Demembranation and reactivation of mammalian spermatozoa from golden hamster and ram

New insights into epididymal function in relation to sperm maturation

Testicular spermatozoa acquire fertility only after 1 or 2 weeks of transit through the epididymis. At the end of this several meters long epididymal tubule, the male gamete is able to move, capacitate, migrate through the female tract, bind to the egg membrane and fuse to the oocyte to result in a viable embryo. All these sperm properties are acquired after sequential modifications occurring either at the level of the spermatozoon or in the epididymal surroundings. Over the last few decades, significant increases in the understanding of the composition of the male gamete and its surroundings have resulted from the use of new techniques such as genome sequencing, proteomics combined with high-sensitivity mass spectrometry, and gene-knockout approaches. This review reports and discusses the most relevant new results obtained in different species regarding the various cellular processes occurring at the sperm level, in particular, those related to the development of motility and egg binding during epididymal transit.

Hyperactivation of monkey spermatozoa is triggered by Ca2+ and completed by cAMP

Digital image analysis of the flagellar movements of cynomolgus macaque spermatozoa hyperactivated by caffeine and cAMP was carried out to understand the change in flagellar movements during hyperactivation. The degree of flagellar bending increased remarkably after hyperactivation, especially at the base of the midpiece. Mainly two beating patterns were seen in the hyperactivated monkey sperm flagella: remarkably asymmetrical flagellar bends of large amplitude and relatively symmetrical flagellar bends of large amplitude. The asymmetrical bends were often seen in the early stage of hyperactivation, whereas the symmetrical bends executed nonprogressive, figure-of-eight movement. Beat frequency of the hyperactivated spermatozoa significantly decreased while wavelength of flagellar waves roughly doubled. To determine the conditions under which the axonemes of hyperactivated sperm flagella have asymmetrical or symmetrical bends, the plasma membranes of monkey spermatozoa were extracted with Triton X-100 and motility was reactivated with MgATP(2-) under various conditions. The asymmetrical flagellar bends were brought about by Ca(2+), whereas the symmetrical flagellar bends resulted from low levels of Ca(2+) and high levels of cAMP. Under these conditions, beat frequency and wavelength of flagellar waves of demembranated, reactivated spermatozoa were similar to those of the hyperactivated spermatozoa. These results suggest that during hyperactivation of monkey spermatozoa intracellular Ca(2+) concentrations first rise, and then decrease while cAMP concentrations increase simultaneously.

Roles of cAMP in regulating microtubule sliding and flagellar bending in demembranated hamster spermatozoa

To understand the mechanism regulating spermatozoa motility, it is important to investigate the mechanism regulating the conversion of microtubule sliding into flagellar bending. Therefore, we analyzed microtubule sliding and its conversion into flagellar bending using a demembranated spermatozoa model in which microtubule sliding and flagellar bending could be analyzed separately by treating the demembranated spermatozoa with and without dithiothreitol, respectively. Using this model, we examined the roles of cAMP and its target molecules in regulating flagellar bending and microtubule sliding. Although flagellar bending did not occur in the absence of cAMP, microtubule extrusion occurred without it, suggesting that cAMP is necessary for the conversion of microtubule sliding into flagellar bending, but not for microtubule sliding itself. The target of cAMP for regulating flagellar bending was not cAMP-dependent protein kinase (PKA), since flagellar bending was still observed in the spermatozoa treated with a PKA-specific inhibitor. Alternatively, the Epac/Rap pathway may be the target. Epac2 and Rap2 were detected in hamster spermatozoa using immunoblotting. Since Rap2 is a GTPase, we investigated the flagellar bending of demembranated spermatozoa treated with GTPgammaS. The treatment markedly increased the beat frequency and bending rate. These results suggest that cAMP activates the Epac/Rap pathway to regulate the conversion of microtubule sliding into flagellar bending.

