Initiation of spermiogenesis in C. elegans: a pharmacological and genetic analysis

Spermiogenesis in Caenorhabditis elegans involves the conversion of spherical, sessile spermatids into bipolar, crawling spermatozoa. In males, spermiogenesis is induced by mating, while in hermaphrodites, spermiogenesis occurs before the first oocytes are fertilized. Alternatively, spermiogenesis can be induced in vitro by treatment with monensin triethanolamine, or pronase. Treatment with the calmodulin inhibitors, trifluoperazine, chlorpromazine, or W7, also induces spermiogenesis in vitro with a half maximal effect at 20 microM. Upon initial activation, spermatids extend long, thin spikes and undergo extensive cellular movements. Eventually, a single motile pseudopod forms through the restructuring of one or more of these spikes. These transient spikes can be prolonged in vitro by removing triethanolamine as soon as the spermatids first form spikes. Spermatids from spe-8 and spe-12 spermatogenesis-defective (spe) mutants activate in vivo with male but not hermaphrodite sperm activator. In vitro, the mutant spermatids arrest spermiogenesis at the spike stage when activated with pronase, but form normal spermatozoa if subsequently or initially treated with monensin or triethanolamine. We present a model of spermiogenesis in which the mutant defects and the action of the pharmacological agents are ordered relative to one another.     

Vesicle fusion,pseudopod extension and amoeboid motility are induced in nematode spermatids by the ionophore monensin

The sodium- and potassium-transporting ionophore monensin induces the maturation of Caenorhabditis elegans spermatids to spermatozoa in vitro. Rearrangement of cytoplasm, fusion of membranous organelles with the plasma membrane and growth of pseudopodia, all characteristic of in vivo spermiogenesis, occur within five minutes after exposure to monensin at concentrations of 0.1–1.0 μM. This activation is dependent upon external Na⁺ and K⁺ ions but not Ca²⁺ ions. Monensin-activated spermatozoa have normal morphology and normal amoeboid motility. During activation spermatids twitch and rotate prior to pseudopod extension. Analysis of intermediates by transmission and scanning electron microscopy reveals that the sequence of morphogenetic events leading from the spherical spermatid to the polarized spermatozoan involves microvilli rearrangement and membranous organelle fusion, cytoplasmic polarization, then pseudopod extension.     

Evidence for a role for cellular alkalinization in the cyclic adenosine 3',5'-monophosphate-mediated initiation of motility in bovine caput spermatozoa

Bicarbonate ion, the local anesthetics procaine and dibucaine, and the ionophores monensin and nigericin have been shown to markedly increase the ability of agents that elevate cyclic adenosine monophosphate (cAMP) levels to initiate motility in bovine caput spermatozoa. A number of other weak bases, including theophylline, D-600 and dipyridamole, elevate cAMP levels maximally in caput sperm at low levels but induce motility only at high levels. These compounds thus appear to have a dual role in the initiation of motility, i.e., they elevate both cAMP levels and internal pH. Confirmation of this view was provided by the demonstration that bicarbonate ion and procaine permit initiation of motility by theophylline, D-600 and dipyridamole at markedly reduced levels. Also, forskolin (a neutral adenylate cyclase activator) elevates cyclic AMP levels in caput sperm but initiates motility only in the presence of bicarbonate or procaine, and the membrane-permeant cAMP analogue 8-bromo-cAMP is capable of inducing motility only in the presence of bicarbonate. Thus, motility in caput sperm is induced only under conditions that elevate both intracellular cAMP and pH, whereas caudal sperm motility is stimulated by an elevation of either cAMP or pH. These data suggest that the epididymal development of motility requires a maturational increase in internal pH. This suggestion was confirmed by direct measurement of the internal pH of caput and caudal sperm; the internal pH of the former was found to be 5.84 +/- 0.1 and the latter 6.27 +/- 0.05.     

