Distribution of calmodulin and calmodulin-binding proteins in membranes from bovine epididymal spermatozoa

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.

Presence of cyclic nucleotide phosphodiesterases PDE1A, existing as a stable complex with calmodulin, and PDE3A in human spermatozoa

Mammalian sperm motility, capacitation, and the acrosome reaction are regulated by signal transduction systems involving cAMP as a second messenger. Levels of cAMP are controlled by two key enzymes, adenylyl cyclase and phosphodiesterases (PDEs), the latter being involved in cAMP degradation. Calmodulin-dependent PDE (PDE1) and cAMP-specific PDE (PDE4) activities were previously identified in spermatozoa via the use of specific inhibitors. Here we report that human sperm PDEs are associated with the plasma membrane (50%-60%) as well as with the particulate fraction (30%-50%) and have more affinity for cAMP than cGMP. Immunocytochemical data indicated that PDE1A, a variant of PDE1, is localized on the equatorial segment of the sperm head as well as on the mid and principal pieces of the flagellum, and that PDE3A is found on the postacrosomal segment of the sperm head. Immunoblotting confirmed the presence of PDE1A and PDE3A isoforms in spermatozoa. Milrinone, a PDE3 inhibitor, increased intracellular levels of cAMP by about 15% but did not affect sperm functions, possibly because PDE3 represents only a small proportion of the sperm total PDE activity (10% and 25% in Triton X-100 soluble and particulate fractions, respectively). PDE1A activity in whole sperm extract or after partial purification by anion-exchange chromatography was not stimulated by calcium + calmodulin. Results obtained with electrophoresis in native conditions indicated that calmodulin is tightly bound to PDE1A. Incubation with EGTA + EDTA, trifluoperazine, or urea did not dissociate the PDE1A-calmodulin complex. These results suggest that PDE1A is permanently activated in human spermatozoa.

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.

Processing of the sperm protein Sp17 during the acrosome reaction and characterization as a calmodulin binding protein

In this study we have demonstrated that the native rabbit sperm protein, Sp17, is a 22- to 24-kDa triplet of proteins in washed ejaculated rabbit spermatozoa and is unaffected by capacitation. However, during the acrosome reaction, Sp17 is processed from a 22- to 24-kDa triplet of proteins to a triplet of proteins at 17-19 kDa by the removal of amino acids from the C-terminal. Recombinant rabbit Sp17 (rRSp17) can also be proteolytically processed by acrosome-reacted spermatozoa in a similar manner. Protease inhibitors prevent the proteolytic processing of Sp17. Both forms of native Sp17 remain associated with acrosome-reacted spermatozoa and are solubilized by ionic detergents. Previously, sequence analysis of Sp17 revealed that Sp17 amino acids 108-137 were 52% identical to the calmodulin binding domain of neuromodulin and contained an IQ motif found in other calmodulin binding proteins. In this study, a truncated recombinant Sp17, rRSp17CB, which lacks amino acids 118-146, including the potential calmodulin binding site, was made. Recombinant rabbit Sp17, but not rRSp17CB, binds to calmodulin in the presence of Ca2+ or EDTA, under reduced or nonreduced conditions in biotinylated-calmodulin overlay assays. In DSS crosslinker experiments, calmodulin bound to rRSp17 in a 1:1 ratio but not to rRSp17CB. Additionally, biotinylated rRSp17 interacts with native sperm calmodulin. We propose that the processing of native Sp17, by removing a C-terminal fragment during the acrosome reaction, might be a mechanism to regulate the calmodulin binding activity of Sp17 and provide calmodulin at specific sites after the acrosome reaction.

Two-dimensional polyacrylamide gel electrophoresis of bovine semen after cryopreservation in half- milliliter straws

