Progesterone-stimulated intracellular calcium increase in human spermatozoa is protein kinase C-independent

Indirect studies suggested that protein kinase C (PKC) has a role in sperm motility and the acrosome reaction. Physiological inducers of the sperm acrosome reaction include progesterone, which can increase intracellular calcium ([Ca2+]i), tyrosine phosphorylation of proteins and chloride efflux in human spermatozoa. PKC may be involved in progesterone-stimulated acrosome reaction, although controversial results have been obtained concerning the effect of PKC inhibition on progesterone-stimulated [Ca2+]i increase. In the present study, we investigated the direct effect of progesterone on the activity of PKC, as well as the effect of a panel of PKC inhibitors on progesterone- stimulated [Ca2+]i increase and tyrosine phosphorylation of proteins. We found that progesterone stimulates sperm PKC activity and that PKC inhibition with staurosporine and bisindolylmaleimide partially reversed the effect of progesterone on acrosome reaction, indicating an involvement of the enzyme in the effect of the steroid. We next evaluated the effect of three different PKC inhibitors (sangivamycin, staurosporine and bisindolylmaleimide) on progesterone-stimulated [Ca2+]i increase. Neither short-term (15 min) nor long-term (90 min) preincubation with any of the three compounds had a substantial effect on the stimulatory effect of progesterone on sperm [Ca2+]i. Nor was responsiveness to progesterone affected by either short-term (determining activation of PKC) or long-term (determining down- regulation of PKC) incubation with the tumour promoter phorbol myristate acetate (PMA), a known non-physiological stimulator of PKC. These results indicate that progesterone-stimulated calcium influx is independent of PKC activation. In addition, we found that preincubation with PKC inhibitors had a stimulatory effect per se on tyrosine phosphorylation of sperm proteins. When compared with the appropriate control, the effect of progesterone on tyrosine phosphorylation was slightly (but not significantly) reduced by the inhibitors, sangivamycin, staurosporine and bisindolylmaleimide, but was significantly inhibited by calphostin C. These results do not permit a final conclusion on the involvement of PKC in progesterone-stimulated tyrosine phosphorylation of sperm proteins. However, the lack of effect of PMA on tyrosine phosphorylation indicates that PKC stimulation is not sufficient to induce this effect. In conclusion, our results indicate that PKC plays a role in progesterone-induced acrosome reaction and that progesterone-stimulated PKC activation is downstream to stimulation of calcium influx by the steroid.      

Modelling human sperm-egg interactions in vitro: signal transduction pathways regulating the acrosome reaction

Recent advances in characterizing sperm surface receptors and ion channels, when combined with the rapidly expanding knowledge of interactions among second messenger systems in somatic cells, permit formulation of a tentative molecular mechanism for the regulation of the human sperm acrosome reaction. As spermatozoa pass through the cumulus mass, progesterone binds to its sperm surface receptor, alkalinizes the sperm head cytosol and potentiates changes in intracellular ionized calcium. Primary binding of spermatozoa to egg involves receptors for mannosyl, N-acetylglucosaminyl and, possibly, fucosyl residues of the glycosylated zona protein, ZP3. These receptors aggregate on multivalent ligand binding, migrate to the equatorial region along an actin filament network formed between the plasma and acrosomal membranes during capacitation, and activate a G protein/protein kinase A/protein kinase C second messenger system and a secondary proteolysis signal. Binding of a receptor tyrosine kinase to ZP3 amino acid residues simultaneous with the sugar recognition event triggers tyrosine phosphorylation signalling. All signals combine to open a voltage-dependent calcium channel. The resulting elevated calcium signal depolymerizes the inter-membrane actin network and activates phospholipases, leading to an acrosome reaction.      

