Low levels of mouse sperm chromatin fragmentation delay embryo development

We previously demonstrated that MnCl2 induces double-stranded DNA breaks in sperm in a process that we term as sperm chromatin fragmentation. Here, we tested if the levels of double-stranded DNA breaks were corelated to the concentration of MnCl2, and we compared this to another agent that causes single-stranded DNA breaks, H2O2. We found that both methods have the advantage of inducing DNA breaks in a concentration-dependent manner. Mouse sperm were treated with varying concentrations of either H2O2 or MnCl2, and the DNA damage was assessed by pulse-field gel electrophoresis, and the alkaline and neutral comet assays. Oocytes were injected with either treated sperm and the resulting embryos analyzed with an embryoscope to detect subtle changes in embryonic development. We confirmed that H2O2 treatment induced primarily single-stranded DNA breaks and MnCl2 induced primarily double-stranded DNA breaks, indicating different mechanisms of damage. These sperm were injected into oocytes, and the development of the resulting embryos followed with an embryoscope equipped with time lapse recording. We found that aberrations in early embryonic development by day 2 with even the lowest levels of DNA damage and that the levels of embryonic aberrations correlated to the concentration of either H2O2 or MnCl2. Low levels of H2O2 caused significantly more aberrations in embryonic development than low levels of MnCl2 even though the levels of DNA damage as measured by comet assays were similar. These data demonstrate that even low levels of sperm DNA damage cause delays and arrests in embryonic development.

Determination of double- and single-stranded DNA breaks in bovine sperm is predictive of their fertilizing capacity

BACKGROUND

The analysis of chromatin integrity has become an important determinant of sperm quality. In frozen-thawed bovine sperm, neither the sequence of post-thaw injury events nor the dynamics of different types of sperm DNA breaks are well understood. The aim of the present work was to describe such sperm degradation aftermath focusing on DNA damage dynamics, and to assess if this parameter can predict pregnancy rates in cattle.

RESULTS

A total of 75 cryopreserved ejaculates from 25 Holstein bulls were evaluated at two post-thawing periods (0-2 h and 2-4 h), analyzing global and double-stranded DNA damage through alkaline and neutral Comet assays, chromatin deprotamination and decondensation, sperm motility, viability, acrosomal status, and intracellular levels of total ROS, superoxides and calcium. Insemination of 59,605 females was conducted using sperm from the same bulls, thus obtaining the non-return to estrus rates after 90 d (NRR). Results showed an increased rate of double-stranded breaks in the first period (0-2 h: 1.29 ± 1.01%/h vs. 2-4 h: 0.13 ± 1.37%/h; P <  0.01), whereas the rate of sperm with moderate + high single-stranded breaks was higher in the second period (0-2 h: 3.52 ± 7.77 %/h vs. 2-4h: 21.06 ± 11.69 %/h; P < 0.0001). Regarding sperm physiology, viability decrease rate was different between the two periods (0-2 h: - 4.49 ± 1.79%/h vs. 2-4 h: - 2.50 ± 3.39%/h; P = 0.032), but the progressive motility decrease rate was constant throughout post-thawing incubation (0-2 h: - 4.70 ± 3.42%/h vs. 2-4 h: - 1.89 ± 2.97%/h; P > 0.05). Finally, whereas no correlations between bull fertility and any dynamic parameter were found, there were correlations between the NRR and the basal percentage of highly-damaged sperm assessed with the alkaline Comet (Rs = - 0.563, P = 0.003), between NRR and basal progressive motility (Rs = 0.511, P = 0.009), and between NRR and sperm with high ROS at 4 h post-thaw (Rs = 0.564, P = 0.003).

CONCLUSION

The statistically significant correlations found between intracellular ROS, sperm viability, sperm motility, DNA damage and chromatin deprotamination suggested a sequence of events all driven by oxidative stress, where viability and motility would be affected first and sperm chromatin would be altered at a later stage, thus suggesting that bovine sperm should be used for fertilization within 2 h post-thaw. Fertility correlations supported that the assessment of global DNA damage through the Comet assay may help predict bull fertility.

Sperm degradation after vasectomy follows a sperm chromatin fragmentation dependent mechanism causing DNA breaks in the toroid linker regions

Vasectomy is a widely used surgical technique creating an obstructive azoospermia. Although sperm cannot be ejaculated, the testis maintains sperm production in vasectomized males. The continuous accumulation of sperm deposited in the epididymis and the vas deferens fraction necessarily need to be degraded and eliminated. While the elimination process is carried out by granulomas that form after vasectomy, the detailed mechanisms of sperm degradation are still not known. The aim was to assess whether sperm chromatin fragmentation (SCF), a mechanism that degrades the entire sperm genome at the toroid linker regions (TLRs), is activated after vasectomy in sperm cells. We vasectomized mice and evaluated the presence of TLR-specific double-strand breaks through pulsed-field gel electrophoresis and the Comet assay at 1, 2 and 3 weeks after surgery. Results for DNA damage (Olive tail moment) at single-cell level showed an increase of double-strand breaks after vasectomy for vas deferens sperm after 1, 2 and 3 weeks postvasectomy (21.78 ± 2.29; 19.71 ± 1.79 and 32.59 ± 1.81, respectively), compared to mock surgery (7.04 ± 1.03; 10.10 ± 1.29 and 8.64 ± 0.85, respectively; P < 0.001). Similar findings were obtained for cauda epididymis sperm (P < 0.001), but not for caput epididymis (P > 0.05). Pulsed-field gel electrophoresis showed the presence of double-stranded breaks between 15 and 145 kb, indicating that DNA breaks were produced mainly in the sperm TLRs. Results presented here suggest that SCF is a mechanism activated in vas deferens after vasectomy to degrade sperm DNA when they cannot be ejaculated, preventing their function.