Calcium/Calmodulin and Calmodulin Kinase II Stimulate Hyperactivation in Demembranated Bovine Sperm1

Hyperactivated motility is observed among sperm in the mammalian oviduct near the time of ovulation. It is characterized by high-amplitude, asymmetrical flagellar beating and assists sperm in penetrating the cumulus oophorus and zona pellucida. Elevated intracellular Ca2+ is required for the initiation of hyperactivated motility, suggesting that calmodulin (CALM) and Ca2+/CALM-stimulated pathways are involved. A demembranated sperm model was used to investigate the role of CALM in promoting hyperactivation. Ejaculated bovine sperm were demembranated and immobilized by brief exposure to Triton X-100. Motility was restored by addition of reactivation medium containing MgATP and Ca2+, and hyperactivation was observed as free Ca2+ was increased from 50 nM to 1 microM. However, when 2.5 mM Ca2+ was added to the demembranation medium to extract flagellar CALM, motility was not reactivated unless exogenous CALM was readded. The inclusion of anti-CALM IgG in the reactivation medium reduced the proportion hyperactivated in 1 microM Ca2+ to 5%. Neither control IgG, the CALM antagonist W-7, nor a peptide directed against the CALM-binding domain of myosin light chain kinase (MYLK2) inhibited hyperactivation. However, when sperm were reactivated in the presence of CALM kinase II (CAMK2) inhibiting peptides, hyperactivation was reduced by 75%. Furthermore, an inhibitor of CAMK2, KN-93, inhibited hyperactivation without impairing normal motility of intact sperm. CALM and CAMK2 were immunolocalized to the acrosomal region and flagellum. These results indicate that hyperactivation is stimulated by a Ca2+/CALM pathway involving CAMK2.

Reducing agents induce microtubule extrusion in demembranated mammalian spermatozoa

To understand the mechanism regulating flagellar bending in spermatozoa, it is important to investigate the regulation of microtubule sliding in the flagellar axoneme. It has been shown that protease treatment following demembranation with Triton X-100 disrupts the connections between microtubules and induces extrusion of microtubules from the flagellar axoneme. This approach enables a direct investigation of the regulation of microtubule sliding; however, the percentage of spermatozoa with protease-induced extrusion was relatively low, probably due to protease digestion of some regulatory motility proteins, as well as proteins connecting the microtubules. In this study, we demonstrate microtubule extrusion in most hamster and mouse demembranated spermatozoa upon treatment with a high concentration of the reducing agents dithiothreitol or 2-mercaptoethanol, without the use of proteases. The extrusion of microtubules occurred when the spermatozoa were treated with concentrations of the reducing agents that were sufficient for the reduction of the disulfide bonds of IgG. These results suggest that the arrangement of the axonemal structures connecting doublet microtubules depends to an important degree on -S-S- bonds. Close observation of the extrusion process using the present method revealed that microtubules were extruded on the same side as that of the curve of the sperm head, and also on the opposite side. Furthermore, we noted that extrusion always started on one side, followed by the other side, but was never initiated on both sides simultaneously.

Motor apparatus in human spermatozoa that lack central pair microtubules

Electron microscopic examination of the spermatozoa from a man suffering from asthenozoospermia (poor or low sperm motility) showed that approximately 92% of the sperm flagella lacked central pair microtubules but possessed dynein arms and radial spokes while a small percentage of the spermatozoa had complete flagella. The characteristics of the motor apparatus of the spermatozoa and the effects of caffeine on the sperm motility were examined, as were the reactivation of demembranated spermatozoa and the sliding of doublet microtubules. Almost all spermatozoa were immotile in a Tyrode solution while only a small percentage of spermatozoa showed slow forward movement or feeble flagellar vibration, whereas addition of caffeine to the sperm suspension induced forward swimming of approximately half of the spermatozoa. The reactivation of demembranated spermatozoa with MgATP(2-) could not succeed because of disintegration of the demembranated flagella. However, when the demembranated spermatozoa were exposed to MgATP(2-) and then treated with elastase, the microtubular doublets of approximately half the number of the flagella slid from the end or middle of the flagella. These results suggest that the motor apparatus in the sperm flagella that lack the central pair microtubules is functionally assembled and intrinsically capable of undergoing flagellar movement but not strong enough to beat normally.