Co-Expression of Mannose-Ligand and Non-Nuclear Progesterone Receptors on Motile Human Sperm Identifies an Acrosome-Reaction Inducible Subpopulation

PROBLEM : To determine whether surface expression of receptors for progesterone and mannose can be used to identify spermatozoa likely to undergo an acrosome reaction after zona binding and to compare the reactivity of these receptors with naturally occurring sperm head-directed anti-sperm antibodies (ASAs). METHOD : Progesterone binding sites on the surface of fresh and capacitated motile human sperm in relation to acrosome status were visualized using a cell-impermeant progesterone. Free progesterone and/or mannose ligands were compared for percent sperm binding and ability to induce an acrosome reaction. Western blots of sperm proteins localized to the plasma membrane and surface proteins precipitated following passive transfer of serum ASAs were probed with progesterone-horseradish peroxidase. The effects of the same ASAs on ligand binding and on the induced acrosome reaction were examined. RESULTS : The two receptors are located in close proximity on a subset of capacitated motile sperm and are coordinately cleared from the plasma membrane overlying the acrosomal cap prior to exocytosis. The surface appearance of functional binding sites for each ligand, however, is regulated by different mechanisms and the progesterone receptor alone is specifically precipitated by ASAs. Passive transfer of ASAs to capacitated sperm selectively inhibits the progesterone-stimulated acrosome reaction but not the ionomycin-induced acrosome reaction or the ability of sperm to bind mannose ligands. CONCLUSIONS : Sperm from fertile donors incubated under capacitating conditions in vitro can be subdivided into acrosome reaction inducible and noninducible subpopulations on the basis of the co-expression or total absence of these receptors. The combined data indicate that reaction of sperm surface progesterone receptors with ASAs contributes to the acrosome reaction insufficiency observed in anti-sperm immune infertility.     

Involvement of osmo-sensitive calcium influx in human sperm activation

Mammalian spermatozoa must undergo capacitation and acrosomereaction before fertilization. To date, the precise mechanismsregulating these complex processes are not well understood butit is generally agreed that they involve an influx of calciumfrom the extracellular space through, as yet, poorly characterizedplasma membrane pathways. Here we present evidence for a novelmechanism to increase intracellular calcium concentration viaa calcium influx pathway activated by sperm cell swelling. Activationof this influx pathway by a mild hypo-osmotic shock and theensuing calcium rise are a potent stimulus for sperm acrosomereaction. Furthermore, hypo-osmolarity-activated spermatozoaare fully competent for oocyte fertilization. During transitalong male and, after ejaculation, female genital tracts spermatozoaare known to be exposed to extracellular fluids of widely differentosmolarity; thus osmo-sensitive calcium influx could have acrucial regulatory role in the cellular events preceding fertilization. acrosome reaction/calcium/human/spermatozoa/stretch-activated channels     

gamma-Aminobutyric acid (GABA) induces the acrosome reaction in human spermatozoa

The sperm acrosome reaction takes place in response to progesterone and zona pellucida. Progesterone may act on more than one type of surface receptor, of which one is a gamma-aminobutyric acid (GABA) type A-like receptor. Although there is direct evidence of GABA initiation of mouse sperm acrosome reaction, there are conflicting results regarding GABA- induced exocytosis in human spermatozoa. We have examined whether GABA would initiate exocytosis in human spermatozoa using the chlortetracycline assay and a zona-free hamster oocyte test. Human spermatozoa preincubated for > or = 3 h in Biggers-Whitten-Whittingham medium with 0.35% bovine serum albumin underwent acrosome reactions in response to GABA, with maximal responses in spermatozoa preincubated for 9 h. The effect was concentration-dependent. Preincubated spermatozoa treated with GABA were able to fertilize a higher proportion of zona-free oocytes, with a higher number of spermatozoa penetrating each oocyte. Exposure of preincubated spermatozoa to GABA and progesterone together resulted in a higher proportion of acrosome reactions than when each agonist was used alone. The effect of GABA was mediated by the influx of extracellular Ca2+ because inclusion of EGTA or the Ca2+ channel antagonist La3+ prevented GABA-induced acrosome reactions. These results indicate that GABA can initiate exocytosis in capacitated human spermatozoa and open up possibilities for studies of signalling mechanisms activated upon occupancy of the GABAA receptor present on the sperm surface.      