Deletion of Orc4 during oogenesis severely reduces polar body extrusion and blocks zygotic DNA replication†

Origin recognition complex subunit 4 (ORC4) is a DNA-binding protein required for DNA replication. During oocyte maturation, after the last oocyte DNA replication step and before zygotic DNA replication, the oocyte undergoes two meiotic cell divisions in which half the DNA is ejected in much smaller polar bodies. We previously demonstrated that ORC4 forms a cytoplasmic cage around the DNA that is ejected in both polar body extrusion (PBE) events. Here, we used ZP3 activated Cre to delete exon 7 of Orc4 during oogenesis to test how it affected both predicted functions of ORC4: its recently discovered role in PBE and its well-known role in DNA synthesis. Orc4 deletion severely reduced PBE. Almost half of Orc4-depleted germinal vesicle (GV) oocytes cultured in vitro were arrested before anaphase I (48%), and only 25% produced normal first polar bodies. This supports the role of ORC4 in PBE and suggests that transcription of the full-length Orc4 during oogenesis is required for efficient PBE. Orc4 deletion also abolished zygotic DNA synthesis. Fewer Orc4-depleted oocytes developed to the metaphase II (MII) stage, and after activation these oocytes were arrested at the two-cell stage without undergoing DNA synthesis. This confirms that transcription of full-length Orc4 after the primary follicle stage is required for zygotic DNA replication. The data also suggest that MII oocytes do not have a replication licensing checkpoint as cytokinesis progressed without DNA synthesis. Together, the data confirm that oocyte ORC4 is important for both PBE and zygotic DNA synthesis.

Functional Aspects of Sperm Chromatin Organization

Sperm nuclei present a highly organized and condensed chromatin due to the interchange of histones by protamines during spermiogenesis. This high DNA condensation leads to almost inert chromatin, with the impossibility of conducting gene transcription as in most other somatic cells. The major chromosomal structure responsible for DNA condensation is the formation of protamine-DNA toroids containing 25-50 kilobases of DNA. These toroids are connected by toroid linker regions (TLR), which attach them to the nuclear matrix, as matrix attachment regions (MAR) do in somatic cells. Despite this high degree of condensation, evidence shows that sperm chromatin contains vulnerable elements that can be degraded even in fully condensed chromatin, which may correspond to chromatin regions that transfer functionality to the zygote at fertilization. This chapter covers an updated review of our model for sperm chromatin structure and its potential functional elements that affect embryo development.

The role of ORC4 in enucleation of Murine Erythroleukemia (MEL) cells is similar to that in oocyte polar body extrusion

The Origin Replication Complex subunit 4 (ORC4) is one in six subunits of the Origin Replication Complexes (ORCs) which is essential for initiating licensing at DNA replication origins and recruiting adaptor molecules necessary for various cellular processes. Previously, we reported that ORC4 also plays a vital role in polar body extrusion (PBE) during oogenesis in which half the chromosomes are extruded from the oocyte. We hypothesized that ORC4 might play a broader role in chromatin elimination. We tested its role in enucleation during the development of erythrocytes. Murine erythroleukemia (MEL) cells can be propagated in culture indefinitely and can be induced to enucleate their DNA by treatment with Vacuolin-1, thereby mimicking normal erythrocyte enucleation. We found that ORC4 appeared around the nuclei of the MEL cells with Vacuolin-1 treatment, gradually increasing in thickness before enucleation. We then tested whether ORC4 was required for MEL enucleation by down regulating ORC4 with siRNA-ORC4 during Vacuolin-1 treatment and found that this prevented MEL enucleation. These data are consistent with the model that ORC4 is required for erythroblast enucleation just as it is for oocyte PBE. They suggest a new model in which ORC4 expression is a marker for the initiation to the enucleation pathway.

Spatial and temporal resolution of mORC4 fluorescent variants reveals structural requirements for achieving higher order self-association and pronuclei entry

The Origin Replication Complex (ORC), which is a multi-subunit protein complex composed of six proteins ORC1-6, is essential for initiating licensing at DNA replication origins. We have previously reported that ORC4 has an alternative function wherein it forms a cage surrounding the extruded chromatin in female meiosis and is required for polar body extrusion (PBE). As this is a highly unexpected finding for protein that normally binds DNA, we tested whether ORC4 can actually form larger, higher order structures, which would be necessary to form a cage-like structure. We generated two fluorescent constructs of mouse ORC4, mORC4-EGFP and mORC4-FlAsH, to examine its spatial dynamics during oocyte activation in live cells. We show that both constructs were primarily monomeric throughout the embryo but self-association into larger units was detected with both probes. However, mORC4-FlAsH clearly showed higher order self-association and unique spatial distribution while mORC4-EGFP failed to form large structures during Anaphase II. Interestingly, both variants were found in the pronuclei suggesting that its role in DNA licensing is still functional. Our results with both constructs support the prediction that ORC4 can form higher order structures in the cytoplasm, suggesting that it is possible to form a cage-like structure. The finding that FlAsH labeled ORC4 formed demonstrably larger higher order structures than ORC4-GFP suggests that ORC4 oligomerization is sensitive to the bulky addition of GFP at its carboxy terminus.

