A Decade of Exploring the Mammalian Sperm Epigenome: Paternal Epigenetic and Transgenerational Inheritance

The past decade has seen a tremendous increase in interest and progress in the field of sperm epigenetics. Studies have shown that chromatin regulation during male germline development is multiple and complex, and that the spermatozoon possesses a unique epigenome. Its DNA methylation profile, DNA-associated proteins, nucleo-protamine distribution pattern and non-coding RNA set up a unique epigenetic landscape which is delivered, along with its haploid genome, to the oocyte upon fertilization, and therefore can contribute to embryogenesis and to the offspring health. An emerging body of compelling data demonstrates that environmental exposures and paternal lifestyle can change the sperm epigenome and, consequently, may affect both the embryonic developmental program and the health of future generations. This short review will attempt to provide an overview of what is currently known about sperm epigenome and the existence of transgenerational epigenetic inheritance of paternally acquired traits that may contribute to the offspring phenotype.

Effects of sperm pretreatment and embryo activation methods on the development of bovine embryos produced by intracytoplasmic sperm injection

The aim of the study was to examine the effects of different embryo activation methods and sperm pretreatments on the activation and development of bovine embryos produced by intracytoplasmic sperm injection (ICSI). Four activation agents, i.e., calcium ionophore (A23187), ionomycin (Ion), electric pulse (EP) and ethanol (Eth) were used in various combinations to activate bovine ICSI embryos. The normal fertilization rate was similar in bovine ICSI embryos activated by A23187+Eth, Ion+Eth, Ion+EP+Eth, and 2-Ion (Ion administered two times)+Eth. Increasing the frequency of ionomycin stimulation from two (2-Ion+Eth) to three times (3-Ion+Eth) significantly (p<0.05) increased the cell number per embryo at the blastocyst stage. In addition, spermatozoa were pretreated with dithiothreitol (DTT), glutathione (GSH) or GSH+lysolecithin (LL) and used for producing bovine ICSI embryos. The blastocyst rate of bovine ICSI embryos produced from sperm pretreated with GSH was significantly (p<0.05) higher than those of embryos produced from sperm pretreated with DTT and GSH+LL. In conclusion, the embryo activation methods and sperm pretreatments examined in the present study did not affect the normal fertilization rate of bovine ICSI embryos. However, activation with 3-Ion+Eth and sperm pretreatment with GSH increased the cell number per embryo at blastocyst stage and the blastocyst rate, respectively, in bovine ICSI embryos.

Expression of human oocyte-specific linker histone protein and its incorporation into sperm chromatin during fertilization

OBJECTIVE

To investigate the expression of oocyte-specific linker histone protein (hH1FOO) in human ovaries and its incorporation into sperm chromatin after intracytoplasmic sperm injection (ICSI).

DESIGN

Laboratory study.

SETTING

University hospital.

PATIENT(S)

Human ovarian tissues were obtained from patients at oophorectomy. Human oocytes and embryos were obtained from infertile patients undergoing IVF and ICSI.

INTERVENTION(S)

A polyclonal rabbit antibody targeting the predicted hH1FOO protein was used for immunohistochemical analysis. Western blot analysis and the reverse transcriptase-nested polymerase chain reaction were done to detect hH1FOO in chromatin of germinal vesicle-stage oocytes through to two-cell embryos.

MAIN OUTCOME MEASURE(S)

The hH1FOO antibody reactivity of oocytes, ovarian tissues, and sperm chromatin after ICSI.

RESULT(S)

hH1FOO protein was localized in all oocytes from primordial to Graafian follicles. In unfertilized oocytes after ICSI, the chromatin of injected sperm was condensed without hH1FOO incorporation in 45.5% of oocytes, condensed with hH1FOO incorporation in 9%, and decondensed with hH1FOO incorporation in 45.5%. None of the oocytes contained decondensed sperm chromatin without hHFOO incorporation.

CONCLUSION(S)

hH1FOO protein was expressed by human oocytes from primordial follicles to early embryogenesis. Sperm nuclei that were still condensed after ICSI could be separated into two categories by hH1FOO incorporation, which might provide valuable information regarding failed fertilization.

Polyspermic penetration in porcine IVM-IVF systems

Although techniques for in vitro production of porcine embryos have proceeded very rapidly during the past decade, polyspermic penetration still remains a persistent obstacle to porcine in vitro fertilization (IVF) systems. Considerable research on in vitro polyspermic penetration in porcine in vitro-matured (IVM) oocytes has been undertaken to try to solve this problem. In the current paper, recent advancements in overcoming the problems of polyspermy in porcine IVF systems are reviewed. Partial induction of the acrosome reaction of boar spermatozoa in IVF media that contain caffeine is likely to be one of the major causes of polyspermy. A reduction in the number of incompletely acrosome-reacted spermatozoa, which can bind tightly to the zona pellucida and mask free sperm receptors of the zona pellucida, could reduce the incidence of polyspermic penetration; however, morphological differences in the reaction of the zona pellucida have been observed between IVM and ovulated oocytes, which suggests that altered zona morphology may be another cause of polyspermic penetration. It has been shown that the developmental ability of polyspermic porcine embryos to the blastocyst stage is similar to that of normal embryos but that developmental competence to term is much lower. To overcome the current problems of polyspermy, it is suggested that future efforts should be focused on controlling boar sperm function and/or sperm-zona binding to achieve the final maturation associated with normal zona modifications of porcine oocytes at fertilization.