Co-localization of the inositol 1,4,5-trisphosphate receptor and calreticulin in the equatorial segment and in membrane bounded vesicles in the cytoplasmic droplet of human spermatozoa

Modulation of the intracellular calcium concentration within mammalian spermatozoa is important in several pre-fertilization events including hyperactivated motility and the acrosome reaction. To identify calcium binding proteins (CBP) potentially regulating these processes, a (45)Ca overlay technique was employed on 2-D blots of human sperm extracts. Microsequencing by Edman degradation and CAD mass spectrometry identified a relatively abundant 60.5 kDa CBP with a pI of 4.2 as calreticulin (CRT). Immunofluorescent labelling with anti-CRT antibodies localized CRT to the acrosome, with highest fluorescence in the equatorial segment, and in the cytoplasmic droplets of 94 and 48% of human spermatozoa respectively. Double immunolabelling experiments demonstrated co-localization of CRT and the inositol 1,4,5-trisphosphate receptor (IP(3)R) in the acrosome, in the equatorial segment, and vesicular structures in the cytoplasmic droplets of the neck region. Electron microscopic immunogold labelling localized CRT to the equatorial segment of acrosome-reacted spermatozoa and to membrane-enclosed vesicles within the cytoplasmic droplet of both acrosome-intact and acrosome-reacted spermatozoa. Localization of the IP(3) receptor to the CRT-containing vesicles, in the sperm neck and to the acrosome, suggests that capacitative calcium entry in human spermatozoa may be regulated from these putative calcium storage sites.     

Identification and localisation of SERCA 2 isoforms in mammalian sperm

Upon binding to the egg's zona pellucida, capacitated spermatozoa will undergo a calcium-dependent exocytotic event called acrosome reaction. During this process, Ca2+ depletion from internal stores is followed by an important rise in [Ca2+]i due to a massive Ca2+ influx. Previous reports have shown that the acrosome can act as a Ca2+ store and that depletion of thapsigargin-sensitive stores induces acrosome exocytosis in capacitated spermatozoa from different mammalian species. The effect of thapsigargin, a specific inhibitor of sarcoplasmic/endoplasmic reticulum Ca2+-ATPases (SERCAs), suggests the presence and implication of SERCA in the active Ca2+ uptake during mammalian sperm capacitation. Although the presence of a thapsigargin-sensitive Ca2+-ATPase has been debated, the aim of this study was to clearly determine whether SERCAs are present in mammalian spermatozoa. Using three different anti-SERCA 2 antibodies, mono- and polyclonal, which recognised the same protein, we successfully identified and localised SERCA 2 in human, mouse and bovine sperm. Western blot analysis suggests that more than one SERCA 2 splice variant are present, one detected in the fraction containing the outer acrosomal membranes and another one present in the subcellular fraction containing the sperm midpiece. These results were confirmed by indirect immunofluorescence where SERCA 2 was observed in the acrosome and midpiece regions of human sperm. SERCA 2 immunohistochemical studies on human testis and PCR-amplification of mRNA encoding for each SERCA 2 splice variant in spermatogenic cells support the presence of this Ca2+-ATPase family in mature spermatozoa. In this paper, we clearly demonstrate, for the first time, the presence of SERCA 2 in mammalian sperm.     