Higher Order Oligomerization of the Licensing ORC4 Protein Is Required for Polar Body Extrusion in Murine Meiosis

We have previously shown that the DNA replication licensing factor ORC4 forms a cage around the chromosomes that are extruded in both polar bodies during murine oogenesis, but not around the chromosomes that are retained in the oocyte or around the sperm chromatin. We termed this structure the ORC4 cage. Here, we tested whether the formation of the ORC4 cage is necessary for polar body extrusion (PBE). We first experimentally forced oocytes to extrude sperm chromatin as a pseudo-polar body and found that under these conditions the sperm chromatin did become enclosed in an ORC4 cage. Next, we attempted to prevent the formation of the ORC4 cage by injecting peptides that contained sequences of different domains of the ORC4 protein into metaphase II (MII) oocytes just before the cage normally forms. Our rationale was that the ORC4 peptides would block protein-protein interactions required for cage formation. Two out of six tested peptides prevented the ORC4 cage formation and simultaneously inhibited PBE, resulting in the formation of two pronuclei (2 PN) that were retained in the oocyte. Together, these data demonstrate that ORC4 oligomerization is required to form the ORC4 cage and that it is required for PBE. J. Cell. Biochem. 118: 2941-2949, 2017. © 2017 Wiley Periodicals, Inc.

A model for the control of DNA integrity by the sperm nuclear matrix

The highly condensed chromatin of mammalian spermatozoa is usually considered to be biologically inert before fertilization. However, we have demonstrated that even in this compacted state, sperm chromatin is subject to degradation at open configurations associated with the nuclear matrix through a process we have termed sperm chromatin fragmentation (SCF). This suggests that a mechanism exists to monitor the health of spermatozoa during transit through the male reproductive tract and to destroy the genome of defective sperm cells. The site of DNA damage in SCF, the matrix attachment sites, are the same that we hypothesize initiate DNA synthesis in the zygote. When sperm that have damaged DNA are injected into the oocyte, the newly created zygote responds by delaying DNA synthesis in the male pronucleus and, if the damage is severe enough, arresting the embryo's development. Here we present a model for paternal DNA regulation by the nuclear matrix that begins during sperm maturation and continues through early embryonic development.

Presence of the Paternal Pronucleus Assists Embryo in Overcoming Cycloheximide Induced Abnormalities in Zygotic Mitosis

After fertilization, the maternal and paternal chromosomes independently proceed through pronuclear formation. These chromatin reconfigurations occur within a shared cytoplasm thus exposing both gametes to the same factors. Here, we report that continuous cycloheximide [40 μg/mL] treatment of parthenogenotes, androgenotes, and ICSI embryos reveals ORC2 pronuclear instability in the maternal (MPN) but not the paternal pronucleus (PPN). When released from CHX after 8 h, the MPN can recover ORC2 and proceed through replication, however, parthenogenotes encounter severe mitotic defects while both ICSI embryos and androgenotes are able to recover and develop at significantly higher rates. Taken together, these data suggest cycloheximide treatment promotes an environment that asymmetrically affects the stability of ORC2 on the MPN, and the ability of the MPN to develop. Furthermore, the presence of the PPN in the zygote can ameliorate both effects. These data suggest further evidence for crosstalk between the two pronuclei during the first cell cycle of the embryo. J. Cell. Biochem. 117: 1806-1812, 2016. © 2016 Wiley Periodicals, Inc.

ORC4 surrounds extruded chromatin in female meiosis

Six proteins, ORC1-6, make up the origin recognition complex (ORC) that initiates licensing of DNA replication origins. We have previously reported that subunit ORC2 is localized between the separating maternal chromosomes at anaphase II just after fertilization and is present in zygotic pronuclei at G1. Here, we found that ORC1, 3, and 5 all localize between the chromosomes at anaphase II, but could not be detected in zygotic G1. ORC6 localized to the periphery of the nucleoli at all zygotic stages. We identified an unexpected potential role for ORC4 in polar body formation. We found that in both female meiotic divisions, ORC4 surrounds the set of chromosomes, as a sphere-like structure, that will eventually be discarded in the polar bodies, but not the chromosomes that segregate into the oocyte. None of the other five ORC proteins are involved in this structure. In Zygotic G1, ORC4 surrounds the nuclei of the polar bodies, but was not detectable in the pronuclei. When the zygote entered mitosis ORC4 was only detected in the polar body. However, ORC4 appeared on both sets of separating chromosomes at telophase. At this point, the ORC4 that was in the polar body also migrated into the nuclei, suggesting that ORC4 or an associated protein is modified during the first embryonic cell cycle to allow it to bind DNA. Our results suggest that ORC4 may help identify the chromosomes that are destined to be expelled in the polar body, and may play a role in polar body extrusion. ORC4 surrounds the chromatin that will be extruded in the polar body in both female meiotic divisions, then makes a transition from the cytoplasm to the chromosomes at zygotic anaphase, suggesting multiple roles for this replication licensing protein.

Luminal fluid of epididymis and vas deferens contributes to sperm chromatin fragmentation

STUDY QUESTION

Do the luminal fluids of the epididymis and the vas deferens contribute to sperm chromatin fragmentation (SCF) in mice?

SUMMARY ANSWER

The luminal fluids of both organs are required for activating SCF in mice, but the vas deferens luminal fluid does this more efficiently than that of the epididymis.

WHAT IS KNOWN ALREADY

Mice sperm have the ability to degrade their DNA in an apoptotic-like fashion when treated with divalent cations in a process termed SCF. SCF has two steps: the induction of reversible double-strand DNA breaks at the nuclear matrix attachment sites, followed by the irreversible degradation of DNA by nuclease. Single stranded DNA breaks accompany SCF.

