Changes in lipid diffusibility in the hamster sperm head plasma membrane during capacitation in vivo and in vitro

Cyclic 3',5'-AMP causes ADAM1/ADAM2 to rapidly diffuse within the plasma membrane of guinea pig sperm

Because sperm cannot synthesize new proteins as they journey to the egg, they use multiple mechanisms to modify the activity of existing proteins, including changes in the diffusion coefficient of some membrane proteins. Previously, we showed that during capacitation the guinea pig heterodimeric membrane protein ADAM1/ADAM2 (fertilin) transforms from a stationary state to one of rapid diffusion within the lipid bilayer. The cause for this biophysical change, however, was unknown. In this study we examined whether an increase in cAMP, such as occurs during capacitation, could trigger this change. We incubated guinea pig cauda sperm with the membrane-permeable cAMP analog dibutyryl cAMP (db-cAMP) and the phosphodiesterase inhibitor papaverine and first tested for indications of capacitation. We observed hypermotility and acrosome-reaction competence. We then used fluorescence redistribution after photobleaching (FRAP) to measure the lateral mobility of ADAM1/ADAM2 after the db-cAMP treatment. We observed that db-cAMP caused roughly a 12-fold increase in lateral mobility of ADAM1/ADAM2, yielding diffusion similar to that observed for sperm capacitated in vitro. When we repeated the FRAP on testicular sperm incubated in db-cAMP, we found only a modest increase in lateral mobility of ADAM1/ADAM2, which underwent little redistribution. Interestingly, testicular sperm also cannot be induced to undergo capacitation. Together, the data suggest that the release of ADAM1/ADAM2 from its diffusion constraints results from a cAMP-induced signaling pathway that, like others of capacitation, is established during epididymal sperm maturation.

Implication of cAMP during porcine sperm capacitation and protein tyrosine phosphorylation

Second messengers are involved in sperm fertilizing potential, as both motility and the acrosome reaction are influenced by cAMP. Moreover, the activity of cyclic nucleotides is implicated in the appearance of tyrosine phosphorylated sperm proteins, which is associated with capacitation in the mammalian spermatozoa. Nevertheless, the involvement of the cAMP/protein kinase A (PK-A) pathway during pig sperm capacitation may be different from that observed in other mammals. The objective of the present study was to clarify the cAMP/PK-A pathway during the capacitation of porcine spermatozoa and to evaluate this impact on the p32 sperm tyrosine phosphoprotein appearance. The presence of p32 was assessed after incubating fresh pig sperm with IBMX/db-cAMP, H-89, a PK-A inhibitor or bistyrphostin, a tyrosine kinase inhibitor, in capacitating (CM) or non-capacitating conditions (NCM) by immunoblotting SDS-extracted and separated sperm proteins using an anti-phosphotyrosine antibody. When pig spermatozoa were incubated in CM supplemented with H-89 (50 microM) or bistyrphostin (1.2 microM), capacitation decreased significantly (P < 0.001). The p32 sperm tyrosine phosphoprotein, previously shown to be associated with capacitation of porcine sperm though not necessarily an end point of this phenomenon, was not modulated by IBMX/db-cAMP (100 microM/1 mM), H-89 (50 microM) nor bistyrphostin (1.2 microM). Our results indicate, therefore, that pig sperm are regulated somewhat differently than as described for other mammals, because although the cAMP/PK-A and tyrosine kinase pathways are involved in capacitation, they do not influence the appearance of p32.

Use of phosphoproteomics to study tyrosine kinase activity in capacitating boar sperm

It is generally accepted that sperm capacitation is associated with the protein kinase A-mediated appearance of tyrosine phosphoproteins, although the substrates and kinase(s) involved have not been identified. We described a Mr 32,000 tyrosine phosphoprotein, "p32", appearing in porcine sperm coincident with capacitation. We also discovered a tyrosine kinase-like enzyme in boar sperm of Mr 32,000 ("TK-32") with enhanced activity during capacitation. The present work was conducted to further characterize and to identify these capacitation-related protein(s). Fresh porcine sperm were incubated to induce capacitation then immunoprecipitation, immunoblotting and proteomic analysis revealed seven tyrosine-phosphorylated proteins aligned in the range of Mr 30,000 with different isoelectric pH values (pI). Therefore, p32 may be composed of several tyrosine phosphoproteins. Three were identified as acrosin-binding sp32 (pI 6.5), and two triosephosphate isomerase isoforms (pI 7.1 and 7.9). At present, however, proteonomic analysis has not revealed any kinase at Mr 32,000. Immunoprecipitation experiments show that p32 and TK-32 are different molecules, as TK-32 activity remains in the supernatant of the antiphosphotyrosine precipitates. Finally, in-gel renaturation and immunoblotting suggest that TK-32 is a mitogen-activated protein kinase (MAPK). The discovery of p32 and the MAPK-like TK-32 provides new insight regarding the mechanisms underlying capacitation in the pig.

