Epididymal protein Rnase10 is required for post-testicular sperm maturation and male fertility

Abstract

Eutherian spermatozoa are dependent on the environment of the proximal epididymis to complete their maturation; however, no specific epididymal factors that mediate this process have so far been identified. Here, we show that targeted disruption of the novel gene Rnase10 encoding a secreted proximal epididymal protein in the mouse results in a binding defect in spermatozoa and their inability to pass through the uterotubal junction in the female. The failure to gain the site of fertilization in the knockout spermatozoa is associated with a gradual loss of ADAM3 and ADAM6 proteins during epididymal transit. In the distal epididymis, these spermatozoa appear to lack calcium-dependent associations with the immobilizing glutinous extracellular material and are released as single, vigorously motile cells that display no tendency for head-to-head agglutination and lack affinity to the oviductal epithelium. In sperm-egg binding assay, they are unable to establish a tenacious association with the zona pellucida, yet they are capable of fertilization. Furthermore, these sperm show accelerated capacitation resulting in an overall in vitro fertilizing ability superior to that of wild-type sperm. We conclude that the physiological role of sperm adhesiveness is in the mechanism of restricted sperm entry into the oviduct rather than in sperm-egg interaction.—Krutskikh, A., Poliandri, A., Cabrera-Sharp, V., Dacheux, J. L., Poutanen, M., Huhtaniemi, I. Epididymal protein Rnase10 is required for post-testicular sperm maturation and male fertility.

In all eutherian mammals, spermatozoa released from the seminiferous epithelium are infertile and display only slight vibratory movements. They attain their fertilizing capacity only during passage through the epididymis where they encounter a continuously changing luminal environment created by the interplay of secretory and absorptive activities of the androgen-dependent epithelium. In this transit, not only do spermatozoa acquire their fertilizing capacity but they are also prepared for prolonged androgen-dependent storage in the distal epididymis, where they accumulate, and from where they are subsequently released at ejaculation in very large numbers. The now mature spermatozoa are able to move forward and ascend the female reproductive tract. However, only a small proportion of the male gametes reach the proximal oviduct where they are able to develop hyperactivated motility, undergo the acrosome reaction and penetrate the oocyte's vestiments to finally fuse with the oolemma.

While the need for sperm maturation in the epididymis generally appears to be related to events in the female reproductive tract, our understanding of this relationship remains limited (1). Exact links, as well as the underlying molecular mechanisms, can be uncovered by applying the methods of reverse genetics and inactivating candidate genes expressed in the epididymis. Of particular interest are the genes whose products are secreted into the lumen; however, the few epididymis-specific knockout (KO) mice that have so far been generated were shown to have normal fertility (2–5).

The early studies involving epididymal duct ligation in the rabbit (6) or high epididymovasostomy in the rat (7) have demonstrated that fertilizing ability is conferred on spermatozoa in the proximal part of the epididymis and that to a large extent, this process appears to be tied to the initial segment (IS).

Through exploration of the mouse epididymal transcriptome, we identified a few novel genes expressed specifically in the proximal epididymis (8). One of these genes has been named Rnase10 because of its homology to the pancreatic Rnase A and its location within the same gene cluster on chromosome 14. It was found to be expressed in the IS and predicted to encode a secreted protein. Its porcine orthologue has been identified as the most abundantly secreted protein in the proximal epididymis (9). On 2-dimensional gel electrophoresis, it appears as a series of variably glycosylated isoforms (called train A), together constituting up to 86% of all adluminally secreted proteins in the two uppermost segments of the porcine epididymis (10). In this species, it lacks ribonucleolytic activity, does not appear to associate with passing spermatozoa, and is secreted and fully reabsorbed within the same zone of the epididymis (11).

We produced a KO mouse for Rnase10 (12) and report here that the encoded product is the first single epididymal secreted protein found to be essential for sperm maturation and normal male fertility, providing evidence that inactivation of function of a single epididymis-specific gene product may be a feasible strategy to develop a male contraceptive.

MATERIALS AND METHODS

Unless otherwise stated, all chemicals were purchased from Sigma-Aldrich (Dorset, UK).

Animals and genotyping

All experimental work involving animals was carried out with approval of the Imperial College London ethics committee and in line with the UK national regulations. The modified allele Rnase10^Cre was produced by targeted gene inactivation in embryonic stem cells so as to disrupt the endogenous Rnase10 translation initiation site in exon 2 with an iCre-Neo cassette (12). The mutated allele was transferred onto, and subsequently maintained on, 2 genetic backgrounds: the inbred C57BL/6 and outbred CD1. Male fertility and sperm transport in the female reproductive tract were assessed on both backgrounds; all other experiments were carried out using CD1 animals. Genotype was established by PCR using the following pairs of primers: 5′-CAGGCAGGCCTTCTCTGAAC-3′ and 5′-CATTCTCCCACCATCGGTGC-3′ for cre; 5′-TGGGGAATGTGAGGAGAAGG-3′ and 5′-GCCAAGTGCCAGACCTTCTG-3′ for intact Rnase10 allele.

