Transcriptional and epigenetic status of protamine 1 and 2 genes following round spermatids injection into mouse oocytes

Mysteries and unsolved problems of mammalian fertilization and related topics

Mammalian fertilization is a fascinating process that leads to the formation of a new individual. Eggs and sperm are complex cells that must meet at the appropriate time and position within the female reproductive tract for successful fertilization. I have been studying various aspects of mammalian fertilization over 60 years. In this review, I discuss many different aspects of mammalian fertilization, some of my laboratory's contribution to the field, and discuss enigmas and mysteries that remain to be solved.

Induced pluripotent stem cells derived from rabbits exhibit some characteristics of naïve pluripotency

Not much is known about the molecular and functional features of pluripotent stem cells (PSCs) in rabbits. To address this, we derived and characterized 2 types of rabbit PSCs from the same breed of New Zealand White rabbits: 4 lines of embryonic stem cells (rbESCs), and 3 lines of induced PSCs (rbiPSCs) that were obtained by reprogramming adult skin fibroblasts. All cell lines required fibroblast growth factor 2 for their growth and proliferation. All rbESC lines showed molecular and functional properties typically associated with primed pluripotency. The cell cycle of rbESCs had a prolonged G1 phase and a DNA damage checkpoint before entry into the S phase, which are the 2 features typically associated with the somatic cell cycle. In contrast, the rbiPSC lines exhibited some characteristics of naïve pluripotency, including resistance to single-cell dissociation by trypsin, robust activity of the distal enhancer of the mouse Oct4 gene, and expression of naïve pluripotency-specific genes, as defined in rodents. According to gene expression profiles, rbiPSCs were closer to the rabbit inner cell mass (ICM) than rbESCs. Furthermore, rbiPSCs were capable of colonizing the ICM after aggregation with morulas. Therefore, we propose that rbiPSCs self-renew in an intermediate state between naïve and primed pluripotency, which represents a key step toward the generation of bona fide naïve PSC lines in rabbits.

Induced pluripotent stem cells derived from rabbits exhibit some characteristics of naïve pluripotency

Not much is known about the molecular and functional features of pluripotent stem cells (PSCs) in rabbits. To address this, we derived and characterized 2 types of rabbit PSCs from the same breed of New Zealand White rabbits: 4 lines of embryonic stem cells (rbESCs), and 3 lines of induced PSCs (rbiPSCs) that were obtained by reprogramming adult skin fibroblasts. All cell lines required fibroblast growth factor 2 for their growth and proliferation. All rbESC lines showed molecular and functional properties typically associated with primed pluripotency. The cell cycle of rbESCs had a prolonged G1 phase and a DNA damage checkpoint before entry into the S phase, which are the 2 features typically associated with the somatic cell cycle. In contrast, the rbiPSC lines exhibited some characteristics of naïve pluripotency, including resistance to single-cell dissociation by trypsin, robust activity of the distal enhancer of the mouse Oct4 gene, and expression of naïve pluripotency-specific genes, as defined in rodents. According to gene expression profiles, rbiPSCs were closer to the rabbit inner cell mass (ICM) than rbESCs. Furthermore, rbiPSCs were capable of colonizing the ICM after aggregation with morulas. Therefore, we propose that rbiPSCs self-renew in an intermediate state between naïve and primed pluripotency, which represents a key step toward the generation of bona fide naïve PSC lines in rabbits.

Methylation profile of the promoters of Nanog and Oct4 in ICSI human embryos

STUDY QUESTION

What is the methylation status of the Nanog and Oct4 promoters in human gametes and ICSI embryos and is abnormal reprogramming of their methylation associated with developmental failure of ICSI embryos?

SUMMARY ANSWER

Developmental failure of human ICSI embryos is associated with high methylation of the Oct4 promoter.

WHAT IS KNOWN ALREADY

Nanog and Oct4 genes play critical roles in the establishment and maintenance of pluripotency during normal early embryonic development, and both are negatively regulated through the methylation of their promoters.