STUDY DESIGN, SIZE, DURATION

Luminal fluids from two reproductive organs of the mouse (B6D2F1 strain), the epididymis and vas deferens, were extracted and tested for SCF activation with divalent cations using four different combinations of the sperm and the surrounding luminal fluids: (i) in situ--sperm were kept in their luminal fluid and activated directly; (ii) reconstituted--sperm were centrifuged and resuspended in their luminal fluid before SCF activation; (iii) mixed--sperm were centrifuged and resuspended in the luminal fluid of the other organ; (iv) no luminal fluid--sperm were centrifuged and reconstituted in buffer. All four experiments were performed without (controls) and with divalent cations (resulting in SCF). For each experimental condition, two different mice were used and the analyses averaged.

PARTICIPANTS/MATERIALS, SETTING, METHODS

DNA damage by SCF was analyzed by three different methods, the sperm chromatin structure assay (SCSA), terminal deoxynucleotidyltransferase-mediated dUTP nick-end labeling (TUNEL) analysis and field inversion gel electrophoresis.

MAIN RESULTS AND THE ROLE OF CHANCE

In all three assays that we used, the vas deferens luminal fluid was much more efficient in stimulating SCF in the sperm from either source than that of the epididymis (P < 0.0001). Vas deferens sperm were capable of initiating lower levels of SCF in the absence of luminal fluid (P < 0.0001).

LIMITATIONS, REASONS FOR CAUTION

Analyses were performed in only one species, the mouse, but we used three separate assays in our analysis.

WIDER IMPLICATIONS OF THE FINDINGS

The data suggest that the luminal fluid of the male reproductive tract interacts with sperm during their transit providing a mechanism to degrade the DNA. We hypothesize that this is part of an apoptotic-like mechanism that allows the reproductive tract to eliminate defective sperm. The SCF model also allowed us to identify differences in the types of DNA lesions that the three tests can identify, providing important background information for the use of these tests clinically.

Double-stranded DNA breaks hidden in the neutral Comet assay suggest a role of the sperm nuclear matrix in DNA integrity maintenance

We used a mouse model in which sperm DNA damage was induced to understand the relationship of double-stranded DNA (dsDNA) breaks to sperm chromatin structure and to the Comet assay. Sperm chromatin fragmentation (SCF) produces dsDNA breaks located on the matrix attachment regions, between protamine toroids. In this model, epididymal sperm induced to undergo SCF can religate dsDNA breaks while vas deferens sperm cannot. Here, we demonstrated that the conventional neutral Comet assay underestimates the epididymal SCF breaks because the broken DNA ends remain attached to the nuclear matrix, causing the DNA to remain associated with the dispersion halo, and the Comet tails to be weak. Therefore, we term these hidden dsDNA breaks. When the Comet assay was modified to include an additional incubation with sodium dodecyl sulfate (SDS) and dithiothreitol (DTT) after the conventional lysis, thereby solubilizing the nuclear matrix, the broken DNA was released from the matrix, which resulted in a reduction of the sperm head halo and an increase in the Comet tail length, exposing the hidden dsDNA breaks. Conversely, SCF-induced vas deferens sperm had small halos and long tails with the conventional neutral Comet assay, suggesting that the broken DNA ends were not tethered to the nuclear matrix. These results suggest that the attachment to the nuclear matrix is crucial for the religation of SCF-induced DNA breaks in sperm. Our data suggest that the neutral Comet assay identifies only dsDNA breaks that are released from the nuclear matrix and that the addition of an SDS treatment can reveal these hidden dsDNA breaks.

Mouse zygotes respond to severe sperm DNA damage by delaying paternal DNA replication and embryonic development

Mouse zygotes do not activate apoptosis in response to DNA damage. We previously reported a unique form of inducible sperm DNA damage termed sperm chromatin fragmentation (SCF). SCF mirrors some aspects of somatic cell apoptosis in that the DNA degradation is mediated by reversible double strand breaks caused by topoisomerase 2B (TOP2B) followed by irreversible DNA degradation by a nuclease(s). Here, we created zygotes using spermatozoa induced to undergo SCF (SCF zygotes) and tested how they responded to moderate and severe paternal DNA damage during the first cell cycle. We found that the TUNEL assay was not sensitive enough to identify the breaks caused by SCF in zygotes in either case. However, paternal pronuclei in both groups stained positively for γH2AX, a marker for DNA damage, at 5 hrs after fertilization, just before DNA synthesis, while the maternal pronuclei were negative. We also found that both pronuclei in SCF zygotes with moderate DNA damage replicated normally, but paternal pronuclei in the SCF zygotes with severe DNA damage delayed the initiation of DNA replication by up to 12 hrs even though the maternal pronuclei had no discernable delay. Chromosomal analysis of both groups confirmed that the paternal DNA was degraded after S-phase while the maternal pronuclei formed normal chromosomes. The DNA replication delay caused a marked retardation in progression to the 2-cell stage, and a large portion of the embryos arrested at the G2/M border, suggesting that this is an important checkpoint in zygotic development. Those embryos that progressed through the G2/M border died at later stages and none developed to the blastocyst stage. Our data demonstrate that the zygote responds to sperm DNA damage through a non-apoptotic mechanism that acts by slowing paternal DNA replication and ultimately leads to arrest in embryonic development.