Effects of progesterone on sperm function: mechanisms of action

Progesterone stimulates sperm functions, e.g. hyperactivation, acrosome reaction, binding to oocyte zona pellucida and penetration rate into the hamster oocyte. The physiological relevance of these effects has been shown using female genital tract fluids which modulate sperm function according to their progesterone content. Progesterone interacts with specific sperm binding sites that, unlike the classic nuclear receptors, are located on the plasma membrane of the spermatozoon. Binding studies have revealed the presence of two classes of progesterone receptors in the human spermatozoon, one class has an elevated affinity constant (nanomolar) and is specific for progesterone, whereas the other class has an affinity constant in the micromolar range and binds equally well other hydroxylated progesterone derivatives. Following exposure to progesterone, the main event is a rapid (within seconds) increase of the intracellular free calcium concentration, followed by a sustained rise lasting for several minutes (plateau phase). Both these calcium transients are dependent upon entry of extracellular calcium. The nature of the calcium channel that mediates the effects of progesterone is, currently, unknown. It has been postulated that it may be: (i) part of the progesterone receptor; (ii) voltage-dependent; or (iii) operated by second messengers following activation of the progesterone receptor. Progesterone also modulates sperm function by stimulating a trypsin-like proteolytic activity, the biosynthesis of polyamine (putrescine and spermidine), phospholipase A2 activity and protein tyrosine kinase activity in the sperm cell. Recent studies have shown that chloride ion efflux is vital for progesterone to promote the acrosome reaction. This effect is achieved by interaction with a sperm membrane receptor which resembles the neuronal GABA(A) receptor. Accordingly, GABA(A) receptors have been found in the spermatozoon plasma membrane and GABA stimulates hyperactivation and promotes the acrosome reaction.     

Regulators of sperm function: Measurement of intracellular pH in human spermatozoa by flow cytometry with the benzo(c)xanthene dye SNAFL-1: a novel, single excitation, dual emission, molecular probe

In numerous animal species the acrosome reaction of spermatozoahas been linked to elevations in intracellular pH (pHi). However,whether or not this is merely a passive consequence of calciumion influx is not known. Studies into the fluctuations in pHiin sperm cells have been hampered by the lack of a pH-sensitiveprobe that could be used in conjunction with flow cytometry.In this study, flow cytometric analysis of pHi in human spermatozoawas accomplished by using one of the new benzo[c]xanthene dyes(SNAFL-1). SNAFL-1 was then observed in situ with conventionalfluorescent microscopy and was found to be located in the post-acrosomalcytoplasm of the head. It was then used to measure the differencesin pHi between acrosome-intact populations of spermatozoa, andpopulations that had been induced to acrosome-react with humanfollicular fluid or the calcium ionophore A23187 to mimic thecalcium influx. It was concluded that the human sperm acrosomereaction is also accompanied by a rise in pHi and the naturalagonist-induced rise could not be accounted for by calcium ioninflux alone. acrosome reaction/flow cytometry/human spermatozoa/intracellular pH/SNAFL-1     

Pregnancy following discontinuation of a calcium channel blocker in the male partner

The fertility potential of human sperm populations can be assessedby the presence of head-directed mannose ligand receptors (mannose-specificlectin) and the occurrence of spontaneous acrosome reactionsafter incubation under capacitating conditions in vitro. Wehave reported previously on the interaction between anti-hypertensivemedications and their effects on these parameters of male fertilitypotential. In this report we document the effects of cessationof calcium ion channel blocker medication on male fertility.Motile spermatozoa from a 30 year old infertile patient on acalcium ion channel blocker as anti-hypertensive treatment hadsubnormal expression of mannose-specific lectin and did notexhibit spontaneous acrosome reactions. Three months followingdiscontinuation of the medications, complete recovery of boththe expression of head-directed mannose ligand receptors andthe acrosome reaction was documented, though sperm motilityand morphology remained unchanged. The couple had 2 years ofinfertility and previously failed to conceive through sevencycles of Pergonal/intra-uterine insemination. Conception occurredon the second Pergonal/intra-uterine insemination cycle afterthe husband discontinued calcium ion channel blocker medication.Calcium ion channel blockers may adversely affect sperm fertilizingpotential. Discontinuation of such medications enhances thechances for conception.     