Quantitative real-time RT-PCR analysis

For the isolation of total RNA, fresh tissues (IS) were processed using the Qiagen RNeasy Mini Kit (Crawley, Sussex, UK), and RNA concentration and purity were determined by measuring OD₂₆₀ and OD₂₈₀ on a spectrophotometer. After DNaseI treatment (Invitrogen, Paisley, UK), first-strand cDNA synthesis was carried out with random primers using the high-capacity cDNA reverse transcription kit (Applied Biosystems, Paisley, UK). Real-time PCR was performed with SYBR Green I dye (Applied Biosystems, Paisley, UK) and the following primers: Rnase10 5′-TAGGAGAGCAGAACTGGGGA-3′ and 5′-ATGCACCAGTGTCACCTTCA-3′; Gapdh 5′-AAGGGCTCATGACCACAGTC-3′ and 5′-GGATGCAGGGATGATGTTCT-3′. Detection of a single amplicon was confirmed by a dissociation curve at the end of real-time PCR cycle. Cycle threshold (C_T) values were analyzed as described previously (13) using Gapdh as an internal control.

Immunoblotting

Protein from washed spermatozoa or homogenized IS was extracted by boiling for 5 min in Tris-buffered saline containing 1% SDS with subsequent removal of insoluble material by centrifugation at 20,000 g for 15 min. Protein extracts resolved by SDS-PAGE were transferred to a nitrocellulose membrane using a semidry blotting method (14). After transfer, membranes were blocked in 5% skimmed milk and 0.1% Tween-20 in Tris-buffered saline, and then incubated with primary antibody overnight at 4°C. Antibodies used were 7C1.2 anti-a disintegrin and metallopeptidase (ADAM) 3 and 9D2.2 anti-ADAM2 monoclonal antibodies (1:700; Millipore, Watford, UK), M-145 anti-ADAM6 polyclonal antibody (1:1000; Santa Cruz Biotechnology, Santa Cruz, CA, USA), 4G10 anti-phosphotyrosine antibody (1:4000; Millipore), and anti-COX4 monoclonal antibody (1:1000; Abcam, Cambridge, UK); Western blot analysis of epididymal epithelium proteins was carried out with a rabbit anti-Rnase10 antiserum (J. L. Dacheux, Institut National de Recherche Agronomique, Nouzilly, France) and monoclonal anti-β-actin antibody (1:5000, Abcam, Cambridge, UK). A corresponding secondary antibody conjugated to horseradish peroxidase was applied at 1:2000 dilution, and protein bands were detected with ECL reagent (GE Healthcare, Little Chalfont, UK). Images shown are representative of ≥3 independent experiments. Densitometric analysis of Western blot bands was carried out using the U.S. National Institutes of Health ImageJ software (http://rsbweb.nih.gov/ij/).

Timed mating and assessment of male fertility

Estrous CD1 females (6–10 wk old; Harlan, Oxon, UK) were detected by observing changes in the appearance of external genitalia and paired with males that had been caged singly for ≥3 d. Typically, mating took place at around midpoint of the dark period of the diurnal cycle, and mated females were detected by observation of copulatory plugs. In the male fertility studies, numbers of fetuses and corpora lutea were counted between estimated d 7.5 and 12.5 of pregnancy, and reproductive rates were calculated. For the calculation of in vivo fertilization rates, eggs collected on the morning postcoitum were examined for the presence of pronuclei in a stereomicroscope.

Studies of sperm transport in the female reproductive tract

Serial 10-μm-thick sections of oviducts recovered ∼2 h postcoitum (preovulatory phase of sperm transport) and 5–6 h postcoitum (periovulatory phase) were stained with hematoxylin and eosin, and sperm in the lumen were quantified on microscopy. To establish sperm numbers per ejaculate, females were killed within 1 h postcoitum, uterine contents were flushed with PBS, and sperm concentration in the lavage from both uterine horns was estimated using a Neubauer chamber.

Collection and preparation of sperm

Caudae epididymides were cannulated via vas deferens and microperfused collecting contents with a glass capillary. In studies of sperm function, the perfusate was added to an aliquot of T6 (15) or M16 (Millipore) medium to allow sperm dispersal and/or capacitation. For immunoblotting, sperm from upper segments of the epididymis and testis were released by disrupting the tubules and purified from epithelial debris using 35% isotonic Percoll (GE Healthcare); purity of isolated spermatozoa was confirmed microscopically.

Assessment of sperm acrosomal status and vitality

Given that the acrosome is unstable and commonly undergoes disintegration soon after cell death (i.e., degenerative acrosome loss), a triple fluorescent staining technique permitting discrimination between live and dead cells was developed for the determination of sperm acrosomal status. Rhodamine-labeled Arachis hypogaea lectin specifically binding to the sperm outer acrosomal membrane and fluorescein-labeled Pisum sativum lectin with affinity to the acrosomal content were used in combination with the membrane semipermeant DNA dye Hoechst 33342 (Invitrogen, Paisley, UK). Numbers of dead spermatozoa detected with Heochst 33342 were found to correlate with those scored using two other supravital staining techniques (eosin-nigrosin, r=0.84, P<0.001 and ethidium bromide, r=0.89, P<0.001). Hoechst 33342 was added to 50 μl of sperm suspension (5×10⁶ cells/ml) at a final concentration of 0.5 μg/ml. Nuclei of dead spermatozoa were stained for 1 min, and the suspension was smeared onto a poly-l-lysine-coated slide. Once air-dried, cells were fixed by immersion in methanol for 2–3 min and stored dry in the dark until needed. Acrosomes were stained in premixed lectins (100 μg/ml in PBS), slides were examined under a fluorescent microscope scoring live acrosome-intact/acrosome-reacted and dead acrosome-intact/acrosome-reacted cells. At least 300 cells were scored for each sample.

Large-scale preparation of zonae pellucidae

Large numbers of zonae pellucidae were isolated by Percoll (GE Healthcare) density gradient centrifugation of ovarian homogenates according to the method of Bleil, as described previously (16). Zonae fragments were washed in T6 medium, and their concentration was estimated microscopically by counting large fragments as halves in a 2-μl drop. Suspension was adjusted to 90 large fragments/μl, and zonae pellucidae were dissolved by heating to 60°C for 4 min. Complete dissolution was confirmed microscopically, and aliquots were stored at −20°C until required.