Unique pattern of ORC2 and MCM7 localization during DNA replication licensing in the mouse zygote

In eukaryotes, DNA synthesis is preceded by licensing of replication origins. We examined the subcellular localization of two licensing proteins, ORC2 and MCM7, in the mouse zygotes and two-cell embryos. In somatic cells ORC2 remains bound to DNA replication origins throughout the cell cycle, while MCM7 is one of the last proteins to bind to the licensing complex. We found that MCM7 but not ORC2 was bound to DNA in metaphase II oocytes and remained associated with the DNA until S-phase. Shortly after fertilization, ORC2 was detectable at the metaphase II spindle poles and then between the separating chromosomes. Neither protein was present in the sperm cell at fertilization. As the sperm head decondensed, MCM7 was bound to DNA, but no ORC2 was seen. By 4 h after fertilization, both pronuclei contained DNA bound ORC2 and MCM7. As expected, during S-phase of the first zygotic cell cycle, MCM7 was released from the DNA, but ORC2 remained bound. During zygotic mitosis, ORC2 again localized first to the spindle poles, then to the area between the separating chromosomes. ORC2 then formed a ring around the developing two-cell nuclei before entering the nucleus. Only soluble MCM7 was present in the G2 pronuclei, but by zygotic metaphase it was bound to DNA, again apparently before ORC2. In G1 of the two-cell stage, both nuclei had salt-resistant ORC2 and MCM7. These data suggest that licensing follows a unique pattern in the early zygote that differs from what has been described for other mammalian cells that have been studied.

Mammalian sperm chromatin as a model for chromatin function in DNA degradation and DNA replication

Reproductive biology is considered a specialty field, however, an argument can be made that it is instead generally applicable to many fields of biology. The one-cell embryo is presented here as a model system for the study of eukaryotic DNA replication, apoptotic DNA degradation, and signaling mechanisms between the cytoplasm and nucleus. Two unique aspects of this system combine to make it particularly useful for the study of chromatin function. First, the evolutionary pressure that lead to the extreme condensation of mammalian sperm DNA resulted in a cell with virtually inert chromatin, no DNA replication or transcription ongoing in the sperm cell, and all of the cells in a G(0) state. This chromatin is suddenly transformed into actively transcribing and replicating DNA upon fertilization. Therefore, the sperm chromatin is poised to become active but does not yet possess sufficient components present in somatic chromatin structure for all these processes. The second unique aspect of this system is that the one cell embryo houses two distinct nuclei, termed pronuclei, through the first round of DNA synthesis. This means the sperm cell can be experimentally manipulated to test the affects of the various treatments on the biological functions of interest. Experimental manipulations of the system have already revealed a certain level of plasticity in the coordination of both the timing of DNA synthesis in the two pronuclei and in the response to cellular signals by each pronucleus involved with the progression through the G1/S checkpoint, including the degradation of DNA in the paternal pronucleus. The fact that two nuclei in the same cytoplasm can undergo different responses infers a level of autonomy in the nuclear control of the cell cycle. Thus, the features of mammalian fertilization can provide unique insights for the normal biology of the cell cycle in somatic cells.

Non-genetic contributions of the sperm nucleus to embryonic development

Recent data from several laboratories have provided evidence that the newly fertilized oocyte inherits epigenetic signals from the sperm chromatin that are required for proper embryonic development. For the purposes of this review, the term epigenetic is used to describe all types of molecular information that are transmitted from the sperm cell to the embryo. There are at least six different forms of epigenetic information that have already been established as being required for proper embryogenesis in mammals or for which there is evidence that it may do so. These are (i) DNA methylation; (ii) sperm-specific histones, (iii) other chromatin-associated proteins; (iv) the perinuclear theca proteins; (v) sperm-born RNAs and, the focus of this review; and (vi) the DNA loop domain organization by the sperm nuclear matrix. These epigenetic signals should be considered when designing protocols for the manipulation and cryopreservation of spermatozoa for assisted reproductive technology as necessary components for effective fertilization and subsequent embryo development.

A novel nuclease activity that is activated by Ca(2+) chelated to EGTA

Most nucleases require a divalent cation as a cofactor, usually Mg(2+) or Ca(2+), and are inhibited by the chelators EDTA and EGTA. We report the existence of a novel nuclease activity, initially identified in the luminal fluids of the mouse male reproductive tract but subsequently found in other tissues,that requires EGTA chelated to calcium to digest DNA. We refer to this unique enzyme as CEAN (Chelated EGTA Activated Nuclease). Using a fraction of vas deferens luminal fluid, plasmid DNA was degraded in the presence of excess Ca(2+) (Ca(2+) :EGTA = 16) or excess EGTA (Ca(2+) :EGTA = 0.25), but required the presence of both. Higher levels of EGTA (Ca(2+) :EGTA = 0.10) prevented activity, suggesting that unchelated EGTA may be a competitive inhibitor. The EGTA-Ca(2+) activation of CEAN is reversible as removing EGTA-Ca(2+) stops ongoing DNA degradation, but adding EGTA-Ca(2+) again reactivates the enzyme. This suggests the possibility that CEAN binds directly to EGTA-Ca(2+). CEAN has a greater specificity for the chelator than for the divalent cation. Two other chelators, BAPTA and sodium citrate, do not activate CEAN in the presence of cation, but chelated EDTA does. EGTA chelated to other divalent cations such as Mn(2+), Zn(2+) , and Cu(2+) activate CEAN, but not Mg(2+) . The activity is lost upon boiling suggesting that it is a protein. These data suggest that EGTA and EDTA may not always protect DNA from nuclease damage.