One of the problems encountered with two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) is the streaking of proteins so that individual spot identification is compromised. This study was conducted to determine whether a low loading dose (50 microg) of protein would permit resolution of more discrete protein spots using megapixel camera technology, and if so, to present a nomenclature for future comparisons of the identified proteins. If the major proteins could be identified in a 50-microg sample we aimed to determine whether they could be identified in the supernatant (seminal plasma plus extender) of cryopreserved semen. Two ejaculates were obtained from each of 6 bulls and bovine seminal plasma (BSP) protein concentration was standardized to 50 microg/10 microl. Isoelectric points (pI) and molecular weights (MWt) of BSP proteins were determined by measuring spot mobility on 2-D PAGE (15% polyacrylamide). Three distinct protein spot constellations (a,b,c) could be readily seen by the naked eye and a faintly stained constellation "d" was identified by the megapixel camera. The image analysis software located 6 protein spots in both constellation "a" (MWt 26 kDa; pI 4.2 to 4.8) and "b" ( MWt 27 kDa; pI 6.6 to 8.0). Constellation "c" contained 13 protein spots distributed in a right-angled triangle with its base towards the acidic end of the gel (MWt 14.7 to 18.8 kDa; pI 5.3 to 7.4). Only spots c(2), c(3), c(5), c(8), and c(13) were present in all 12 samples. Streaking can be eliminated by using 50 microg protein for 2-D PAGE, and the major protein spots are readily identified by megapixel camera technology. Protein spots c(3), c(5), c(13) and constellation "a" appear to correspond with Manjunath's proteins (BSP-A(1), -A(2); -A(3); -30 kDa). Killian's 2 low fertility proteins may lie in the "c" constellation, and 1 of the high fertility proteins may lie in the "b" constellation. The 3 major BSP proteins can be visualized in the supernatant of cryopreserved semen. We believe that the technique may prove useful for retrospective analysis of processed semen batches that achieve less than satisfactory results in the field.

Localization of intracellular calcium during the acrosome reaction in ram spermatozoa

Calcium was identified by a pyroantimonate-osmium fixation technique in ram spermatozoa undergoing a spontaneous acrosome reaction induced by incubation of diluted semen at 39 degrees C. Intracellular calcium was only detected in diluted spermatozoa and increased in amount and distribution over 4 hr At 4 hr, the majority of the spermatozoa displayed ultrastructural evidence of an acrosome reaction. Calcium was initially evident on the outer acrosomal membrane in multiparticulate clusters, which were seen to be located on scalloped crests of acrosomal membrane as fusion developed; it was also located in the region of the acrosomal ridge beneath the outer acrosomal membrane. Vesiculation commenced just anterior to the equatorial segment and proceeded anteriorly. As vesiculation advanced, calcium particles became associated with the periphery of the vesicles attached in the region of the fusion between the two membranes, but were never seen inside the vesicles. The equatorial segment was not labelled until much later in the reaction, at which time calcium particles were also evident on the nuclear membrane; vesiculation of the equatorial segment was also noted at this time. Dense labelling of the postacrosomal dense lamina was seen in all incubated spermatozoa. At the anterior margin of this structure the labelling was seen to be in a "sawtooth" arrangement. The disposition of the calcium both temporally and spatially is discussed in relation to its possible mechanisms in bringing about membrane fusion.

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.

Cytoskeletal elements in mammalian spermiogenesis and spermatozoa

Identification of the cytoskeletal elements and their role in the formation as well as the maintenance of head membrane compartmentalization is a much debated issue in mammalian spermatozoa. Data which have emerged during the last ten years are summarized. Those which have converged in a common opinion, such as the distribution of actin in mammalian spermiogenesis, are distinguished from those which have to be confirmed, such as the role of actin related proteins and actin in mature spermatozoa.

Changes in calmodulin compartmentalization throughout capacitation and acrosome reaction in guinea pig spermatozoa

Calmodulin has been postulated as a mediator in the calcium-dependent processes that culminate in the acrosome reaction. Changes in calmodulin compartmentalization as a consequence of the increased permeability to extracellular calcium during capacitation and acrosome reaction have been suggested. In the present study the temporal localization of calmodulin in guinea pig spermatozoa was studied during in vitro capacitation and acrosome reaction by indirect immunofluorescence. Capacitation was achieved by incubation in Tyrode medium supplemented with pyruvate, lactate, and glucose in the presence and in the absence of calcium. Acrosome reaction was elicited in three different conditions: 1) by transfer to minimal culture medium containing pyruvate and lactate (MCM-PL) after in vitro capacitation 2) by 0.003% Triton-X 100 treatment, and 3) by A 23187 addition to sperm samples incubated in MCM-PL. During capacitation, calmodulin was observed both in the acrosome and in the flagellum; this localization seemed to be independent of the presence of extracellular calcium and of exogenous substrates. Throughout the acrosome reaction, different stages of calmodulin compartmentalization were observed. It became clustered around the equatorial region just before or a little after the acrosome reaction had occurred. Later, it was observed around the postacrosomal region in the acrosome-reacted sperm. The changes in calmodulin distribution were found to be dependent on the stage in the acrosome reaction.