In vitro fertilization (IVF) and sperm competition assay

Superovulation was induced in young CD1 females with 5 U of pregnant mare's serum gonadotropin (i.p.) followed by 5 U of human chorionic gonadotropin (hCG; i.p.) after 46–48 h. Females were killed 12–14 h after hCG injection, and oviducts with ovulated cumulus-oocyte complexes were excised. Most females responded to the hormonal treatment, and cumulus masses were visible in the oviductal ampullae. Experiments were carried out in 300-μl drops of medium M16 preequilibrated with 5% CO₂ in humidified air (pH 7.4), under mineral oil. Sperm from single-KO and wild-type (WT) caudae epididymides were capacitated at 37°C, 5% CO₂ in M16 and were added to the fertilization drops at different ratios to make up a total final concentration of 10⁵ cells/ml. The following WT:KO sperm ratios were used in order to determine whether the gene mutation conferred a competitive advantage or disadvantage on sperm in IVF: 1:0, 10:1, 1:1, 1:10, and 0:1. One pair of males of each genotype was used in each individual IVF experiment. Cumulus-enclosed oocytes from 3 or 4 oviducts were added to each fertilization drop, and dishes were placed in the incubator for 6 h. After incubation, morphologically normal oocytes were selected, and fertilization was initially assessed by observing pronuclei formation at this stage. Following a further incubation for 24 h in 50-μl drops of M16, 2-cell embryos were scored, washed of any additional spermatozoa in 1 mM EDTA, and subjected to nested PCR analysis for the presence of Rnase10^Cre allele. In the first step, amplification of a cre fragment was carried out with 5′-AGCACCTGGGCCAGCTCAACAT-3′ and 5′-TGCGCAGCAGGGTGTTGTAG-3′ to generate a template for the second round of amplification with cre-specific primers used for animal genotyping. Three IVF experiments were carried out. The rate of parthenogenesis was determined from the proportion of cre-negative 2-cell embryos following insemination with KO spermatozoa and was estimated at 1.4 ± 0.9%.

Zona penetration assay

Sperm were preincubated in M16 for 60 min and added to drops of medium as in the IVF experiments. Eggs within cumulus masses were then introduced and sampled at 30, 60, 90, 120, 150, and 180 min of coincubation. Granulosa cells and spermatozoa were removed from the outer zona surface by a brief incubation first in 0.3 mg/ml hyaluronidase and subsequently in 1 mM EDTA supplemented with 4 mg/ml polyvinylpyrrolidone-40. Following fixation in 1% paraformaldehyde, eggs were transferred to a staining drop containing 0.2% Triton X-100, 20 μg/ml FITC-labeled soybean agglutinin (Vector Laboratories, Peterborough, UK) and 10 μg/ml Hoechst 33342 in PBS. Eggs were then analyzed by confocal microscopy, counting Hoechst 33342-stained sperm nuclei within the confines of fluorescein-stained zona pellucida. For each time point, ≥60 eggs were examined in 3 independent experiments.

Sperm-egg binding assay

Binding of spermatozoa to zonae pellucidae was assessed as per the method of Bleil and Wassarman (17) with modifications. After superovulation, eggs were released from oviductal ampullae into medium M2 (15), and cumulus oophorus/corona radiata cells were dispersed by a brief incubation in hyaluronidase solution (0.3 mg/ml in M2) at room temperature. Cumulus-free oocytes (20–25) were placed in 100-μl drops of T6 medium containing ∼5 × 10⁶ motile spermatozoa/ml (preincubated for 2 h). Several 2-cell stage embryos were also introduced. After a 20-min incubation in humidified atmosphere of 5% CO₂ in air at 37°C, the oocytes and embryos with associated spermatozoa were transferred to a drop of M2 medium. Using a micropipette with an internal diameter of ∼300 μm, loosely associated spermatozoa were washed off the oocytes until 1 or 2 sperm remained bound to the embryos. The oocytes were then transferred to a drop of fixative (1% paraformaldehyde in PBS supplemented with 4 mg/ml polyvinylpyrrolidone-40) and photographed.

Statistical analysis

Numeric data from ≥3 independent experiments were analyzed and are shown as means ± se. In the male fertility study, differences between genotypes were determined using the Kruskal-Wallis analysis with Dunn's post hoc test. In all other studies, Student's t test was used. A value of P < 0.05 was considered statistically significant.

RESULTS

Gene inactivation was achieved by disrupting the Rnase10 coding sequence with a cre-neo replacement vector by means of homologous recombination in embryonic stem cells (12). Real-time RT-PCR analysis of total RNA isolated from the proximal epididymis showed that KO mice completely lacked the Rnase10 transcript (Fig. 1A). The level of Rnase10 mRNA in heterozygous males was approximately half of that of WT mice, indicating biallelic expression of the gene. A similar effect of allelic dosage was found in Western blotting of protein extracts from the proximal epididymides of WT, heterozygous, and KO males (Fig. 1B). When whole-tissue lysates of testis and 10 consecutive segments of the epididymis were probed in Western blotting, we found that Rnase10 protein specific to the IS was completely absent from the male reproductive tract in the KO mice (Fig. 1C). Furthermore, absence of Rnase10 in the downstream segments of the epididymis in the WT mice indicates that this protein does not associate with spermatozoa passing through the IS.

Figure 1.