Regulation of sperm flagellar motility by calcium and cAMP-dependent phosphorylation

There is substantial evidence that cAMP-dependent phosphorylation is involved in the activation of motility of spermatozoa as they are released from storage in the male reproductive tract. This evidence includes observations that in vivo activation of motility can be inhibited by protein kinase inhibitors, can be reversed by protein phosphatase treatment of demembranated spermatozoa, and is associated with phosphorylation of sperm proteins, and observations that spermatozoa that have not been activated in vivo can be activated in vitro by cAMP-dependent phosphorylation. Activation in vivo can often be triggered by conditions that increase intracellular pH, but the relevance of this to in vivo activation under natural conditions and the steps between pH increase and cAMP increase have not been fully established. The relationships between changes in the protein substrates for cAMP-dependent phosphorylation and changes in axonemal function are still unknown. Sperm chemotaxis to egg secretions is widespread; in the sea urchin Arbacia, the egg jelly peptide resact has been identified as a chemoattractant. Response to chemoattractants involves changes in asymmetry of flagellar bending waves, and similar changes in asymmetry can be produced in vitro by increases in [Ca++]. Temporal changes in resact receptor occupancy might lead to transient changes in intracellular [Ca++] and the asymmetry of flagellar bending, but many links in this hypothetical sequence remain to be established. Both of these signalling systems offer immediate opportunities for investigations of biochemical pathways leading to easily assayable biological responses. However, complications resulting from interactions between these two systems need to be considered.     

秀丽线虫精子发生和精子受精的研究进展

秀丽线虫精子发生过程包括减数分裂和精子活化两个阶段, 通过早期特异基因的表达和后期蛋白分子的翻译后修饰, 精原细胞发育成为具有运动能力的精子。其受精阶段包括精子运动、精子竞争、精卵信号通讯以及精卵融合等过程。通过突变体筛选目前已经获得了一些影响精子发生或受精的突变体, 并且对其中一些突变体进行了基因克隆和功能分析的研究。这些研究不仅对于阐明精子发生和受精的机理具有重大的理论意义, 而且对男性不育的治疗和男性无毒避孕药物的研发可能提供重要的依据。文章阐述了目前在线虫精子发生和精子受精两个方面的研究进展。     

The effects of extracellular sodium on acid release and motility initiation in rat caudal epididymal spermatozoa in vitro

When rat caudal epididymal spermatozoa were incubated in a sodium-free solution, they suffered a progressive fall in motility, and by 40 min the motility was completely suppressed. However, upon resuspending the spermatozoa in a sodium containing solution, motility was completely restored within 15 min. During this reinitiation period, H⁺ ions were found to be released from spermatozoa. Both motility reinitiation and acid release were found to be closely dependent on the extracellular sodium. Of the other monovalent cations studied, only NH₄⁺ could replace Na⁺ in these events. Both processes were partially inhibited by amiloride (10^?4-10^?3 M) and ouabain (10^?4-10^?3 M) but was unaffected by acetazolamide (10^?4 M). The motility activation and acid release were studied under various conditions. It was found that there was a close correlation between the two processes. The H⁺ efflux during motility activation was accompanied by a rise in the intracellular pH of the sperm. It is proposed that the requirement of Na⁺ for the motility initiation is attributed to an increase in the intracellular pH via the Na⁺-H⁺ exchange. An intracellular pH shift might be involved in motility activation in mammalian sperm.     

Regulation of sperm activation by SWM-1 is required for reproductive success of C. elegans males

BACKGROUND: Sexual reproduction in animals requires the production of highly specialized motile sperm cells that can navigate to and fertilize ova. During sperm differentiation, nonmotile spermatids are remodeled into motile spermatozoa through a process known as spermiogenesis. In nematodes, spermiogenesis, or sperm activation, involves a rapid cellular morphogenesis that converts unpolarized round spermatids into polarized amoeboid spermatozoa capable of both motility and fertilization. RESULTS: Here we demonstrate, by genetic analysis and in vivo and in vitro cell-based assays, that the temporal and spatial localization of spermiogenesis are critical determinants of male fertility in C. elegans, a male/hermaphrodite species. We identify swm-1 as a factor important for male but not hermaphrodite fertility. We show that whereas in wild-type males, activation occurs after spermatids are transferred to the hermaphrodite, swm-1 mutants exhibit ectopic activation of sperm within the male reproductive tract. This ectopic activation leads to infertility by impeding sperm transfer. The SWM-1 protein is composed of a signal sequence and two trypsin inhibitor-like domains and likely functions as a secreted serine protease inhibitor that targets two distinct proteases. CONCLUSIONS: These findings support a model in which (1) proteolysis acts as an important in vivo trigger for sperm activation and (2) regulating the timing of proteolysis-triggered activation is crucial for male reproductive success. Furthermore, our data provide insight into how a common program of gamete differentiation can be modulated to allow males to participate in reproduction in the context of a male/hermaphrodite species where the capacity for hermaphrodite self-fertilization has rendered them nonessential for progeny production.     