Decrease in order of human sperm lipids during capacitation

Ejaculated mammalian sperm must undergo a final maturation (capacitation) before they can acrosome-react and fertilize eggs. Loss of the sperm sterols, cholesterol and desmosterol, is an obligatory step in the capacitation of human sperm. Because sterols can increase the order of membrane phospholipids, it has been suggested that the importance of sterol loss is that it decreases membrane lipid order. The present study tested the hypotheses that sterol loss decreases sperm membrane lipid order during capacitation and that lipid disorder is a sufficient stimulus for capacitation. Steady-state fluorescence anisotropy of the membrane probe, 1,6-diphenyl-1,3,5-hexatriene, decreased during capacitation, indicating a decrease in lipid order. The decrease was dependent on the loss of sperm sterols, suggesting that it reflected diminished sterol-mediated phospholipid ordering. However, the lipid-fluidizing agents, benzyl alcohol and 2-(2-methoxyethoxy)ethyl 8-(cis-2-n-octylcyclopropyl) octanoate, did not cause sperm capacitation or overcome inhibition by cholesterol. In summary, loss of sperm sterols caused a significant decline in lipid order during capacitation; however, decreased bulk lipid order was not sufficient to trigger the subsequent events that complete capacitation.

Structural features of sterols required to inhibit human sperm capacitation

Ejaculated mammalian sperm must undergo a final maturation (capacitation) before they can acrosome-react and fertilize eggs. Loss of cholesterol is an essential step in the capacitation of human sperm. Experimentally maintaining a high level of cholesterol inhibits capacitation, but the mechanism is unknown. The present study investigated the structural features that are required for cholesterol's inhibitory activity. Human sperm also contain much desmosterol, which is lost from sperm during capacitation. Preventing the loss of desmosterol inhibited capacitation (as assessed by acrosomal responsiveness), with an effectiveness approximately equal to cholesterol's inhibitory activity. Other structural analogs were added to the incubation medium to replace sperm cholesterol and desmosterol. Most inhibited capacitation, including those that lacked cholesterol's 3beta-OH group (cholesteryl methyl ether and epicholesterol) and those with modified C17 groups (ergosterol and diosgenin). Two steroids did not inhibit capacitation well. Coprostanol, which has a nonplanar steroid nucleus, had low inhibitory activity that could be explained by an elevated endogenous cholesterol concentration. Epicoprostanol, which has a nonplanar ring structure and a 3alpha-OH group, promoted rather than inhibited capacitation. The inhibitory activity of the analogs was correlated with their ability to promote order of egg phosphatidylcholine as measured by fluorescence anisotropy. In summary, a planar ring structure is required for sterol inhibitory activity, but a 3beta-OH group and a saturated cholesterol-like aliphatic tail on C17 are not required. The present results support the hypothesis that sperm sterols block capacitation by increasing order of phospholipids.

Porcine sperm capacitation and tyrosine kinase activity are dependent on bicarbonate and calcium but protein tyrosine phosphorylation is only associated with calcium

Mammalian sperm undergo capacitation in the female reproductive tract or under defined conditions in vitro. Although capacitation is now considered to be mediated by intracellular signaling events, including protein phosphorylation, the regulation of the transduction mechanisms is poorly understood. The objective of the present study was to evaluate the importance of medium components on capacitation of porcine sperm, the appearance of an M(r) 32 000 sperm protein (p32), and activity of a tyrosine kinase (TK-32). As determined by the ability of the sperm to undergo the A23187-induced acrosome reaction, pig sperm require bicarbonate and calcium but not BSA for capacitation in vitro. The appearance of p32 was assessed by immunoblotting SDS-extracted and separated sperm proteins using an anti-phosphotyrosine antibody. The appearance of p32 requires calcium, although p32 appears even in the absence of bicarbonate in the incubation medium, demonstrating that the appearance of this tyrosine phosphoprotein is not a final end point of pig sperm capacitation. An in-gel tyrosine kinase renaturation assay showed that TK-32 activity depends on calcium and bicarbonate in the incubation medium. Immunoprecipitation experiments using an anti-phosphotyrosine antibody and inhibitor demonstrated that p32 and TK-32 are different proteins. These data indicate that the signal transduction mechanisms of capacitation in pig sperm are different from those in other mammals, suggesting that certain species specificity may exist with respect to this phenomenon.