Effect of gene knockout on Rnase10 expression. A) Quantitative real-time RT-PCR analysis of Rnase10 mRNA levels in the initial segments of WT (+/+), heterozygous (+/−), and null (−/−) mutant epididymides; values are means ± se from 3 animals/genotype. B) Western blot analysis of SDS protein extracts from epididymal initial segments of WT, heterozygous, and KO males with a polyclonal antibody raised against recombinant mouse Rnase10; each lane contains 2 μg total protein. C) Immunodetection of Rnase10 in whole-tissue SDS protein extracts from testis (T) and successive segments of the epididymis (E1-E10) of WT (+/+) and KO (−/−) males; actin was used as a loading control.

Rnase10 is required for male fertility

Irrespective of the genotype, both male and female mice were generally healthy and normal in appearance and behavior. Examination of KO epididymides did not reveal any abnormality in the anatomy or histology, and the cauda filling with sperm was normal. While female reproduction was not affected by the genotype, KO males only rarely sired offspring on a mixed 129/Sv;C57BL/6 genetic background. We subsequently established the mutated allele on 2 genetic backgrounds, the inbred C57BL/6 and outbred CD1, by crossing to respective congenic WT mice. While male fertility was significantly affected in C57BL/6 knockouts, null mutant CD1 males sired normal numbers of pups. In a focused male fertility study, we found that, when spermatozoa passed through the epididymis completely devoid of Rnase10 in C57BL/6 males, approximately one-half of the matings did not result in conception, while in the other half only a small proportion of the eggs released at ovulation underwent development, giving a median reproductive rate of 0% (Fig. 2A). Reproductive rates did not differ significantly between heterozygous (median 80.6%) and WT (median 87.9%) males, indicating haplosufficiency of Rnase10, at least in a noncompetitive mating system. To determine whether the reproductive failure in the KO was due to a fertilization defect, we examined, in a separate experiment, eggs in the oviduct following natural mating. We found that not only did the KO males produce similarly reduced fertilization rates (Fig. 2B) but also there was a difference in the status of cumulus oophorus. When the females were mated to WT or heterozygous males, on d 1 postcoitum, cumulus was nearly completely dispersed, and most of the freed eggs were fertilized. Conversely, following mating with KO males, the oviductal ampullae contained intact cumulus masses with mostly unfertilized eggs. Interestingly, while there was no difference in both reproductive rates and fertilization rates between Rnase10 genotypes of CD1 males (Fig. 2C, D), when females were mated to CD1 KO males, cumulus masses in the oviduct remained intact, and no sperm were seen within them, suggesting a possible abnormality in KO sperm transport.

Figure 2.

Effect of Rnase10 disruption on male fertility in C57BL6 (A, B) and CD1 (C, D) genetic backgrounds. A, C) Reproductive rate was calculated as number of fetuses per eggs ovulated (determined by counting corpora lutea) in each reproductive event. Fetuses between estimated d 7.5 and 12.5 of pregnancy rather than newborns were counted to exclude the effect of cannibalism. Dots represent individual reproductive events, when a copulatory plug was observed after mating; events by individual males are grouped into subcolumns separated by vertical dashed lines; n, total number of reproductive events (females)/total number of males; horizontal lines represent medians. B, D) In vivo fertilization rates are the percentage of eggs with pronuclei in the oviduct following natural mating; the status of the cumulus oophorus was noted and is indicated at bottom. Kruskal-Wallis analysis with Dunn's post hoc test: a vs. c and b vs. c, P < 0.0001; a vs. b, not significant; d vs. f and e vs. f, P < 0.001; d vs. e, not significant; g vs. h vs. i, not significant; j vs. k vs. l, not significant; differences between individual males of the same genotype were not significant.

Rnase10 knockout spermatozoa fail to ascend the female reproductive tract

In the mouse, spermatozoa are ejaculated into the uterus, and a small proportion of them subsequently enter the oviduct to populate the caudal isthmus, where they become immobilized at the endosalpinx and await ovulation. When WT females were mated to heterozygous or WT males, spermatozoa were seen entrapped in crypts, as well as in the canal of the uterotubal junction (UTJ) during the preovulatory phase (Fig. 3A), and later, at around the time of ovulation, in folds of the caudal isthmus attached to the epithelium (Fig. 3C). However, very few or no spermatozoa were observed to have traversed the UTJ in the females examined after mating with Rnase10 null males (Fig. 3B, D). Quantification of sperm ascent showed that significantly fewer spermatozoa were present within the oviductal lumen after mating with KO males in both CD1 (246±19 vs. 8470±350 in WT) and C57BL/6 (only 3.33±0.88 vs. 2708±217 in WT) genetic backgrounds (Fig. 3E). The difference was physiologically more significant in the C57BL/6 males, whose sperm appeared to populate the oviduct in smaller numbers even in the WT situation. This was found to be in accord with smaller ejaculates of the C57BL/6 males (Fig. 3F).

Figure 3.

Failure of spermatozoa from Rnase10 KO males to traverse the UTJ and populate the lower oviductal isthmus. A–D) Longitudinal sections of colliculi tubarii 2 h postcoitum (A, B) and cross sections of isthmic regions of fallopian tubes 5 or 6 h postcoitum (C, D). Insets: enlarged view of boxed area (A, C). Arrows indicate sperm; hematoxylin-and-eosin staining. Scale bars = 150 μm. E, F) Quantitative analysis of sperm ascent in the female reproductive tract: E) Sperm numbers in periovulatory oviduct 5 or 6 h postcoitum. ***P < 0.0001, unpaired t test. F) Ejaculated sperm numbers in postcoital uterus in CD1 and C57BL6 males.