Evidence for a delta pH-driven Ca2+ uptake in EGTA-treated bovine spermatozoa

Calcium efflux from ejaculated bovine spermatozoa occurred upon incubation in Ca2+/EGTA buffers with Ca2+ ion concentrations ranging from 0.1 microM to 1 nM. Both total cellular calcium and cytosol free Ca2+ concentrations, the latter measured with Quin 2, were inversely correlated with the Ca2+ activity of the medium. An influx of radioactive 45Ca2+ parallel to a net efflux of calcium took place in spermatozoa incubated in 45Ca2+/EGTA buffers with 45Ca2+ activity of 0.01 microM or 0.1 microM. The uptake of the radioactive isotope was higher in spermatozoa incubated at pH 7.8 than that found at pH 6.8, increased in the presence of acetate or amiloride but decreased when ammonium chloride or monensin was added to the incubation mixture. Addition of acetate produced a decrease of the cytoplasmic pH, determined with the indicator carboxyfluorescein, whereas addition of NH4Cl or monensin caused a pH increase. Addition of either nigericin or monensin to spermatozoa suspended in a choline medium containing low concentrations of Na+, K+ and Ca2+ produced a cytosolic acidification, the subsequent addition of Ca2+ caused a cytosolic alkalinization parallel to an increase of the cytosolic free Ca2+. Addition of CaCl2 to EGTA-pretreated spermatozoa resuspended in a poorly buffered medium induced an evident decrease of extracellular pH suggesting a cellular proton extrusion. Both monensin and nigericin caused an increase of the calcium transport in spermatozoa suspended in a choline medium containing a physiological concentration of 1.5 mM CaCl2. Taken together the present results indicate that, under the experimental conditions used, a delta pH-driven Ca2+ uptake occurs in ejaculated bovine spermatozoa and suggest that Ca2+ is taken up in exchange with H+.     

Motility of fish spermatozoa: from external signaling to flagella response

For successful fertilization, spermatozoa must access, bind, and penetrate an egg, processes for which activation of spermatozoa motility is a prerequisite. Fish spermatozoa are stored in seminal plasma where they are immotile during transit through the genital tract of most externally fertilizing teleosts and chondrosteans. Under natural conditions, motility is induced immediately following release of spermatozoa from the male genital tract into the aqueous environment. The nature of an external trigger for the initiation of motility is highly dependent on the aquatic environment (fresh or salt water) and the species’ reproductive behavior. Triggering signals include osmotic pressure, ionic and gaseous components of external media and, in some cases, egg-derived substances. Extensive study of environmental factors influencing fish spermatozoa motility has led to the proposal of several mechanisms of activation in freshwater and marine fish. However, the signal transduction pathways initiated by these mechanisms remain clear. This review presents the current knowledge with respect to (1) membrane reception of the activation signal and its transduction through the spermatozoa plasma membrane via the external membrane components, ion channels, and aquaporins; (2) cytoplasmic trafficking of the activation signal; (3) final steps of the signaling, including signal transduction to the axonemal machinery, and activation of axonemal dyneins and regulation of their activity; and (4) pathways supplying energy for flagellar motility.     

Effects of ATP, cAMP and pH on the initiation of flagellar movement in demembranated models of rat epididymal spermatozoa

Demembranated model of rat epididymal spermatozoa was employed to establish the conditions for the initiation of flagellar movement. Extensive initiation of the flagellar movement required 0.5 mM ATP, 1 μM cAMP and pH 7.9. The requirement for ATP was highly specific and can partially be replaced by 2′-deoxy-ATP only, but not by analogs of ATP or other nucleoside triphosphates. In contrast, the cAMP requirement was less specific and can partially be replaced by other cyclic nucleotides and cAMP-analogs except 2′-deoxy-cAMP and 2′,3′-cAMP. The data implied that the intracellular pH rise, not the cAMP increase, was the probable trigger for the initiation of sperm motility after ejaculation. During sperm maturation, the sperm motile apparatus appeared unchanged with respect to the above conditions of reactivation.     