Function of sperm chromatin structural elements in fertilization and development

Understanding how DNA is packaged in the mammalian sperm cell has important implications for human infertility as well as for the cell biology. Recent advances in the study of mammalian sperm chromatin structure and function have altered our perception of this highly condensed, inert chromatin. Sperm DNA is packaged very tightly to protect the DNA during the transit that occurs before fertilization. However, this condensation cannot sacrifice chromosomal elements that are essential for the embryo to access the correct sequences of the paternal genome for proper initiation of the embryonic developmental program. The primary levels of the sperm chromatin structure can be divided into three main categories: the large majority of DNA is packaged by protamines, a smaller amount (2-15%) retains histone-bound chromatin and the DNA is attached to the nuclear matrix at roughly 50 kb intervals. Current data suggest that the latter two structural elements are transferred to the paternal pronucleus after fertilization where they have important functional roles. The nuclear matrix organization is essential for DNA replication, and the histone-bound chromatin identifies genes that are important for embryonic development. These data support the emerging view of the sperm genome as providing, in addition to the paternal DNA sequence, a structural framework that includes molecular regulatory factors that are required for proper embryonic development.

Asynchronous DNA replication and origin licensing in the mouse one-cell embryo

To prevent duplicate DNA synthesis, metazoan replication origins are licensed during G1. Only licensed origins can initiate replication, and the cytoplasm interacts with the nucleus to inhibit new licensing during S phase. DNA replication in the mammalian one-cell embryo is unique because it occurs in two separate pronuclei within the same cytoplasm. Here, we first tested how long after activation the oocyte can continue to support licensing. Because sperm chromatin is licensed de novo after fertilization, the timing of sperm injection can be used to assay licensing initiation. To experimentally skip some of the steps of sperm decondensation, we injected mouse sperm halos into parthenogenetically activated oocytes. We found that de novo licensing was possible for up to 3 h after oocyte activation, and as early as 4 h before DNA replication began. We also found that the oocyte cytoplasm could support asynchronous initiation of DNA synthesis in the two pronuclei with a difference of at least 2 h. We next tested how tightly the oocyte cytoplasm regulates DNA synthesis by transferring paternal pronuclei from zygotes generated by intracytoplasmic sperm injection (ICSI) into parthenogenetically activated oocytes. The pronuclei from G1 phase zygotes transferred into S phase ooplasm were not induced to prematurely replicate and paternal pronuclei from S phase zygotes transferred into G phase ooplasm continued replication. These data suggest that the one-cell embryo can be an important model for understanding the regulation of DNA synthesis.

Mouse spermatozoa contain a nuclease that is activated by pretreatment with EGTA and subsequent calcium incubation

We demonstrated that mouse spermatozoa cleave their DNA into approximately 50 kb loop-sized fragments with topoisomerase IIB when treated with MnCl(2) and CaCl(2) in a process we term sperm chromatin fragmentation (SCF). SCF can be reversed by EDTA. A nuclease then further degrades the DNA in a process we term sperm DNA degradation (SDD). MnCl(2) alone could elicit this activity, but CaCl(2) had no effect. Here, we demonstrate the existence of a nuclease in the vas deferens that can be activated by ethylene glycol tetraacetic acid (EGTA) to digest the sperm DNA by SDD. Spermatozoa were extracted with salt and dithiothreitol to remove protamines and then incubated with EGTA. Next, the EGTA was removed and divalent cations were added. We found that Mn(2+), Ca(2+), or Zn(2+) could each activate SDD in spermatozoa but Mg(2+) could not. When the reaction was slowed by incubation on ice, EGTA pretreatment followed by incubation in Ca(2+) elicited the reversible fragmentation of sperm DNA evident in SCF. When the reactions were then incubated at 37 degrees C they progressed to the more complete degradation of DNA by SDD. EDTA could also be used to activate the nuclease, but required a higher concentration than EGTA. This EGTA-activatable nuclease activity was found in each fraction of the vas deferens plasma: in the spermatozoa, in the surrounding fluid, and in the insoluble components in the fluid. These results suggest that this sperm nuclease is regulated by a mechanism that is sensitive to EGTA, possibly by removing inhibition of a calcium binding protein.

The sperm nuclear matrix is required for paternal DNA replication

The mammalian sperm nucleus provides an excellent model for studying the relationship between the formation of nuclear structure and the initiation of DNA replication. We previously demonstrated that mammalian sperm nuclei contain a nuclear matrix that organizes the DNA into loop domains in a manner similar to that of somatic cells. In this study, we tested the minimal components of the sperm nucleus that are necessary for the formation of the male pronucleus and for the initiation of DNA synthesis. We extracted mouse sperm nuclei with high salt and dithiothreitol to remove the protamines in order to form nuclear halos. These were then treated with either restriction endonucleases to release the DNA not directly associated with the nuclear matrix or with DNAse I to digest all the DNA. The treated sperm nuclei were injected into oocytes, and the paternal pronuclear formation and DNA synthesis was monitored. We found that restriction digested sperm nuclear halos were capable of forming paternal pronuclei and initiating DNA synthesis. However, when isolated mouse sperm DNA or sperm DNA reconstituted with the nuclear matrices were injected into oocytes, no paternal pronuclear formation or DNA synthesis was observed. These data suggest that the in situ nuclear matrix attachment organization of sperm DNA is required for mouse paternal pronuclear DNA synthesis.

Most human Alu and murine B1 repeats are unique

Alus and B1s are short interspersed repeat elements (SINEs) indirectly derived from the 7SL RNA gene. While most researchers recognize that there exists extensive variability between individual elements, the extent of this variability has never been systematically tested. We examined all Alu elements over 200 nucleotides and all B1 elements over 100 nucleotides in the human and mouse genomes, and analyzed the number of copies of each element at various stringencies from 22 nucleotides to full length. Over 98% of 923,277 Alus and 365,377 B1s examined were unique when queried at full length. When the criterion was reduced to half the length of the repeat, 97% of the Alus and 73% of the B1s were still found to be a single copy. All single and multi-copy sequences have been mapped and documented. Access to the data is possible using the AluPlus website http://www.ibr.hawaii.edu.