Function of Rnase10 KO sperm in vitro

Phase-contrast microscopy showed no apparent structural defect in the KO spermatozoa. A mature form of flagellation was observed at all times, with 42 ± 7% and 41 ± 4% of sperm displaying sustained progressive movement at 1 and 3 h of incubation in medium T6 (15), respectively (vs. 41±4 and 40±5% in heterozygous males).

We next examined the ability of the spermatozoa to undergo the acrosome reaction, a prerequisite for both zona penetration and fusion with the oolemma. Similar trends in the rates of spontaneous acrosome loss were observed in spermatozoa from KO and heterozygous males. While the proportion of live acrosome-reacted sperm was maintained at ∼15%, the subpopulation of live acrosome-intact cells was gradually becoming exhausted over the 4-h investigation (Fig. 4A). Neither solubilized zona pellucida nor progesterone was found to elicit an appreciable acrosome response in capacitated spermatozoa, irrespective of donor genotype (Fig. 4B). No increase in the proportion of acrosome-reacted spermatozoa was seen even when progesterone was used at concentrations as high as 50 μM, and with zona pellucida at concentrations sufficient to completely saturate zona binding sites on sperm as demonstrated in sperm-egg binding assay. The divalent cation ionophore A23187, on the other hand, induced a vast majority of the spermatozoa to undergo acrosome reaction, with no difference between sperm from KO and heterozygous males.

Figure 4.

In vitro analysis of sperm function. A) Spontaneous acrosome loss. Sperm from caudae epididymides of +/− and −/− males were incubated in complete medium T6 and sampled at times indicated. Graphs show mean ± sd values of 3 independent experiments. Comparison was made by Student's t test using calculated area under curve (AUC) values: AUC_live 55.5 ± 19.6 (+/−) vs. 42.7 ± 12.1 (−/−), P = 0.39; AUC_total 126.5 ± 11.6 (+/−) vs. 114.5 ± 29.9 (−/−), P = 0.55. B) Induced acrosome reaction. Sperm from caudae epididymides were preincubated in complete medium T6 for 90 min, and cell suspensions at a concentration of 5 × 10⁶ cells/ml were treated over 1 h with vehicle (T6), divalent cation ionophore A23187 at 10 ng/ml, and increasing concentrations of heat-solubilized murine zonae pellucidae and progesterone as shown. Dead spermatozoa were excluded from analysis. Data from 3 independent experiments (shown as means±se) were analyzed using Student's t test: no difference was found between the two genotypes (P>0.116). Increasing concentrations of solubilized zona pellucida were probed in parallel in sperm-egg binding assay; degree of inhibition of sperm binding is shown: +, mild but noticeable inhibition; ++, moderate inhibition; +++, complete inhibition of sperm binding. C) Tyrosine phosphorylation status of spermatozoa after 10, 30, 60, and 90 min of incubation in complete medium T6 (Western blot representative of 3 independent experiments; each lane contains SDS lysate of 10⁵ sperm). D, E) Sperm competition assay: WT eggs were inseminated with a mixture of WT and KO sperm at ratios indicated at the bottom following a period of capacitation for 90 min (D) or 120 min (E); IVF rates are the proportion of eggs that progressed to 2-cell stage following culture; solid boxes represent the proportion of 2-cell embryos lacking the modified allele (cre-negative in PCR); x vs. x′, P < 0.05. F, G) Zona penetration study with WT (solid squares) and knockout (circles) spermatozoa. F) Proportion of eggs penetrated by ≥1 sperm over time during gamete coincubation. G) Mean number of sperm per egg; data are means ± se of 3 independent experiments. *P < 0.05.

Induction of the acrosome reaction by calcium ionophore can easily be achieved in fresh caudal epididymal spermatozoa or under noncapacitating conditions, and therefore, it cannot be taken to indicate the ability of sperm to undergo capacitation. One such indicator may be tyrosine phosphorylation of sperm proteins, which has been correlated with the attainment of a capacitated state (18). As assessed by Western blotting (Fig. 4C), there were no apparent differences in protein tyrosine phosphorylation in the +/− and −/− spermatozoa, nor was there any difference in the kinetics of this process between spermatozoa from the two genotypes.

The ability of KO spermatozoa to undergo capacitation and ultimately to fertilize was further investigated in IVF experiments. In preliminary experiments, we found that the KO spermatozoa consistently produced better fertilization rates. We further investigated this phenomenon in sperm competition assays, where sperm from WT and KO males were premixed at different ratios, and the resulting embryos were genotyped for the presence of the mutant allele. As shown in Fig. 4D, KO spermatozoa had a significant competitive advantage at fertilization over their WT counterparts, with the majority of embryos bearing the mutated allele even when the fraction of KO sperm in the inseminate was as small as 1/10. Interestingly, when spermatozoa were allowed to capacitate for a longer period of time, WT sperm gained competitiveness (Fig. 4E). This phenomenon was further investigated by studying egg penetration. We found that KO spermatozoa were able to begin zona penetration sooner after insemination compared to WT control sperm, and they also produced higher overall penetration rates (Fig. 4F). Furthermore, when eggs were inseminated with KO sperm, increasing numbers of supernumerary spermatozoa were observed within the perivitelline space in the course of fertilization (Fig. 4G), although only 2 pronuclei were typically observed within the vitellus.