Initiation,prolongation, and reactivation of the motility of salmonid spermatozoa

The motility of salmonid spermatozoa initiated by dilution of the milt with ovarian fluid or isotonic saline is brief duration; it was believed that it can be activated only once in the life of the spermatozoon. Dilution of the milt with an equal volume of isotonic saline (0.12 M-NaCl) containing 5 mM-3-isobutyl-1-methylxanthine (MIX) prolonged and intensified sperm motiliy. When motility had stopped after initial mobilization with saline or ovarian fluid, it could be reactivated by addition of MIX; reactivated spermatozoa fertilized eggs. Dilution with saline containing K⁺ (24 mEq/liter) did not initiate sperm motility even in the presence of MIX. The spermatozoa were mobilized by subsequent with 0.12 M-NaCl. The concentration of adenosine triphosphate (ATP) in sperm suspensions dropped on dilution with saline and rose as motility ceased, but declined without subsequent recovery following dilution with MIX-saline. The concentration of cyclic adenosine monophosphate (cAMP) rose and fell sharply on initiation of motility and rose again after motility had declined. While salmonid spermatozoa can be mobilized by dilition with saline alone, the effectiveness of MIX in reactivating “spent” spermatozoa supports the assumption that cAMP plays a role in the initiation of sperm motility.     

Effects of deep freezing on the energy metabolism of bovine spermatozoa during in vitro capacitation: A cytochemical approach

In order to study the effects of deep freezing on the energy metabolism of bovine spermatozoa, a cytochemical quantitative study was carried out by a microdensitometric method on cytochrome oxidase and lactate dehydrogenase (LDH) activities. These were evaluated in situ on individual frozen-thawed bull spermatozoa collected at different times during in vitro capacitation. The results showed that in bull spermatozoa both the initiation of motility and capacity to fertilize eggs were associated with the anaerobic rather than aerobic glycolysis. The freezing-thawing processes and storage in liquid nitrogen induced a general enhancement of both the enzymatic activities examined. The high ionic strength treatment gave rise to a significant but reversible decrease in both the cytochrome oxidase and LDH activities in the fresh as well as in the frozen-stored sperm. The findings, based on cytochemical observations of energy metabolism of spermatozoa and evaluated during in vitro capacitation, suggest that the respiration and the anaerobic glycolysis of spermatozoa seem to be slightly impaired by the freezing-thawing and storage processes.     

Changes in guinea pig sperm intracellular sodium and potassium content during capacitation and treatment with monovalent ionophores

Guinea pig spermatozoa were collected from the caudae epididymides in various isotonic solutions and the intracellular sodium and potassium content was determined by atomic absorption spectroscopy. The sperm intracellular Na and K content was found to be influenced by large variations in the extracellular concentrations of these ions. Treatment of spermatozoa suspended in a saline-based solution with the monovalent ionophores monensin or nigericin caused an approximate 2-fold increase in the intracellular Na content and a 3–6 fold decrease in the intracellular K content. Incubation of the spermatozoa in a K⁺-free minimal culture medium (MCM-PL) at a pH of 7.6 or 8.3 for 2 hr caused an approximate 2-fold increase in the sperm intracellular Na content and a 5-fold decrease in the intracellular K content. The motile spermatozoa incubated for 2 hr at pH 7.6 showed less than 5% acrosome reactions, compared with 30–40% acrosome reactions after incubation at pH 8.3, in response to the addition of 5 mM Ca²⁺. Changes in the sperm intracellular elemental composition during culture in vitro, which may lead to an acrosome reaction, are discussed.     