Capacitation is associated with tyrosine phosphorylation and tyrosine kinase-like activity of pig sperm proteins

Capacitation represents the final maturational steps that render mammalian sperm competent to fertilize, either in vivo or in vitro. Capacitation is defined as a series of events that enables sperm to bind the oocyte and undergo the acrosome reaction in response to the zona pellucida. Although the molecular mechanisms involved are not fully understood, sperm protein phosphorylation is associated with capacitation. The hypothesis of this study is that protein tyrosine phosphorylation and kinase activity mediate capacitation of porcine sperm. Fresh sperm were incubated in noncapacitating or capacitating media for various times. Proteins were extracted with SDS, subjected to SDS-PAGE, and immunoblotted with an antiphosphotyrosine antibody. An M(r) 32 000 tyrosine-phosphorylated protein (designated as p32) appeared only when the sperm were incubated in capacitating medium and concomitant with capacitation as assessed by the ionophore-induced acrosome reaction. The p32 was soluble in Triton X-100. Fractionation of sperm proteins with Triton X-114 demonstrated that after capacitation, this tyrosine phosphoprotein is located in both the cytosol and the membrane. Enzyme renaturation of sperm proteins was conducted in gels with or without either poly glu:tyr (a tyrosine kinase substrate) or kemptide (a protein kinase A substrate). An M(r) 32 000 enzyme with kinase behavior was observed in all gels but was preferentially phosphorylated on tyrosine, as assessed by phosphorimagery and by thin layer chromotography to identify the phosphoamino acids. Indirect immunolocalization showed that the phosphotyrosine residues redistribute to the acrosome during capacitation, which is an appropriate location for a protein involved in the acquisition of fertility.

Guinea pig fertilin exhibits restricted lateral mobility in epididymal sperm and becomes freely diffusing during capacitation

The guinea pig sperm protein fertilin functions in sperm-egg plasma membrane binding. Fertilin is initially present in the plasma membrane of the whole head in testicular sperm, then becomes concentrated into the posterior head domain during epididymal passage. Fertilin remains localized to the posterior head plasma membrane following the acrosome reaction, when it functions in sperm-egg interaction. Fluorescence redistribution after photobleaching was used to examine the lateral mobility of fertilin in both acrosome-intact and acrosome-reacted sperm. Fertilin exhibited highly restricted lateral mobility in both testicular and epididymal sperm (D < 10(-10) cm(2)/s). However, fertilin in acrosome-reacted sperm was highly mobile within the membrane bilayer (D = 1.8 x 10(-9) cm(2)/s and %R = 84). Measurement of the lateral mobility of fertilin in capacitated, acrosome-intact sperm revealed two populations of cells. In approximately one-half of the cells, lateral mobility of fertilin was similar to sperm freshly isolated from the cauda epididymis; while in the other half fertilin was highly mobile. The release of fertilin from interactions that restrict its lateral mobility may regulate its function in sperm-egg interaction.

Lipid dynamics in the plasma membrane of ram and bull spermatozoa after washing and exposure to macromolecules BSA and PVP

Seminal plasma proteins and macromolecules in the external medium have a major influence on the functionality of sperm plasma membranes. In this investigation we have examined their effects on lipid diffusion in the surface membrane of ram and bull spermatozoa as measured by fluorescence recovery after photobleaching (FRAP). Results show that progressive removal of seminal plasma from ram spermatozoa by repeated centrifugation and resuspension in media +/- 4% bovine serum albumin (BSA) or 0.4% polyvinlypyrrolidone (PVP) causes a reduction in lipid diffusion in all regions of the membrane. By contrast, bull sperm membranes respond with an increase in diffusion in all regions. Repeated washing of bull spermatozoa whose membranes were previously immobile (i.e., showed no recovery after FRAP) restored lipid diffusion suggesting an inhibitory effect of seminal plasma proteins. Further analysis by atomic force microscopy revealed a close association between BSA and the plasma membrane. It is concluded that diffusion of lipids in the plasma membrane of ejaculated ram and bull spermatozoa is influenced by seminal plasma proteins and the composition of the suspending medium. Mol. Reprod. Dev. 59:306-313, 2001.

