Characterization of a calmodulin-stimulated adenylate cyclase from abalone spermatozoa

Is sperm capacitation analogous to early phases of Ca2+-triggered membrane fusion in somatic cells and viruses?

An important feature of male fertility is the physiological priming of spermatozoa by a multifaceted process collectively referred to as capacitation. The end point of this evasive process is the hyperactivated spermatozoa capable of binding to terminal sugar residues on the egg's extracellular coat, the zona pellucida (ZP), and undergoing acrosomal exocytosis (i.e., induction of the acrosome reaction). The hydrolytic action of acrosomal enzymes released at the site of zona binding, along with the enhanced thrust generated by the hyperactivated beat pattern of the bound spermatozoa, are important factors that regulate the penetration of ZP and fertilization of the egg. Despite many advances in identifying sperm components that promote capacitation, the mechanism underlying the calcium-triggered process remains elusive. The purpose of this review article is to focus on new advances that have enhanced our understanding of in vivo/in vitro capacitation, a prerequisite event resulting from a dramatic modification and reorganization of the sperm membrane molecules. Special emphasis has been laid on accumulating evidence suggesting potential similarities between the sperm capacitation and early phases of calcium-triggered membrane fusion (i.e., tethering and docking) during secretory and endocytotic pathways among eukaryotes.

A sperm-activating peptide controls a cGMP-signaling pathway in starfish sperm

Peptides released from eggs of marine invertebrates play a central role in fertilization. About 80 different peptides from various phyla have been isolated, however, with one exception, their respective receptors on the sperm surface have not been unequivocally identified and the pertinent signaling pathways remain ill defined. Using rapid mixing techniques and novel membrane-permeable caged compounds of cyclic nucleotides, we show that the sperm-activating peptide asterosap evokes a fast and transient increase of the cGMP concentration in sperm of the starfish Asterias amurensis, followed by a transient cGMP-stimulated increase in the Ca(2+) concentration. In contrast, cAMP levels did not change significantly and the Ca(2+) response evoked by photolysis of caged cAMP was significantly smaller than that using caged cGMP. By cloning of cDNA and chemical crosslinking, we identified a receptor-type guanylyl cyclase in the sperm flagellum as the asterosap-binding protein. Sperm respond exquisitely sensitive to picomolar concentrations of asterosap, suggesting that the peptide serves a chemosensory function like resact, a peptide involved in chemotaxis of sperm of the sea urchin Arbacia punctulata. A unifying principle emerges that chemosensory transduction in sperm of marine invertebrates uses cGMP as the primary messenger, although there may be variations in the detail.

Calmodulin antagonists differentially affect capacitation-associated protein tyrosine phosphorylation of mouse sperm components

Sperm capacitation in vitro is thought to be correlated with the increased protein tyrosine phosphorylation of a subset of sperm components. Our group recently used a pharmacological approach to demonstrate that calmodulin (CaM), a 17 kDa calcium sensor protein, has a role in sperm capacitation. In the present study, we have used several CaM antagonists in an attempt to characterize further the role of CaM in capacitation-associated protein tyrosine phosphorylation of sperm components. Our data demonstrate, first, that mouse spermatozoa incubated in a medium that favors capacitation undergo increased protein tyrosine phosphorylation in a time-dependent manner. Second, inclusion of six CaM antagonists individually in an in vitro incubation medium prevented sperm capacitation, as demonstrated by their diminished ability to undergo agonist-induced acrosome reaction. Third, half of the CaM antagonists (compound 48/80, W13 and CaM-binding domain) had no effect on protein tyrosine phosphorylation or sperm motility. Fourth, by contrast, three CaM antagonists (W7, ophiobolin A and calmidazolium) significantly inhibited protein tyrosine phosphorylation of sperm components (42, 56, 66, 82 and 95 kDa) and adversely affected their motility without altering viability as assessed by propidium iodine staining. Finally, inclusion of purified CaM in the capacitation medium significantly increased tyrosine phosphorylation of 82 kDa and 95 kDa components. Combined, these data suggest that CaM antagonists prevent capacitation by interfering with multiple regulatory pathways, and do so either with or without adverse effects on sperm motility and protein tyrosine phosphorylation.