Protamines and male infertility

Protamines are the major nuclear sperm proteins. The human sperm nucleus contains two types of protamine: protamine 1 (P1) encoded by a single-copy gene and the family of protamine 2 (P2) proteins (P2, P3 and P4), all also encoded by a single gene that is transcribed and translated into a precursor protein. The protamines were discovered more than a century ago, but their function is not yet fully understood. In fact, different hypotheses have been proposed: condensation of the sperm nucleus into a compact hydrodynamic shape, protection of the genetic message delivered by the spermatozoa, involvement in the processes maintaining the integrity and repair of DNA during or after the nucleohistone-nucleoprotamine transition and involvement in the epigenetic imprinting of the spermatozoa. Protamines are also one of the most variable proteins found in nature, with data supporting a positive Darwinian selection. Changes in the expression of P1 and P2 protamines have been found to be associated with infertility in man. Mutations in the protamine genes have also been found in some infertile patients. Transgenic mice defective in the expression of protamines also present several structural defects in the sperm nucleus and have variable degrees of infertility. There is also evidence that altered levels of protamines may result in an increased susceptibility to injury in the spermatozoan DNA causing infertility or poor outcomes in assisted reproduction. The present work reviews the articles published to date on the relationship between protamines and infertility.

Effects of dithiothreitol and boar on pronuclear formation and embryonic development following intracytoplasmic sperm injection in pigs

The objective of this study was to improve normal fertilization, male pronuclear formation and embryonic development following intracytoplasmic injection of dithiothreitol (DTT)-treated boar spermatozoa. To determine the effect of DTT treatment, frozen-thawed boar spermatozoa were treated with DTT for 0, 10, 30, and 60 min, and injected into porcine oocytes. The effects of DTT and male difference on normal fertilization and embryonic development were investigated. The mean normal fertilization rate in the groups treated with DTT for 30 min (73.8%) and 60 min (74.9%) was higher (P < 0.05) than that in the control group (49.3%). The mean blastocyst formation rate in the group treated with DTT for 30 min (23.2%) was higher (P < 0.05) than that in the other groups (8.7-10.9%). Among boars there was no difference in normal fertilization, but there was a significant difference between the non-treated and the DTT-treated groups. The mean rate of blastocyst formation was different (P < 0.05) among boars, and between the non-treated and DTT-treated groups. The mean number of cells in blastocysts was similar among the boars and between the non-treated and the DTT-treated groups. In conclusion, DTT treatment for 30 min increased the rate of normal fertilization and embryonic development to the blastocyst stage. Furthermore, the rate of blastocyst formation of oocytes injected with spermatozoa differed among boars.

Further evidence on the role of heparan sulfate as protamine acceptor during the decondensation of human spermatozoa

BACKGROUND

Human spermatozoa decondense in vitro upon exposure to heparin and glutathione. Glutathione is also the disulfide bond reducer in vivo, and heparan sulfate, a functional analogue of heparin, has been proposed as the protamine acceptor. The aim of this study was to evaluate the decondensing ability of chemically modified heparins and different glycosaminoglycans (GAGs) on isolated sperm nuclei in vitro, and to analyse the possible role of different GAGs as protamine acceptors.

METHODS

Capacitated spermatozoa and isolated sperm nuclei from normospermic semen samples were decondensed in the presence of heparin (or its equivalent) and glutathione. After fixation with glutaraldehyde, the percentage of decondensed spermatozoa and nuclei was determined under phase-contrast. Proteins were extracted from sperm nuclei previously incubated in the presence of gluhathione and different GAGs by incubation with urea-beta-meracptoethanol-NaCl, and analysed by acid polyacrylamide gel electrophoresis.

RESULTS

The ability of desulfated heparins and other GAGs to decondense isolated nuclei mirrored exactly the decondensation of capacitated spermatozoa, the only difference being the level of maximum decondensation achieved. Heparan sulfate and heparin, but not other GAGs, were able to release protamines from sperm chromatin.

CONCLUSIONS

Heparan sulfate could be functioning as protamine acceptor in vivo during human sperm nuclear decondensation.

Bovine embryo development following ICSI: effect of activation, sperm capacitation and pre-treatment with dithiothreitol

The development of bovine embryos obtained by intracytoplasmic sperm injection (ICSI) was studied in relation to various treatments applied to the sperm and to the early embryo. We investigated the effect of different activation protocols on ICSI-embryos and the influence of sperm capacitation with heparin and D-penicillamine, hypotaurine, and epinephrine (PHE) prior to ICSI. Finally, we studied the effect of dithiothreitol (DTT) pre-treatment of sperm or of injected oocytes. The activation of ICSI-embryos by ionomycin (Io)-cycloheximide (CHX) and sperm pre-treatment with heparin in combination with PHE did not increase the developmental capacity of ICSI-embryos. By contrast, the treatment of injected oocytes with 2 mM DTT resulted in increased cleavage and blastocyst rates in the group of non-activated embryos and in acceleration of blastocyst development in the group of activated embryos. Similarly, pre-treatment of sperm with DTT, followed by ICSI and activation, determined an increase of embryo development on Day 7 although the total number of blastocysts recorded on Day 8 was not different from untreated controls. The transfer of 11 ICSI-blastocysts, produced without activation, in six recipients gave rise to two pregnancies of which one went to term with the birth of an healthy calf.