Analgesic action of loperamide,an opioid agonist,and its blocking action on voltage-dependent Ca2+ channels

We investigated the relationship between the antinociceptive effect of the opiate agonist loperamide at the spinal level and its inhibitory effect on calcium influx. Intrathecal administration of loperamide showed a significant antinociceptive effect in the formalin test, which was not prevented by naloxone. On the other hand, no significant effects were observed by nicardipine, an L-type specific blocker, or by BAY K8644, an L-type specific agonist, suggesting no significant role of L-type calcium channels in nociceptive signal transduction. Loperamide suppressed the calcium influx in dorsal root ganglion neurons. As the antinociceptive effect of loperamide was not affected by naloxone or other calcium channel blocking toxins, and loperamide showed a direct inhibitory effect on calcium-influx, the analgesic effect of intrathecally injected loperamide might be due to its blockade of the voltage-dependent calcium channels at the terminals of the primary afferent fibers.     

慢性缺氧对大鼠肺动脉平滑肌细胞胞内钙的影响及其机制研究

目的:研究慢性缺氧对大鼠肺动脉平滑肌细胞(PASMCs)胞内钙浓度(［Ca²⁺］_i)的影响及L-型钙通道和胞内钙库的作用,为缺氧性肺动脉高压(HPH)发病机制的进一步研究提供理论依据。 方法:复制大鼠缺氧性肺动脉高压动物模型，利用Fura－2/AM钙离子成像方法测定PASMCs在不同钙离子浓度细胞外液及L-型钙通道阻滞剂nifedipine和IP3R钙通道抑制剂肝素干预前后 ［Ca²⁺］_i变化。 结果:(1)缺氧＋含钙外液组PASMCs ［Ca²⁺］_i 显著高于对照＋含钙外液组（P<0.05）。缺氧＋含钙外液组PASMCs ［Ca²⁺］_i显著高于缺氧＋无钙外液组（P<0.05）。(2)缺氧nifedipine组PASMCs［Ca²⁺］_i在加药前后无显著差异（P>0.05）。（3）缺氧未干预组与缺氧肝素组PASMCs ［Ca²⁺］_i无明显差异（P>0.05）。 结论:慢性缺氧可使PASMCs的［Ca²⁺］_i增加。慢性缺氧引起［Ca²⁺］_i增加可能与细胞外钙内流有关，L-型钙通道和IP3R钙通道在调节［Ca²⁺］_i的过程中可能不独立发挥作用。     

Calcium channels in embryonic chick skeletal muscle cells after cultivation with calcium channel blocker.

The effects of chronic treatment with calcium channel blockers were studied on the expression of voltage-dependent calcium channels (VDCCs) in chick skeletal muscle cells developing in culture. Myotubes were treated after 2 days in culture with either 20 microM D600 or 10 microM nifedipine, and measurements were made of the maximum rate of rise (M.R.R.) of the two components of action potential, operated by T- and L-type VDCCs, respectively. Treatment with either blocker reduced the M.R.R. of the action potential component operated by the L-type VDCC throughout the culture period examined. The M.R.R. of the T-type VDCC component, on the other hand, was unaffected by either treatment. The reduction in the M.R.R. of the L-type component in blocker-treated cells is thought to be due to the down-regulation of the expression of L-type VDCC. Thus, it appears that the expression of L-type VDCC in the chick skeletal muscle cells can be regulated by a function of L-type VDCC, which mediate the entry of Ca2+ into the cells. The physiological significance of the L-type VDCC, which expressed prominently early in the development of skeletal muscle cells, for the differentiation of excitability is discussed.     

Dihydropyridine block of voltage-dependent K currents in rat dorsal root ganglion neurons