Calcium-binding proteins of boar spermatozoan plasma membranes: identification and partial characterization

Calcium-binding proteins (CBPs) of boar spermatozoa and boar seminal plasma were identified by using a 45Ca overlay technique to detect these proteins on transblots of PAGE-separated proteins. A single CBP (Mr approximately 300 kDa) was detected in seminal plasma. This protein binds specifically to the plasma membrane overlying the principal segment and is removed from sperm during capacitation. The protein was purified for further characterization by anion exchange chromatography and gel filtration. In addition, six major proteins (30, 35, 38, 42, 52, and 66 kDa) which do not originate from accessory gland secretions were found to be strongly associated with the plasma membrane. Most of these proteins are not integral to the membrane and appear to develop an association with the plasma membrane during epididymal maturation. Similarly, calmodulin-binding proteins appear to develop strong associations with the plasma membrane during epididymal transit.

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+.

Effect of calmodulin antagonists on calcium pump of ram spermatozoa plasma membrane

Plasma membranes isolated from ram spermatozoa contain calmodulin, which represents approximately 0.03% of the total sperm calmodulin and 0.025% of the membrane protein. When membranes were isolated in the presence of ethylene glycol (beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA), the amount of calmodulin associated with the plasma membranes was reduced by only 20%. The ATP-dependent calcium transport activity of the isolated plasma membranes is not enhanced by adding calmodulin and not inhibited by the calmodulin antagonists trifluoperazinc (TFP), compound 48/80, or calmidazolium. In fact, there is an enhancement of calcium uptake by the calmodulin antagonists and this enhancement can be blocked by the Ca2+-channel blocker D-600. It is suggested that the ATP-dependent calcium transport activity in the plasma membrane of ram spermatozoa is not regulated by calmodulin.

Concentrations of calmodulin in sperm in relation to their motility in fertile euspermic and infertile asthenozoospermic men

The relationship between calmodulin and sperm motility was assessed in euspermic and asthenozoospermic men using radioimmunoassay and time-lapse photography, respectively. There was a significant decrease in the % sperm motility, mean sperm velocity, motility index and % of progressively motile sperm in the asthenozoospermic group when compared with euspermic men. The former also exhibited a higher % of sperm with erratic or circular motility. Calmodulin concentration in sperm from the asthenozoospermic men was 4.8 +/- 1.4 micrograms/mg protein compared with 12.6 +/- 2.3 in euspermic men (P less than 0.0005). The differences observed in sperm motility characteristics between the two groups may, thus, be due to the observed differences in the concentration of calmodulin.

Actin and calmodulin coexist in the equatorial segment of ejaculated boar sperm

The direct overlay technique with iodinated calmodulin on boar sperm fractions evidenced a strong bond in the 45,000 molecular weight, which is the region recognized by anti-actin antibodies. This result and the close codistribution of the two staining patterns for calmodulin and actin with immunofluorescence and immunoelectronmicroscopy suggest a possible interaction between calmodulin and actin in boar sperm.

Sperm surface changes during the acrosome reaction as observed by freeze-fracture

The mammalian acrosome reaction is an exocytotic process that can be analyzed by the technique of freeze-fracture; only sperm cells capacitated in vitro or treated to elicit the acrosome reaction in vitro have been studied, and all pictures published are from material fixed before freezing. All the authors point out the appearance of particle-free areas in the plasma membrane of the acrosomal region during capacitation and before any fusion. This is interpreted as an increase in membrane fluidity as suggested by studies on membrane lipid composition in guinea-pig sperm. We have recently described the induced acrosome reaction in ram spermatozoa. Fusion starts at the limit of the anterior and equatorial segments and progresses forward in the anterior segment along ramified paths, resulting in a fenestration gradient of the acrosomal cap. Fusion propagation may be controlled by fluidity increase in the plasma membrane of the anterior segment, and it is probably inhibited in the equatorial segment by the ordered structure of the acrosomal membrane.