Loss of calcium in human spermatozoa via EPPIN, the semenogelin receptor

The development of a new male contraceptive requires a transition from animal model to human and an understanding of the mechanisms involved in the target's inhibition of human spermatozoan fertility. We now report that semenogelin (SEMG1) and anti-EPPIN antibodies to a defined target site of 21 amino acids on the C terminal of EPPIN cause the loss of intracellular calcium, as measured by Fluo-4. The loss of intracellular calcium explains our previous observations of an initial loss of progressive motility and eventually the complete loss of motility when spermatozoa are treated with SEMG1 or anti-EPPIN antibodies. Thimerosal can rescue the effects of SEMG1 on motility, implying that internal stores of calcium are not depleted. Additionally, SEMG1 treatment of spermatozoa decreases the intracellular pH, and motility can be rescued by ammonium chloride. The results of this study demonstrate that EPPIN controls sperm motility in the ejaculate by binding SEMG1, resulting in the loss of calcium, most likely through a disturbance of internal pH and an inhibition of uptake mechanisms. However, the exact steps through which the EPPIN-SEMG1 complex exerts its effect on internal calcium levels are unknown. Anti-EPPIN antibodies can substitute for SEMG1, and, therefore, small-molecular weight compounds that mimic anti-EPPIN binding should be able to substitute for SEMG1, providing the basis for a nonantibody, nonhormonal male contraceptive.     

Aquaporin inhibition changes protein phosphorylation pattern following sperm motility activation in fish

Our previous studies demonstrated that osmolality is the key signal in sperm motility activation in Sparus aurata spermatozoa. In particular, we have proposed that the hyper-osmotic shock triggers water efflux from spermatozoa via aquaporins. This water efflux determines the cell volume reduction and, in turn, the rise in the intracellular concentration of ions. This increase could lead to the activation of adenylyl cyclase and of the cAMP-signaling pathway, causing the phosphorylation of sperm proteins and then the initiation of sperm motility. This study confirms the important role of sea bream AQPs (Aqp1a and Aqp10b) in the beginning of sperm motility. In fact, when these proteins are inhibited by HgCl₂, the phosphorylation of some proteins (174 kDa protein of head; 147, 97 and 33 kDa proteins of flagella), following the hyper-osmotic shock, was inhibited (totally or partially). However, our results also suggest that more than one transduction pathways could be activated when sea bream spermatozoa were ejaculated in seawater, since numerous proteins showed an HgCl₂(AQPs)-independent phosphorylation state after motility activation. The role played by each different signal transduction pathways need to be clarified.     

Comparison of sperm velocity, motility and fertilizing ability between firstly and secondly activated spermatozoa of common carp (Cyprinus carpio)

The objective of the study was to compare carp sperm motility performances (sperm velocity and motility rates) from 10 males including fertilizing ability (hatching rates from 10 males and eight females) as a function of time elapsed after sperm exposure to activation medium in two situations: firstly activated sperm and sperm which had terminated swimming and was ‘re‐activated’ after incubation in a K⁺ rich (200 mm KCl) non‐swimming solution. In case of both initial (first) and secondly activated spermatozoa, the motility was triggered in hatchery solution (HAS, 11.2 mOsmol) and in carp activation solution (CAS, 128.9 mOsmol) containing 45 mm NaCl, 5 mm KCl, 30 mm Tris–HCl while also adjusted to a pH of 8.0. First time activated sperm showed significantly higher relative motility, sperm velocity and fertilizing ability compared to re‐activated sperm. The carp spermatozoa (in either first or second activation) rapidly lost their fertilizing ability as a function of exposure time of sperm to diluents prior to addition to eggs: this shows that spermatozoa must be in contact with eggs as soon as their motility is triggered. When sperm was firstly activated in CAS and also activated a second time in CAS (labeled CASCAS) the hatching rate was significantly higher at egg contact after 10, 20, 30, and 120 s of activation. Also at 20 s after the second activation of the sperm higher sperm motility was observed compared to the first activation. This study showed that incubation of spermatozoa in a K⁺‐rich incubation medium can mitigate the affects of structural damages occurring in re‐activated sperm, which may help spermatozoa to increase their motility and fertilization. To our knowledge, the results presented in this study document for the first time that fertilization can be achieved with sperm re‐activated a second time while being exposed to a incubation medium that permits ATP reloading within the flagellum. Previous studies have show the potential for recovery of motility, however, the effect on possible fertilization is hitherto unknown. It critical outcome of the study clearly indicated the need for avoiding the use of different, subsequent activation media (e.g. first and second activation) but only on the same medium for both steps (see above CASCAS).     