Sperm-zona adhesion defect

While conducting the IVF experiments, it was found that on completion of cumulus cell dispersal, conspicuously fewer spermatozoa were associated with zonae pellucidae when oocytes had been inseminated with sperm from the KO males. This phenomenon was consistently observed early after insemination in all experiments and in all eggs; thus, it could not be attributed to the development of the postfertilization zona block to binding. This feature of Rnase10 KO spermatozoa was further examined in the sperm-zona pellucida binding assay.

When cumulus-free oocytes were introduced to sperm suspensions in medium T6, copious numbers of spermatozoa from heterozygous or WT males associated with the surface of zona pellucida, and a large proportion of them appeared to be firmly bound after removal of loosely attached sperm by washing through a wide-bore pipette (Fig. 5A). The development of tenacious sperm-zona associations was observed both in complete medium T6 supporting capacitation and in N-(2-hydroxyethyl)piperazine-N′-(2-ethanesulfonic acid) (HEPES)-buffered T6 medium without bicarbonate, and larger numbers of spermatozoa appeared to bind tightly to zonae after a period of preincubation. No tenacious binding of spermatozoa occurred in a Ca²⁺-free T6 medium. In sharp contrast with spermatozoa from heterozygous males, Rnase10 KO sperm were unable to establish a firm association with the surface of the zona pellucida (Fig. 5B).

Figure 5.

Loss of sperm adhesiveness in the Rnase10 KO male. A, B) Sperm from Rnase10 KO males are unable to bind tenaciously to the zona pellucida (sperm-egg binding assay). Phase-contrast microscopy. C–F) Failure of KO spermatozoa to autoagglutinate (C, D) and to bind to oviductal epithelial cells (E–F) in vitro. Spermatozoa were released from caudae epididymides of +/− (A, C, E) and −/− (B, D, F) males into complete medium T6. Isthmic regions of oviducts were disrupted mechanically and added to sperm suspensions. After incubation for 30 min, smears were prepared on poly-l-lysine-coated slides, air-dried, fixed in methanol, and stained with hematoxylin and eosin. Scale bars = 25 μm (A, B); 30 μm (C–F).

Caudal epididymal sperm dispersal and activation of motility

Several important features of Rnase10 KO spermatozoa were noticed through direct observation of caudal epididymal sperm preparations. Sperm suspensions were normally prepared by piercing/slicing caudae epididymides in a drop of T6 medium and squeezing the contents out of the tubules. Quiescent caudal spermatozoa were then allowed to disperse for ∼10 min as their motility was activated on dilution. While sperm suspensions could be obtained using this technique from both WT/heterozygous and KO males, there was a difference in the character of sperm release.

Spermatozoa from heterozygous and WT epididymides were released in clumps that subsequently disaggregated, forming a heterogeneous suspension of single and autoagglutinated spermatozoa. During incubation, some of the free-swimming cells began to exhibit hyperactivated motility, and the proportion of such cells increased over time. In contrast, KO spermatozoa were invariably released as single, already vigorously motile cells, forming a homogeneous opaque suspension.

The difference in sperm release appeared to be more pronounced when bicarbonate was omitted from the medium. In PBS supplemented with glycolysis substrates (5.56 mM glucose, 25 mM lactate and 0.5 mM pyruvate), spermatozoa from heterozygous males exhibited slow tail movement and adhered to one another by way of their heads, forming large, slowly moving conglomerates and trains of cells; the proportion of free-swimming cells was negligible (Supplemental Video S1). A suspension of single, actively motile cells could be obtained by the addition of 1 mM EDTA, suggesting calcium-dependence of the sperm associations with the immobilizing caudal epididymal extracellular material. In contrast, KO spermatozoa, again, formed a homogeneous suspension of single, vigorously motile cells, and no conglomerate formation was observed in the absence of bicarbonate (Supplemental Video S2).

After dispersal of epididymal contents in complete medium T6, spermatozoa of WT and heterozygous males showed a tendency for head-to-head agglutination, and a large proportion of them existed in rosettes (Fig. 5C). When the latter were disaggregated by vigorous pipetting, spermatozoa tended to reassociate during subsequent incubation. Moreover, if a clump of epididymal epithelial cells was present in the suspension, it was always richly covered with spermatozoa bound to its surface by their heads. In contrast, KO spermatozoa showed no tendency for autoagglutination (Fig. 5D), nor did they adhere to epididymal epithelial cells. When oviductal epithelial cells were introduced to sperm suspensions, heterozygous spermatozoa readily adhered (Fig. 5E), whereas KO spermatozoa similarly showed no affinity toward them (Fig. 5F).

Loss of ADAM3 and ADAM6 from sperm surface in the Rnase10 KO epididymis

Both loss of affinity to the zona pellucida and inability to migrate into the oviduct have been described for spermatozoa in calmegin (19), fertilin β/ADAM2 (20), fertilin α/ADAM1a (21), cyritestin/ADAM3 (22, 23), calsperin (24), Tpst2 (25), and Pdilt (26) KO mice. A common feature in the molecular phenotypes of these mouse lines is the lack of ADAM3 protein in mature spermatozoa, and for this reason, we examined the status of this protein in Rnase10 KO sperm. This member of the ADAM family is synthesized in round and elongating spermatids as a protein with a molecular mass of 110 kDa. It is found in spermatozoa collected from the caput epididymidis as a 42-kDa protein (27). Its proteolysis, therefore, likely takes place during sperm transit through the IS. When protein extracts from spermatozoa isolated from the testis and five consecutive parts of the epididymis were probed with an anti-ADAM3 antibody in Western blotting, we found that while proteolysis of ADAM3 in the IS was not affected, the truncated form of ADAM3 was gradually lost from KO spermatozoa during their passage toward the cauda (Fig. 6).