Evidence for the capacitation-associated membrane priming of mouse spermatozoa

An important feature of male fertility is the physiological priming of mammalian spermatozoa by a multifaceted process referred to as capacitation. It is a prerequisite event before spermatozoa can bind to the egg's extracellular coat, the zona pellucida, and undergo a signal transduction cascade. The net result is the fusion of the plasma membrane (PM) and underlying outer acrosomal membrane at multiple sites and the release of acrosomal contents (i.e., glycohydrolases, proteinases, etc.) at the site of sperm-zona binding. In this study, we have used an indirect immunofluorescence (IIF) assay and other staining approaches to examine capacitation-associated membrane priming of mouse spermatozoa. For IIF studies, we used affinity-purified antibodies against two glycohydrolases that cross-reacted with the acrosomal enzymes only when the uncapacitated spermatozoa were permeabilized. Incubation of spermatozoa in a medium that favors in vitro capacitation induced membrane priming that allowed the antibodies to cross-react with the acrosomal enzymes in capacitating acrosome-intact spermatozoa without permeabilization, as revealed by the appearance of several distinct fluorescent patterns, including an initial immunopositive lining over the acrosome cap to an intense immunopositive reaction throughout the acrosome. These early immunopositive patterns were followed by the appearance of intense fluorescent spots (droplets) that seem to establish contact with the PM in a time-dependent manner. Inclusion of calmodulin, a 17-kDa Ca(2+)-binding protein which promotes capacitation, in the incubation medium did not alter the overall rate of capacitation; however, its presence accelerated the initial stages of membrane priming. The potential similarities between sperm capacitation and early events of Ca(2+)-triggered membrane fusion among eukaryotes and among various stations of the secretory and endocytotic pathways are discussed.

Distribution and localization of calmodulin-binding proteins in bull spermatozoa

Previous studies from our laboratory have shown that a decrease in the calmodulin binding properties of a few sperm proteins occurs during the capacitation process, an effect associated with a decrease in intracellular calmodulin concentrations. Using biotinylated-calmodulin nitrocellulose overlay assay on protein extracts of subcellular fractions of bull spermatozoa, one of these proteins (p32) is detected in the flagellar-enriched fractions, whereas p30 is found in the fraction enriched with sperm heads. This latter calmodulin binding protein, p30, appears to be associated with the perinuclear theca. None of these binding proteins was solubilized by nonionic detergents. Sodium dodecyl sulfate was effective solubilizing p32, whereas p30 was extracted only in conditions reported to isolate the perinuclear theca. Cellular localization of calmodulin binding proteins was also achieved by incubating spermatozoa fixed on slides with biotinylated calmodulin and revealed in a further step by fluorescein-conjugated streptavidin. Using this procedure, it was found that calmodulin binds to the sub- and postacrosomal areas of the sperm head along with the midpiece in the presence of Ca(2+). Only a sharp band of fluorescence at the subacrosomal area was observed when this procedure was performed in the absence of Ca(2+) in the presence of EGTA. The pattern of cellular calmodulin binding was highly decreased when spermatozoa were incubated under capacitating conditions, in the presence of heparin, in agreement with the published effect of capacitation on calmodulin binding proteins.

Evidence for the involvement of calmodulin in mouse sperm capacitation

Although Ca(2+) is of fundamental importance in mammalian sperm capacitation, its downstream targets have not been definitively demonstrated. The purpose of this study was to use the calmodulin (CaM) antagonists W7 and calmidazolium (CZ) to investigate the possible role of CaM, a Ca(2+)-specific binding protein, in capacitation. Sperm membrane changes associated with capacitation were assessed by the B pattern after chlortetracycline staining and by the ability to undergo the acrosome reaction (AR) in response to lysophosphatidylcholine (LPC). The percentage of B pattern sperm was significantly inhibited by W7 or CZ in a concentration-dependent manner. At 100 microM W7 or 10 microM CZ, these inhibitors also significantly reduced the sperm's ability to undergo the LPC-induced AR. Inhibition of the B pattern and the LPC-induced AR was overcome by exogenous cAMP analogues. Treatment of the sperm with 100 microM W7 also resulted in a significant decrease in their ability to fertilize eggs in vitro. At 100 microM, W5, a less potent dechlorinated W7 analogue, had no effect on the B pattern, LPC-induced AR, or fertilization competence. Sperm viability and protein tyrosine phosphorylation were not substantially affected by 100 microM W7 (relative to 100 microM W5) or 10 microM CZ; however, the percentages of motile and hyperactivated sperm were significantly reduced. The antagonist-inhibited sperm motility was restored by dilution in control medium, but not by cAMP analogues. These results suggest that CaM participates in the regulation of membrane changes important for mouse sperm capacitation, at a point upstream from cAMP, and that this pathway is at least partially separable from pathways controlling tyrosine phosphorylation and hyperactivation.