The dihydropyridines nifedipine, nimodipine and Bay K 8644 are widely used as pharmacological tools to assess the contribution of L-type voltage-gated Ca²⁺ channels to a variety of neuronal processes including synaptic transmission, excitability and second messenger signaling. These compounds are still used in neuronal preparations despite evidence from cardiac tissue and heterologous expression systems that they block several voltage-dependent K⁺ (Kv) channels. Both because these compounds have been used to assess the relative contribution of L-type Ca²⁺ channels to several different processes in dorsal root ganglion (DRG) neurons and because a relatively wide variety of Kv channels present in other neuronal populations is present in DRG neurons, we determined the extent to which dihydropyridines block Kv currents in these neurons. Standard whole cell patch clamp techniques were used to study acutely disassociated adult rat DRG neurons. All three dihydropyridines tested blocked Kv currents in DRG neurons; IC₅₀ values (concentration resulting in an inhibition that is 50% of maximum) for nifedipine and nimodipine-induced block of sustained Kv currents were 14.5 and 6.6 μM, respectively. The magnitude of sustained current block was 44±1.6%, 60±2%, and 56±2.9% with 10 μM nifedipine, nimodipine and Bay K 8644, respectively. Current block was occluded by neither 4-aminopyridine (5 mM) nor tetraethylammonium (135 mM). Dihydropyridine-induced block of Kv currents was not associated with a shift in the voltage-dependence of current activation or inactivation, the recovery from inactivation, or voltage dependent block. However, there was a small use-dependence to the dihydropyridine-induced block. Our results suggest that several types of Kv channels in DRG neurons are blocked by mechanisms distinct from those underlying block of Kv channels in cardiac myocytes. Importantly, our results suggest that if investigators wish to explore the contribution of L-type Ca²⁺ channels to neuronal function, they should consider alternative strategies for the manipulation of these channels than the use of dihydropyridines.     

Participation of the sperm proteasome in human fertilization

BACKGROUND: Fertilization in mammals comprises the sequential interactions of the sperm with the cumulus oophorus, zona pellucida, and oocyte plasma membrane. Here we investigate proteasome activity in human sperm and its possible involvement during the fertilization process. METHODS: Proteasome activity was measured in intact sperm and in sperm extracts using the fluorogenic substrate Suc-Leu-Leu-Val-Tyr-AMC, in the presence or absence of the specific proteasome inhibitor, clasto-lactacystin beta-lactone. The participation of the proteasome was evaluated during (i) sperm-zona binding using the hemizona assay; (ii) zona pellucida-induced acrosome reactions with a pulse and chase design; (iii) progesterone-induced acrosome reactions incubating overnight capacitated sperm with progesterone; and (iv) progesterone-induced Ca(2+) influx using fura-2AM. RESULTS: Intact sperm and sperm extracts possessed proteasome activity, which was susceptible to inhibition by clasto-lactacystin beta-lactone. Sperm-zona binding was not inhibited by clasto-lactacystin beta-lactone. However, both zona pellucida- and progesterone-induced acrosome reactions were inhibited by clasto-lactacystin beta-lactone. The proteasome inhibitor also blocked the sustained phase of the Ca(2+) influx provoked by progesterone but not the peak. CONCLUSION: The human sperm proteasome is involved in the exocytosis of the acrosome, perhaps in events upstream of the plateau phase of the Ca(2+) influx.     

Use of mannose ligands in IVF screens to mimic zona pellucida-induced acrosome reactions and predict fertilization success

A predictive test for determining whether motile populations of human spermatozoa will fertilize eggs in vitro has been an elusive goal of clinical research. We have developed an assay for the ability of motile human spermatozoa to bind fluorescein isothiocyanate-labelled mannosylated bovine serum albumin (Man-FITC-BSA) as a test for the presence of sperm surface receptors (lectins) for mannose ligands. Mannosylated ligands are present on the human zona pellucida and are involved in the species-specific binding of human spermatozoa to the zona pellucida. We now demonstrate in prospective blinded analysis that the fractional increase in acrosome loss following a mannose ligand challenge is highly correlated with the rate of fertilization in vitro. Using an incremental increase of acrosome exocytosis of >0.1 as a threshold to predict which specimens will yield normal fertilization, the assay has a sensitivity of 97.8%, a specificity of 83.3%, a positive predictive value of 95.7% and a negative predictive value of 90.7%. These data indicate that testing for a mannose-induced acrosome reaction may be useful in assessment of sperm function prior to in- vitro fertilization in order to assign males to conventional insemination or intracytoplasmic sperm injection protocols.