Evidence of Alu and B1 expression in dbEST

Alus and B1s are short interspersed repeat elements (SINEs) derived from the 7SL RNA gene. Alus and B1s exist in the cytoplasm as non-coding RNA indicating that they are actively transcribed, but their function, if any, is unknown. Transcription of individual SINEs is a prerequisite for retroposition, but it is also possible that individual Alu and B1 elements have some cellular functions. Previous studies suggest that transcription of Alu elements depends on the presence of an RNA polymerase-III bipartite promoter and the poly-A tail. Sequencing of small RNAs has demonstrated that the members of the Y and S subfamily are expressed. We analyzed almost one million Alu sequences longer than 200 nucleotides for the presence of RNA polymerase-III bipartite promoter sequences. More than half contained a promoter indicating some potential for expression. We searched 7.7 million human EST sequences in dbEST for the presence of Alu non-coding RNAs and found evidence for the expression of 452. Analysis of mouse spermatogenic dbEST libraries revealed an apparent relationship between the level of differentiation and the level of B1-related sequences in the EST library.

A model for the function of sperm DNA degradation

In this review, we present our recent evidence suggesting, but not yet proving, that mammalian spermatozoa contain a mechanism by which they can digest their own DNA when exposed to a stressful environment. We discuss our recent data that demonstrate that when mammalian spermatozoa are treated in a variety of ways, the paternal chromosomes in the zygote, or the sperm DNA itself, are degraded into large, chromosome-sized fragments. These published data support the existence of nuclease activity in spermatozoa. We suggest that this nuclease activity is part of a mechanism the spermatozoon uses when it encounters a stressful environment to prevent fertilisation and to avoid the transmission of potentially damaged DNA to the embryo. We propose a model based on sperm chromatin structure by which this nuclease can digest the highly condensed sperm chromatin.

Function of the sperm nuclear matrix

Mammalian spermatozoa contain some of the most highly compact chromatin. This is due to the DNA binding proteins, the protamines, which replace most of the histones during spermiogenesis. This chromatin, however, shares some features with somatic cell chromatin. One of these is the organization of DNA into loop domains attached at their bases to a proteinaceous nuclear matrix. Several groups have shown that the sites at which DNA associates with the sperm nuclear matrix contain chromatin structures that are linked with specific functions. Recent data also suggest that the sperm nuclear matrix plays essential roles in the paternal pronucleus of the newly fertilized oocyte, suggesting that the sperm cell provides more information to the new embryo than solely the genetic material it delivers. Here, we will review these data which together give insight into the functional significance and requirements of sperm nuclear structure.

Sperm DNA fragmentation: awakening the sleeping genome

We have recently demonstrated that mammalian spermatozoa have the ability to degrade their DNA by a mechanism that is similar to apoptosis in somatic cells. When this mechanism is activated, the DNA is first degraded into loop-sized fragments by TOP2B (topoisomerase IIB). This degradation, termed sperm chromatin fragmentation, can be reversed by EDTA, which causes TOP2B to religate the double-stranded breaks it originally produced. Under certain conditions, a nuclease then degrades the sperm DNA further, digesting the entire sperm genome. When mouse spermatozoa which have been treated to induce TOP2B-mediated DNA breaks are injected into oocytes, the paternal DNA is specifically and completely degraded. This total digestion of paternal DNA occurs at the time of DNA synthesis initiation. In the present study, we explore the significance of an active TOP2B in the nucleus for mouse sperm function.

Paternal pronuclear DNA degradation is functionally linked to DNA replication in mouse oocytes

We recently demonstrated that mouse spermatozoa contain a mechanism to degrade their DNA into loop-sized fragments of about 50 kb, mediated by topoisomerase IIB, termed sperm chromatin fragmentation (SCF). SCF is often followed by a more complete digestion of the DNA with a sperm nuclease. When SCF-induced spermatozoa are injected into oocytes, the paternal pronuclei degrade their DNA after the initiation of DNA synthesis, but the maternal pronuclei are unaffected and replicate normally. Here, we tested whether the nuclease activity changes in spermatozoa of different maturation stages, and whether there is a functional relationship between the initiation of DNA synthesis and paternal DNA degradation induced by SCF in the zygote. We found that spermatozoa from the vas deferens have a much higher level of SCF activity than those from the cauda epididymis, suggesting that spermatozoa may acquire this activity in the vas deferens. Furthermore, paternal pronuclei formed in zygotes from injecting oocytes with SCF-induced vas deferens spermatozoa degraded their DNA, but this degradation could be inhibited by the DNA synthesis inhibitor, aphidicolin. Upon release from a 4 h aphidicolin-induced arrest, DNA synthesis was initiated in maternal pronuclei, while the paternal pronuclei degraded their DNA. Longer aphidicolin arrest resulted in the paternal pronuclei replicating their DNA, suggesting that delaying the initiation of DNA synthesis allowed the paternal pronuclei to overcome the SCF-induced DNA degradation pathway. These results suggest that the paternal DNA degradation, in oocytes fertilized with SCF-induced spermatozoa, is coupled to the initiation of DNA synthesis in newly fertilized zygotes.