Capacitation of mouse spermatozoa. II. Protein tyrosine phosphorylation and capacitation are regulated by a cAMP-dependent pathway

In the accompanying report (Visconti, P.E., Bailey, J.L., Moore, G.D., Pan, D., Olds-Clarke, P. and Kopf, G.S. (1995) Development, 121, 1129–1137) we demonstrated that the tyrosine phosphorylation of a subset of mouse sperm proteins of M(r) 40,000-120,000 was correlated with the capacitation state of the sperm. The mechanism by which protein tyrosine phosphorylation is regulated in sperm during this process is the subject of this report. Cauda epididymal sperm, when incubated in media devoid of NaHCO3, CaCl2 or bovine serum albumin do not display the capacitation-associated increases in protein tyrosine phosphorylation of this subset of proteins. This NaHCO3, CaCl2 or bovine serum albumin requirement for protein tyrosine phosphorylation can be completely overcome by the addition of biologically active, but not inactive, cAMP analogues. Addition of the active cAMP analogues to sperm incubated in media devoid of NaHCO3, CaCl2 or bovine serum albumin overcomes the inability of these media to support capacitation, as assessed by the ability of the cells to acquire the pattern B chlortetracycline fluorescence, to undergo the zona pellucida-induced acrosome reaction and, in some cases, to fertilize metaphase II-arrested eggs in vitro. The effects of the cAMP analogues to enhance protein tyrosine phosphorylation and to promote capacitation appears to be at the level of the cAMP-dependent protein kinase (PKA), since two specific inhibitors of this enzyme (H-89 and Rp-cAMPS) block the capacitation-dependent increases in protein tyrosine phosphorylation in sperm incubated in media supporting capacitation. Capacitation, as assessed by the aforementioned endpoints, also appears to be inhibited by H-89 in a concentration-dependent manner. These results provide further evidence for the interrelationship between protein tyrosine phosphorylation and the appearance of the capacitated state in mouse sperm. They also demonstrate that both protein tyrosine phosphorylation and capacitation appear to be regulated by cAMP/PKA. Up-regulation of protein tyrosine phosphorylation by cAMP/PKA in sperm is, to our knowledge, the first demonstration of such an interrelationship between tyrosine kinase/phosphatase and PKA signaling pathways.

Adenylate cyclase activity in cyanobacteria: activation by Ca(2+)-calmodulin and a calmodulin-like activity

An adenylate cyclase activity was partially characterized in the cyanobacterium Anabaena sp. The enzyme activity is found in soluble cell fractions and shows an apparent molecular weight of about 183,400. This adenylate cyclase is activated by Ca2+ and bovine brain or spinach calmodulin and it is inhibited by EGTA and some phenothiazine derivatives. Furthermore, Anabaena sp. extracts contain a calmodulin-like activity which stimulates bovine brain cyclic AMP phosphodiesterase and the Anabaena adenylate cyclase. EGTA and phenothiazine derivatives block the cyanobacterial modulator effect.

Bicarbonate dependence of cAMP accumulation induced by phorbol esters in hamster spermatozoa