Hyperactivated motility of bull sperm is triggered at the axoneme by Ca2+ and not cAMP

Hyperactivated motility, a swimming pattern of mammalian sperm in the oviduct, is essential for fertilization in vivo. It is characterized by high-amplitude flagellar waves and, usually, highly asymmetrical flagellar beating. It had been suggested, but not tested, that Ca2+ and cAMP switch on hyperactivation by directly affecting the flagellar axoneme. In this study, the direct affects of these agents on the axoneme were tested by using detergent-demembranated bull sperm. As confirmed by TEM, treatment of sperm with 0.2% Triton X-100 disrupted the plasma, acrosomal, and inner mitochondrial membranes, leaving axonemes intact. In the presence of 2 mM ATP, the percentage of reactivated sperm that were hyperactivated increased to 80% when free Ca2+ was increased from 50 to 400 nM. The effect of the Ca2+ in this range was to increase beat asymmetry by increasing the curvature of the principal bend. No additional increases were observed above 400 nM free Ca2+, but motility was suppressed at 1 mM. The ability of Ca2+ to produce hyperactivation depended on ATP availability, such that more ATP was required to produce the high amplitude flagellar bends characteristic of hyperactivated motility than to produce activated motility. Cyclic AMP was not required for reactivation, nor for hyperactivation. Production of hyperactivated motility also required an alkaline environment (pH 7.9-8.5). These results suggest that, provided sufficient ATP is present and pH is sufficiently alkaline, Ca2+ switches on hyperactivation by enabling curvature of the principal bends to increase.

Maintenance of motility in mouse sperm permeabilized with streptolysin O

One approach to studying the mechanisms governing sperm motility is to permeabilize sperm and examine the regulation of motility by manipulating the intracellular milieu of the cell. The most common method of sperm permeabilization, detergent treatment, has the disadvantage that the membranes and many proteins are extracted from the cell. To avoid this problem, we have developed a method that uses streptolysin O to create stable pores within the plasma membrane while leaving internal membranes intact. Sperm were permeabilized, preincubated, and then treated with 0.6 U/ml of streptolysin O. Permeabilization was assessed by fluorescent dye technologies and endogenous protein phosphorylation using exogenously added [gamma-32P]ATP. Streptolysin O-induced permeabilization rendered the sperm immotile, and the effect was Ca2+-dependent. When the cells were treated simultaneously with a medium containing ATP, streptolysin O-treated sperm maintained flagellar movement. These results demonstrate that the streptolysin O permeabilization model system is a useful experimental method for studying the mechanisms that regulate sperm motility since it allows the flagellar apparatus to be exposed to various exogenously added molecules.

Reactivation of demembranated, cytosol-free ram spermatozoa

A procedure for preparing cytosol-free ram sperm models was developed. Sperm are introduced to a Triton X-100-containing demembranation medium layered on top of a discontinuous Percoll gradient. After brief exposure to the demembranating solution, the sperm are separated from the detergent-soluble components by centrifugation through a 55% Percoll layer, finally collecting on top of a 90% Percoll cushion from where they are recovered. Optimum conditions consisted of Triton X-100 at 0.20% and a demembranation time of 35 sec. Cross-sections of midpieces and principal pieces of the demembranated sperm were examined by electron microscopy. With 0.20% Triton X-100 in the demembranation medium, 86% of the cross-sections showed no plasma membranes and the rest had broken plasma membranes. The remaining tail structures appeared to be morphologically intact. Assay of phosphoglucose isomerase as a marker enzyme confirmed that at least 98% of the cytosolic protein was removed. Ram sperm models obtained by this procedure could be reactivated, and the percent motility and beat parameters were similar to those of the intact sperm. Reconstitution with the detergent-soluble components was neither required for, nor enhanced, reactivation. Therefore, demembranated ram sperm do not require a detergent-soluble protein factor for reactivation.