebachiensis, and Lytechinus pictus. The effects of TPCK on sperm in subsequent steps of fertilization were also investigated. First, gamete binding assays were performed on fixed eggs. This precluded any effects TPCK might have had on egg-derived secretions (e.g., proteases). Binding of prereacted sperm occurred with both fixed and living eggs. However, fertilization of living eggs in the presence of TPCK was greatly reduced, even though sperm had been prereacted with egg jelly. Vitelline coats were then removed from eggs by trypsin treatment. Eggs in TPCK fertilized and developed normally after the above treatment. These observations are consistent with the hypothesis of a sperm protease participating in the acrosome reaction and the penetration of the egg vitelline coat in the sea urchin.