Phorbol esters stimulate cyclic adenosine 3',5'-monophosphate (cAMP) accumulation in hamster spermatozoa under conditions for in vitro capacitation. The 20-50-fold elevation of cAMP levels induced by 1 microM phorbol 12-myristate 13-acetate (PMA) in spermatozoa depends on the presence of sodium bicarbonate in the medium (ED50: 15 mM) and it is independent of extracellular pH. Sodium bicarbonate stimulates adenylate cyclase activity in membrane preparations by 4-fold (ED50: 40 mM). After solubilization, the bicarbonate-sensitive moiety elutes as a single peak of 55 kDa in a gel filtration column. Blockers of bicarbonate chloride antiporters diisothiocyanate stilbene 2,2'-disulfonic acid (DIDS) or acetamido 4'-isothiocyanate stilbene 2,2'-disulfonic acid (SITS) inhibit the bicarbonate dependent PMA effect on cAMP in living spermatozoa (ED50: 100 microM). Maximal (85%) inhibition in cAMP accumulation is observed at 1 mM. Motility is inhibited only at high concentrations of the blockers. Pretreatment of living cells with 1 mM DIDS does not affect membrane adenylate cyclase activity which remains responsive to bicarbonate. These results suggest that controlled transport of bicarbonate through the sperm plasma membrane could be associated to the regulation of cAMP synthesis.

Calmodulin modulates thymocyte adenylate cyclase activity through the guanine nucleotide regulatory unit

We have previously demonstrated in rat thymocyte plasma membranes that adenylate cyclase activity and its stimulation by 3,5,3'-triiodothyronine (T3) are influenced by calmodulin, and that these effects of calmodulin require calcium. In the present study, the mechanism by which calmodulin exerts its action was examined, in situ, in fresh plasma membranes isolated from rat thymocytes. Adenylate cyclase activity was potentiated by guanyl nucleotides, NaF and forskolin. Calmodulin did not affect basal adenylate cyclase activity. However, calmodulin influenced the guanyl nucleotide- and forskolin-stimulated adenylate cyclase activity, but had no effect on the fluoride-stimulated enzyme activity. This was evident from experiments with inhibitors of calmodulin: trifluoperazine, calmidazolium, and antibodies against calmodulin. The three inhibitors did not change basal adenylate cyclase activity, but all produced a marked decrease in the guanyl nucleotide- and forskolin-stimulated adenylate cyclase activity. The inhibitory effect of all three agents was reversed completely by the addition of calmodulin. The three inhibitors, however, failed to affect the fluoride-stimulated adenylate cyclase activity. In addition, T3, like the calmodulin inhibitors, did not change basal adenylate cyclase activity, increased the guanyl nucleotide- and forskolin-stimulated enzyme activity, but had no effect on the fluoride-stimulated enzyme activity. From these results I suggest that in the rat thymocyte calmodulin activation, and thereby T3 stimulation of the calcium-sensitive adenylate cyclase system is mediated through the guanine nucleotide regulatory unit.

Calmodulin-binding proteins are developmentally regulated in gametes and embryos of fucoid algae

Calcium-binding proteins and calmodulin-binding proteins were identified in gametes and zygotes of the marine brown algae Fucus vesiculosus, Fucus distichus, and Pelvetia fastigiata using gel (SDS-PAGE) overlay techniques. A calcium current appears to be important during cell polarization in fucoid zygotes (K.R. Robinson and L.F. Jaffe, 1975, Science 187, 70-72; K.R. Robinson and R. Cone, 1980, Science 207, 77-78), but there are no biochemical data on calcium-binding proteins in these algae. By using a sensitive 45Ca2+ overlay method designed to detect high-affinity calcium-binding proteins, at least 9-11 polypeptides were detected in extracts of fucoid gametes and zygotes. All samples had calcium-binding proteins with apparent molecular weights of about 17 and 30 kDa. A 17-kDa calcium-binding protein was purified by calcium-dependent hydrophobic chromatography and was identified as calmodulin by immunological and enzyme activator criteria. A 125I-calmodulin overlay assay was used to identify potential targets of calmodulin action. Sperm contained one major calmodulin-binding protein of about 45 kDa. Eggs lacked major calmodulin-binding activity. A 72-kDa calmodulin-binding protein was prominent in zygotes from 1-65 hr postfertilization. Both calmodulin-binding proteins showed calcium-dependent binding activity. Overall, the data suggest that the appearance and distribution of certain calcium-binding and calmodulin-binding proteins are under developmental regulation, and may reflect the different roles of calcium during fertilization and early embryogenesis.