Figure 6.

Immunodetection of ADAM3, ADAM6, and ADAM2 in the testis and epididymal spermatozoa of Rnase10 KO (−/−) and heterozygous (+/−) mice. SDS-lysates were analyzed by Western blotting using antibodies against ADAM3, ADAM6, and ADAM2; COX4 was used as loading control. Lanes: T, testis; I–V, sperm from 5 consecutive regions of the epididymis: proximal caput excluding the IS (I), distal caput (II), corpus (III), proximal cauda (IV), and distal cauda (V). Mature ADAM3, ADAM6, and ADAM2 are detected in spermatozoa as 40, 80, and 45-kDa bands, respectively; their testicular precursors are recognized as the higher molecular weight bands. Western blots (lanes I–V) from 3 independent experiments were analyzed by densitometry; graphs at right show COX-4-normalized band densities. *P < 0.05.

It has been suggested that ADAM3 exists on the sperm surface in protein complexes with ADAM2 (28) and ADAM6 (29), both of which are also proteolytically processed in the epididymis. Therefore, we hypothesized that if ADAM3 is lost, both ADAM2 and ADAM3 would also be affected on Rnase10 KO sperm. ADAM2, synthesized in the testis as a 100-kDa protein, undergoes partial proteolysis in the proximal epididymis and is found in mature spermatozoa as a 45-kDa protein (30). As shown in Fig. 6, this protein was, however, not affected in the Rnase10 KO male. On the other hand, ADAM6, an 80-kDa protein that is cleaved in the epididymis from a testicular precursor, is lost from sperm in the Rnase10 KO epididymis together with ADAM3 (Fig. 6).

DISCUSSION

Our study demonstrates that inactivation of Rnase10 results in a reproductive failure consequent to an inability of the KO spermatozoa to traverse the UTJ and gain the site of fertilization, while their fertilizing ability per se is enhanced. We also found that these spermatozoa appear to lack associations with the sperm-coating material in the cauda epididymidis and are released as single, vigorously motile cells incapable of binding to both the endosalpinx and the zona pellucida. These findings have important implications for our understanding of the role of epididymal sperm maturation in eutherian conception and for the interpretation of previous studies of fertilization conducted in vitro.

In particular, the ability of Rnase10 KO spermatozoa to fertilize without establishing a firm association with the zona surface contradicts the presently prevailing view of fertilization in the mouse. The sequence of events leading to syngamy in this species is currently defined as weak attachment of sperm to the surface of the zona pellucida by way of their intact acrosomes, followed by the development of a tenacious association mediated by the zona glycoprotein ZP3, which also induces the acrosome reaction in the bound spermatozoon to enable penetration through the zona (31). This model of sperm-egg interaction has largely been derived from observations of sperm behavior in zona-binding assays. However, as shown in the present study and in at least 3 other KO models (ADAM1a, ADAM3 and PDILT; refs. 21, 26), the avid binding of acrosome-intact spermatozoa to the zona demonstrable in sperm-egg binding assays is not necessary for fertilization and may simply be a laboratory artifact. Sperm from these KOs are unable to bind to the zona, yet they are fully capable of fertilizing eggs in vitro.

Previous studies in vitro bypassed the need for sperm to ascend the female reproductive tract and generally ignored the importance of the fact that the sperm-egg ratio at the site and moment of fertilization in vivo is close to unity (32, 33). In the mouse, large numbers of spermatozoa populate the uterus, from where a small proportion of them pass through the mucus of the UTJ into the caudal portion of the oviduct. Bound to the epithelial lining here, they await ovulation, and very few of them at a time are released to proceed to the ampulla, where fertilization occurs (33).

As argued by Bedford (1, 34), eutherian mammals appear to have developed a fertilization strategy quite distinct from that described for marine invertebrates, and its evolution seems to have been determined by the small alecithal egg intolerant to polyspermy. Transition to internal fertilization concomitant to terrestriality in amniotes was coupled with the change of cleavage pattern from holoblastic to meroblastic with complete loss of polyspermy-preventing mechanisms in fewer but large, yolky eggs where the entry of more than one sperm is physiological (35, 36). In embryo-retaining eutherian mammals, the secondary reduction of yolk accumulation and extreme reduction in oocyte size necessitated reintroduction of the egg's polyspermy-preventing mechanisms and/or more stringent control of sperm numbers at the site of fertilization. This is the context in which eutherian epididymal function seems to have evolved. Although there are no examples of alecithality in live-bearing reptiles, the idea of the role of the epididymis in eutherian transition to viviparity is supported by the lack of complexity of epididymal function in the monotremes with megalecithal eggs (37, 38).

Eutherian spermatozoa do not require any egg components for the induction of acrosome reaction under physiological conditions (39, 40), or when such conditions are mimicked in vitro (41). Therefore, securing diploidy at fertilization by reduction of sperm numbers in the ampulla appears to be particularly important in species whose eggs lack effective block to polyspermy (42).

In the present study, we were unable to induce acrosome reaction with heat-solubilized zonae pellucidae, and early electron microscopic studies of eggs recovered after natural matings in the rabbit (43, 44) and hamster (45–47) showed that acrosome breakdown occurred within the layer of cumulus oophorus/corona radiata cells. A recent work by Jin et al. (48) suggests that the situation may not be dissimilar in the mouse. In the rabbit studies, association with the zona surface in the penetrating spermatozoa was shown to be mediated by the remainder of the already reacted acrosome, while male gametes with intact acrosomes were found adherent to denuded areas of the zona pellucida only as an artifact (49). Furthermore, in a recent study Inoue et al. (50) demonstrated that acrosome-reacted mouse spermatozoa recovered from the perivitelline space are capable of passing through the cumulus-enclosed zona a second time to fertilize another egg.