Sperm motility in the horseshoe crab. IV. Extracellular ions and intracellular pH are not mediators of motility initiation

Upon dilution into sea water, Limulus spermatozoa undergo a brief flurry of motility (duration < 60 sec), after which they are nonmotile until encountering a sperm motility initiating peptide (SMI) that emanates from eggs. Utilizing highly purified SMI extracts and simplified seawater formulations (from which individual ions have been deleted), we found that no specific extracellular ion is required for either dilution-initiated or SMI-initiated motility. Indeed, deletion of one ion (Na⁺) produced dilution-initiated motility of very long duration (several hours). When motility is initiated by SMI (in normal seawater) there is an increase in intracellular pH (pH_i), as indicated by the fluorescent probe, 9-amino acridine; however, this pH, change is not a trigger for motility. As a more general method examining ion movements, the fluorescent probe diS-C₃-(5) was used to qualitatively measure changes in the membrane potential of spermatozoa. Although crude SMI extracts caused membrane depolarization, further purification resulted in an almost complete separation of this activity from SMI, thus showing that SMI activation is apparently an electroneutral event. (The membrane-depolarizing factor has a molecular weight > 30,000 and does not initiate acrosome reactions.) Experiments utilizing the ionophore A23187 and Ca⁺²-blocking agents (verapamil and TMB-8) provided tentative evidence that mobilization of intracellular Ca⁺² may be required for motility initiation. These results show that neither changes in pH_i nor the influx of specific extracellular ions are direct mediators of SMI-initiated motility; however, experiments with pharmacologic agents indicate a possible role for intracellular Ca⁺².     

A volatile inhibitor immobilizes sea urchin sperm in semen by depressing the intracellular pH

Sea urchin spermatozoa are normally immotile in semen, but motility can be initiated by increasing gas flow over the semen--for example, by blowing N2 gas over a thin layer of semen. This result indicates that sperm motility is not O2 limited and suggests that seminal fluid contains a volatile inhibitor of motility which is responsible for the paralysis of sperm in semen. This inhibitor might be carbon dioxide, which reversibly immobilizes sperm. 31P-NMR measurements of pH show that the sperm intracellular pH (pHi) increases by 0.36 pH unit upon dilution of semen into seawater. Since previous studies have shown that this magnitude of pH increase is sufficient to trigger sperm motility, we suggest that the volatile inhibitor is inhibiting sperm motility in semen by depressing the pHi. A simple hypothesis that explains these observations is that the volatile motility inhibitor is CO2, which could acidify pHi as a diffusable weak acid. In this regard, sperm diluted into seawater release acid, and this acid release is related to the pHi increase and motility initiation. In fact, nearly half of the acid released by sperm upon dilution is volatile and may therefore be due to CO2 efflux. Most of the acid, however, cannot be attributed to CO2 release because it is not volatile. Thus, when sperm are diluted into seawater, they raise their pHi by releasing CO2 and protons from the cytoplasm into the surrounding seawater.     

Roles of pH and cyclic adenosine monophosphate in the acquisition of potential for sperm motility during migration from the sea to the river in chum salmon

In the natural process of the migration of chum salmon from the sea to the river, spermatozoa moved from the testis to the sperm duct, and the pH value of seminal plasma, concentration of cyclic adenosine monophosphate (AMP) in the sperm cells, and potential for sperm motility increased. Cyclic AMP levels and the potential for motility gradually increased when testis spermatozoa with no capacity for movement were incubated in the artificial seminal plasma of which the pH was much the same as, or higher than, the pH of natural seminal plasma from the sperm duct. Such correlation in motility, pH, and cyclic AMP suggests that the increases in seminal pH and intracellular cyclic AMP level during passage of spermatozoa from the testis to the sperm duct cause the acquisition of potential for motility. Motility of testicular spermatozoa demembranated with Triton X-100 was very low in fish caught in the sea, while motility of spermatozoa from the posterior portion of the sperm duct was much higher in fish caught in the river. Furthermore, nondemembranated, intact spermatozoa showed a lag in the timing of the acquisition of potential for motility vs. demembranated spermatozoa: The demembranated sperm exhibited the potential earlier than the nondemembranated sperm. These data suggest that increase in activity of the motile apparatus, the axoneme, is a prerequisite, in part, for the acquisition of sperm motility, whereas the development of some function of the plasma membrane also contributes to this phenomenon. © 1993 Wiley-Liss, Inc.