Analysis of calmodulin acceptor proteins and the influence of calmodulin antagonists on human spermatozoa

The possible role of calmodulin in regulating a number of calcium-dependent functions exhibited by human spermatozoa was investigated by using the antagonists trifluoperazine and calmidazolium. At high doses both antagonists inhibited the motility of human spermatozoa and induced a concomitant rise in [Ca2+]i and a decline in cAMP. Lower doses of these antagonists, particularly calmidazolium, suppressed the ability of human spermatozoa to generate reactive oxygen species and exhibit sperm-oocyte fusion, without influencing [Ca2+]i, cAMP, or motility. This inhibition of sperm-oocyte fusion was effective even if the spermatozoa were subsequently exposed to A23187, suggesting that calmodulin may regulate this aspect of human sperm function at a point downstream from calcium influx. Both radiolabelling and affinity chromatography techniques were used to detect a number of calcium-dependent and calcium-independent calmodulin acceptor proteins in the human spermatozoon. The major calcium-dependent acceptor proteins exhibited Mr values of 32,000 and 22,000-27,000, respectively, and did not appear to be associated with the sperm plasma membrane.

Calmodulin-stimulated cyclic nucleotide phosphodiesterases in plasma membranes of bovine epididymal spermatozoa

Cyclic nucleotide phosphodiesterase in the plasma membranes of bovine epididymal spermatozoa was stimulated by added Ca2+ and calmodulin. The rate of hydrolysis and responsiveness toward calmodulin was greater for cAMP than for cGMP. The kinetic analysis of the activity revealed two forms of phosphodiesterase with apparent Km values of 7.5 and 95 microM for cAMP. Calmodulin stimulated both of the activities by increasing the Vmax without affecting the Km's. The activity response with respect to Ca2+ concentration appears to be biphasic in both the absence and presence of added calmodulin. Trifluoperazine inhibited the Ca2+- and calmodulin-sensitive enzyme activity in a dose-dependent manner. The calmodulin-stimulated phosphodiesterase activity in the sperm plasma membranes can be solubilized and absorbed to a Calmodulin-Sepharose affinity column in the presence of Ca2+.

The evolutionary origin of eukaryotic transmembrane signal transduction

1. A comparison was made of transmembrane signal transduction mechanisms in different eukaryotes and prokaryotes. 2. Much attention was given to eukaryotic microbes and their signal transduction mechanisms, since these organisms are intermediate in complexity between animals, plants and bacteria. 3. Signal transduction mechanisms in eukaryotic microbes, however, do not appear to be intermediate between those in animals, plants and bacteria, but show features characteristic of the higher eukaryotes. 4. These similarities include the regulation of receptor function, adenylate cyclase activity, the presence of a phosphatidylinositol cycle and of GTP-binding regulatory proteins. 5. It is proposed that the signal transduction systems known to operate in present-day eukaryotes evolved in the earliest eukaryotic cells.

Calmodulin regulation of adenylate cyclase activity

Calmodulin-dependent stimulation of adenylate cyclase was initially thought to be a unique feature of neural tissues. In recent years evidence to the contrary has accumulated, calmodulin-dependent stimulation of adenylate cyclase now being demonstrated in a wide range of structurally unrelated tissues and species. Demonstration of the existence of calmodulin-dependent adenylate cyclase has in nearly all instances required the removal of endogenous calmodulin. It is not yet clear whether calmodulin-dependent and calmodulin-independent forms of the enzyme exist and whether some tissues (such as heart) lack a calmodulin-dependent adenylate cyclase. The presence of calmodulin appears largely responsible for the ability of the adenylate cyclase enzyme to be stimulated by submicromolar concentrations of calcium; it may not be relevant to the inhibition of the enzyme which occurs at higher concentrations of calcium. The physical relationship of calmodulin to the plasma membrane bound enzyme (or to the soluble forms of the enzyme) is not known nor is the mechanism of adenylate cyclase activation by calmodulin clear; current data suggest some involvement with both the N and C units of the enzyme. Finally, it is possible that in vivo calcium contributes to the duration of the hormone stimulated cyclic AMP signal. Thus current in vitro data suggest that optimal hormonal activation of calmodulin-dependent adenylate cyclase occurs at very low intracellular calcium concentrations, comparable to those found in the resting cell; conversely the enzyme is inhibited as intracellular calcium increases, following for example agonist stimulation of the cell. These higher calcium concentrations would then activate calmodulin-dependent phosphodiesterase. Such differential effects of calcium on adenylate cyclase and phosphodiesterase would ultimately restrict the duration of the hormone-induced cyclic AMP signal.