Identification of capacitation-associated phosphoproteins in porcine sperm electroporated with ATP-gamma-(32)P

Our objectives were to incorporate ATP-gamma-(32)P into boar sperm to radiolabel endogenous phosphoproteins and compare phosphorylation patterns from sperm incubated in capacitating (CM) and non-capacitating conditions (NCM). Sperm were electroporated (1000 V/cm, 125 microF/cm, 65 Omega/cm, 0.3 msec) with ATP-gamma-(32)P which moderately decreased sperm viability (P < 0.01), but did not affect motility (P = 0.34) or the appearance of spontaneous acrosome reactions (P = 0.49). Sperm incubated in CM for 3 hr underwent capacitation, determined by the ability to undergo ionophore-induced acrosome reactions (P </= 0.05). Furthermore, more sperm in CM than in NCM exhibited chlortetracycline (CTC) pattern B (capacitated) fluorescence (P </= 0.01). SDS-PAGE, autoradiography and phosphoimagery of extracted, (32)P-labeled sperm proteins revealed a subset of phosphoproteins (Mr 28,000-60,000) from cells incubated in CM, whereas only two phosphorylated proteins were evident from sperm in NCM (44 and 57 kDa). The appearance of phosphoproteins increased concomitant with capacitation (P </= 0.05). In NCM, the 44 kDa protein was unaffected by time (P > 0.05) and the 57 kDa phosphoprotein increased after capacitation (P </= 0.05). Computer-assisted analysis revealed that the percentage of motile sperm in either medium decreased with time, and CM only transiently maintained motility over NCM (P >/= 0.02). ATP-gamma-(32)P can, therefore, be incorporated into porcine sperm to radiolabel endogenous phosphoproteins, and the different profiles from sperm incubated in NCM versus CM suggest that capacitation is mediated by signaling events involving protein phosphorylation.

Dynamics in the membrane organization of the mammalian sperm cell and functionality in fertilization

The capacitation process of sperm cells involves complex changes in the composition and orientation of molecules at the surface of the sperm cell. Here we focus on the lipid architecture in the sperm plasma membrane and demonstrate that the sperm plasma membrane is not static but is an extremely dynamic structure. Advanced fluoroscopic techniques enabled continuous monitoring of lipid organization in living cells and extremely rapid lipid movements were observed. The orientation of lipids in the sperm plasma membrane changed under capacitative treatments, was found to be sensitive for temperature and also changed upon binding of sperm cells to the zona pellucida. The changes in membrane properties coincided with an activation of protein kinases resulting in tyrosine phosphorylation of specific plasma membrane proteins. The detected membrane changes relate to intrinsic membrane properties such as fluidity, permeability, adhesiveness and fusibility. We think that these results may provide a physiological basis for new assays, able to discriminate between functional and non-physiological sperm cells.

Lipid dynamics in the plasma membrane of fresh and cryopreserved human spermatozoa

Preserving the integrity of the plasma membrane of spermatozoa is crucial for retention of their fertilizing capacity, especially after stressful procedures such as freezing and storage. In this investigation we have measured lipid diffusion in different regions of the plasma membrane of fresh and cryopreserved human spermatozoa using a sensitive, high resolution fluorescence photobleaching technique (FRAP) with 5-(N-octadecanyl)aminofluorescein as reporter probe. Results show that diffusion was significantly faster on the plasma membrane overlying the acrosome and decreased progressively in the postacrosome, midpiece and principal piece. The midpiece plasma contains a higher proportion of immobile lipids than other regions. In cryopreserved spermatozoa, lipid diffusion in the plasma membrane was significantly reduced on the acrosome, postacrosome and midpiece relative to fresh spermatozoa. Diffusion, however, could be restored to normal levels by washing spermatozoa in a medium containing 0.4% polyvinylpyrrolidine but not in medium alone or in medium containing 0.4% albumin. These results suggest that (i) lipid dynamics in the plasma membrane of human spermatozoa varies significantly between surface regions; (ii) in-plane diffusion is adversely affected by cryopreservation; and (iii) washing frozen spermatozoa in 0.4% polyvinylpyrrolidine restores membrane lipid fluidity to normal levels. The latter finding has important implications for improving the fertility of human spermatozoa following cryopreservation.