Topoisomerase II-Mediated Breaks in Spermatozoa Cause the Specific Degradation of Paternal DNA in Fertilized Oocytes1

We have demonstrated that mouse spermatozoa can cleave their DNA into 50-kb fragments when treated with Triton X-100, MnCl(2), and CaCl(2). This cleavage, which is termed sperm chromatin fragmentation (SCF), is mediated by topoisomerase IIB (TOP2B) following stimulation by a factor in the epididymal fluid, most likely a nuclease, and can be at least partially religated by EDTA. When the protamines are removed, this DNA breakage is followed by digestion of the DNA by a nuclease(s). We tested whether the oocyte could repair TOP2B-induced sperm DNA breaks and whether partial religation by EDTA would allow spermatozoa to fertilize the oocytes normally. Oocytes injected with untreated spermatozoa developed normally. However, oocytes injected with spermatozoa treated with MnCl(2) and CaCl(2) to induce SCF, with or without subsequent EDTA treatment, failed to develop. In both of these treatment groups, the maternal pronuclei developed normally and replicated their DNA. However the paternal pronuclei did not replicate their DNA and this DNA began to disappear 6 h postinjection, which corresponded approximately to the time at which maternal DNA replication was initiated. These data suggest that when TOP2B is induced to cleave sperm DNA before fertilization, the paternal DNA is subsequently degraded by a highly regulated mechanism that does not affect the maternal chromatin. Furthermore, partial religation by EDTA of TOP2B-induced breaks prevents neither the inhibition of DNA synthesis nor DNA degradation.

Topoisomerase IIB and an Extracellular Nuclease Interact to Digest Sperm DNA in an Apoptotic-Like Manner1

We previously demonstrated that mammalian spermatozoa contain a nuclease activity that cleaves DNA into loop-sized fragments. We show here that this activity is mediated by a nuclear matrix-associated topoisomerase IIB (TOP2B) interacting with an extracellular Mn2+/Ca2+-dependent nuclease. Together, these enzymes cleave all of the DNA into fragments of 50 kb, and this cleavage can be reversed by EDTA. If dithiothreitol is included, the nuclease digests the DNA, and if the protamines are removed the DNA is completely digested. A similar, TOP2B-mediated, chromatin fragmentation, which is reversible, followed by digestion of the DNA by an intracellular nuclease occurs in somatic cells during apoptosis. The extracellular location of the sperm nuclease made it possible to reconstitute the fragmentation activity in isolated spermatozoa, thus allowing us to identify two novel aspects of the mechanism. First, the fragmentation of all of the DNA to 50 kb by TOP2B required the addition of the extracellular nuclease or factor. Second, the subsequent, complete digestion of the DNA by the nuclease could be inhibited by etoposide, suggesting that the nuclease digestion requires TOP2B religation of the cleaved DNA. These data are the first demonstration of an active TOP2B in spermatozoa, suggesting this inert chromatin may be more active than previously thought. They also show that the unique chromatin structure of spermatozoa may provide an important model to study the regulated degradation of chromatin by TOP2B and associated nucleases.

An endogenous nuclease in hamster, mouse, and human spermatozoa cleaves DNA into loop-sized fragments

Recent work from our laboratory provided evidence for the existence of a nuclease in hamster spermatozoa. This endogenous nuclease cleaves sperm chromatin at the bases of DNA loop domains into large fragments with an average size of roughly 50 kb. Here, we demonstrate that this sperm nuclease is present in the sperm nucleus and that it is activated by the presence of both calcium and magnesium much more efficiently than with either ion alone, resulting in DNA degradation in 30 minutes. We also show that similar nucleases are present in mouse and human spermatozoa. The human nuclease can be activated by freeze-thawing spermatozoa in noncryoprotective media. The activity of the sperm nuclease in all 3 species resembles that of a group of somatic cell DNAses that also require both calcium and magnesium and that digest the chromatin into loop-sized fragments during apoptosis.

A model for the function of sperm DNA degradation

In this review, we present our recent evidence suggesting, but not yet proving, that mammalian spermatozoa contain a mechanism by which they can digest their own DNA when exposed to a stressful environment. We discuss our recent data that demonstrate that when mammalian spermatozoa are treated in a variety of ways, the paternal chromosomes in the zygote, or the sperm DNA itself, are degraded into large, chromosome-sized fragments. These published data support the existence of nuclease activity in spermatozoa. We suggest that this nuclease activity is part of a mechanism the spermatozoon uses when it encounters a stressful environment to prevent fertilisation and to avoid the transmission of potentially damaged DNA to the embryo. We propose a model based on sperm chromatin structure by which this nuclease can digest the highly condensed sperm chromatin.

Ability of hamster spermatozoa to digest their own DNA

Mammalian sperm chromatin is bound by protamines into highly condensed toroids with approximately 50 kilobases (kb) of DNA. It is also organized into loop domains of about the same size that are attached at their bases to the proteinaceous nuclear matrix. In this work, we test our model that each sperm DNA-loop domain is condensed into a single protamine toroid. Our model predicts that the protamine toroids are linked by chromatin that is more sensitive to nucleases than the DNA within the toroids. To test this model, we treated hamster sperm nuclei with DNase I and found that the sperm chromatin was digested into fragments with an average size of about 50 kb, by pulse-field gel electrophoresis (PFGE). Surprisingly, we also found that spermatozoa treated with 0.25% Triton X-100 (TX) and 20 mM MgCl2 overnight resulted in the same type of degradation, suggesting that sperm nuclei have a mechanism for digesting their own DNA at the bases of the loop domains. We extracted the nuclei with 2 M NaCl and 10 mM dithiothreitol (DTT) to make nuclear halos. Nuclear matrices prepared from DNase I-treated spermatozoa had no DNA attached, suggesting that DNase I digested the DNA at the bases of the loop domains. TX-treated spermatozoa still had their entire DNA associated with the nuclear matrix, even though the DNA was digested into 50-kb fragments as revealed by PFGE. The data support our donut-loop model for sperm chromatin structure and suggest a functional role for this type of organization in that sperm can digest its own DNA at the sites of attachment to the nuclear matrix.