The loss of affinity to the endosalpingeal epithelium coupled with the inability to ascend the oviduct in the otherwise fertile Rnase10 KO spermatozoa brings out the confusing parallels between sperm-zona and sperm-endosalpingeal interactions and places sperm adhesiveness in a more physiological perspective.

Sperm awaiting ejaculation in the cauda epididymidis are held together by the immobilizing matrix in a calcium-dependent manner, and the release of single, vigorously motile spermatozoa into a bicarbonate-free medium in the KO indicates a lack of sperm associations with this “decapacitating” glutinous extracellular material. Although the KO sperm are unable to penetrate the zona immediately, their requirement for capacitation is reduced. If the KO spermatozoa attain a capacitated state in the uterus, where the concentration of bicarbonate is low, this alone might suffice to limit their entry into the oviduct (51, 52). In this vein, the need for the inhibition of sperm fertilizing ability in the distal epididymis (i.e., the requirement for capacitation) may be to enable restricted sperm entry into the oviduct.

In the WT situation, bicarbonate stimulates motility and promotes detachment of spermatozoa from the caudal matrix. The shedding of the coating material, however, seems to be incomplete, and the residual caudal epididymal electron-dense material has been shown to mediate the formation of sperm agglutinates or rosettes (53). The physiological relevance of this phenomenon is difficult to establish, given that sperm do not normally encounter high bicarbonate concentrations until in the oviduct. The role of bicarbonate in the regulation of sperm adhesion and transport in vivo should, therefore, be considered in the context of the ascending gradient of this anion concentration in the female tract created by the regionalized expression of endosalpingeal carbonic anhydrases (54).

The observation that Rnase10 KO spermatozoa display precocious capacitation, while the kinetics of tyrosine phosphorylation in these cells are unchanged, indicates that these two phenomena are not related, as has previously been suggested (18).

The mechanism whereby Rnase10 confers adhesiveness onto spermatozoa in the proximal epididymis has yet to be uncovered, although the loss of the mature form of ADAM3 and ADAM6 from spermatozoa suggests Rnase10 involvement in the processing and stabilization of these proteins within the sperm plasmalemma. The relationship between various reproductive ADAMs appears to be complex and has not been fully elucidated (29). ADAM3 seems to be a key molecule with a role in sperm adhesion, as all of the KO models with a lack of this protein share the same phenotype, i.e., the inability of sperm to bind to the zona pellucida and to pass through the UTJ. In Clgn, Adam1a, Adam3, Calr3 and Pdilt KO mice (19–22, 24, 26), the loss of ADAM3 is pre-epididymal. Adam2 KO sperm also lack ADAM3 (21), and in an attempt to explain its loss from sperm, a model has been proposed where ADAM3 is stabilized on the sperm surface in a complex with ADAM2 (28). However, in the present study, we found that ADAM2 protein levels remain intact on the Rnase10 KO sperm that have lost most of ADAM3, therefore, challenging the idea of an ADAM2/ADAM3 complex. Furthermore, in an earlier study (21) ADAM3 has been shown in immunoprecipitation to be incapable of forming complexes with ADAM2, and there seems to be no discernible functional role for ADAM2 on mature sperm, as Adam1b KO mice completely lacking ADAM2 on sperm do not exhibit any reproductive failure and have normal levels of ADAM3 (55). The loss of ADAM3 in the Adam2 KO appears to be pre-epididymal and related to the lack of ADAM2/ADAM1a heterodimer in spermatids (21).

There is currently no Adam6 KO model, but ADAM6 protein is lost form both Adam2 and Adam3 KO sperm and coimmunoprecipitates with ADAM3 from testicular lysates (29). The disappearance of ADAM6 seems to precede the loss of ADAM3 in the Rnase10 KO sperm (Fig. 6). The direct functional role of ADAM3 and ADAM6 proteins on the sperm surface remains elusive at present. As a result of proteolytic processing in the initial segment, the mature form of ADAM3 lacks the metalloproteinase domain (56). In contrast, epididymal processing of ADAM6 leaves the metalloproteinase domain that, however, lacks a consensus sequence for an active protease site (29), and we are not aware of any studies demonstrating its proteolytic activity biochemically. Whether these ADAMs are directly involved in sperm adhesion or passage through the UTJ is unclear, even though the affinity of ADAM3 toward zona pellucida glycoproteins has been demonstrated in a protein-binding assay (57). While the nonsticky sperm of Clgn, Clr3, Pdilt, Adam1a, Adam2 and Adam3 KOs lack ADAM3 (21, 24, 26, 58), the adhesion defect in Ace and Pgap1 KO mice is not associated with a loss of ADAM3 (58, 59), suggesting the possibility of other molecular defects in the Rnase10 KO sperm.

In summary, our findings suggest that Rnase10 action in the proximal epididymis is required for the acquisition of spermatozoal adhesiveness, the feature of murine spermatozoa that appears to have a bearing on the mode of sperm transport in the female reproductive tract. This study is the first to demonstrate that interference with a single secreted epididymal protein can have a major impact on sperm maturation and fertility. Besides adding to a better understanding of the role of epididymal sperm maturation in eutherian conception, these findings should also help select and define strategies for the development of methods of post-testicular male contraception. Such an approach would require the identification of an epididymal gene product (e.g., receptor, enzyme, or ion channel) that could be blocked by a small molecule pharmacological inhibitor.