STUDY DESIGN, SIZE AND DURATION

We analysed the methylation profile of Nanog and Oct4 promoters in 5 control sperm from normally fertile men, 70 metaphase II oocytes, 21 4-cell control ICSI embryos, 7 control blastocysts and 45 ICSI embryos arrested at 2- to 8-cell stage following prolonged culture.

PARTICIPANTS, MATERIALS, SETTING AND METHODS

Embryos and gametes were donated for research by patients from the Department of Reproductive Medicine at the Hôpital Femme Mère Enfant (Bron, France) and the Clinique du Tonkin (Villeurbanne, France) after giving their informed consent.

MAIN RESULTS

For both promoters, high methylation was observed in sperm cells. Although, in general, the promoters were unmethylated in oocytes, the methylation of some alleles was observed, particularly in oocytes from women with known infertility. Both gene promoters were hypomethylated in control blastocyst ICM (inner cell mass) and in control 2-8-cells embryos obtained from 6 out of 8 couples. However, they appeared highly methylated in embryos obtained from the other two couples. In most arrested ICSI embryos, the Nanog promoter was unmethylated while the Oct4 promoter was highly methylated. High methylation of the Oct4 promoter was significantly more pronounced in embryos from couples where a male factor was the only known cause of infertility. When the embryos were heterozygous for a G/A single nucleotide polymorphism, both alleles could be methylated, each likely representing a paternally inherited or a maternally inherited copy.

LIMITATIONS AND REASONS FOR CAUTION

The study was done on a limited number of oocytes and embryos and the gametes of the couples were not available.

WIDER IMPLICATIONS OF THE FINDINGS

These results provide new insight regarding the roles of epigenetic abnormalities in early developmental failure in humans.

STUDY FUNDING/COMPETING INTEREST(S)

No external funding was obtained for this study. There was no competing interest.

Protamine-1 represents a sperm specific gene transcript: a study in Callithrix jacchus and Bos taurus

Spermatozoa are transcriptionally inactive cells, but contain acetylated histones, normally a characteristic of transcriptionally active cells. Acetylgroups are thought to represent epigenetic marks that are transmitted to the oocyte and are involved in starting gene expression in the zygote and in regulating gene expression during early embryogenesis. We performed reverse transcription polymerase chain reaction (RT-PCR) in the common marmoset monkey (Callithrix jacchus) and in bovine spermatozoa, oocytes, zygotes, two- and four-cell embryos to evaluate the presence of specific transcripts known to play a role during fertilisation and early embryo development, namely protamine-1 (PRM1), protamine-2 (PRM2), histone H1 (H1), histone H3 (H3), histone H4 (H4), cAMP-responsive element modulator (CREM), DNA methyltransferase-1 (DNMT1), TATA box-binding protein (TBP). All transcripts tested were present in spermatozoa of the common marmoset, while bull spermatozoa lack PRM2. Marmoset oocytes exhibited transcripts for H1, H3, H4 and TBP, whereas bovine oocytes revealed H1, H3, H4, CREM, DNMT and TBP mRNAs. In zygotes, we amplified H1, H4, TBP (marmoset) and PRM1, H1, H3, H4, CREM, DNMT1 and TBP (bovine). Two-cell embryos showed PCR products for H1, H3 and TBP in the marmoset. In the bovine, all transcripts could be observed except PRM2. In four-cell embryos, PCR signals were obtained for PRM1, H1, H3, H4 and TBP in the marmoset. In the bovine, all transcripts were detected except PRM2. Our data suggest that, in both C. jacchus and Bos taurus, PRM1 transcripts are delivered by the spermatozoon to the oocyte.

Methods for incorporating the hypermutability of CpG dinucleotides in detecting natural selection operating at the amino acid sequence level

In detecting natural selection operating at the amino acid sequence level by comparing the rates of synonymous (r(S)) and nonsynonymous (r(N)) substitutions, the rates of synonymous and nonsynonymous mutations are assumed to be approximately the same. In reality, however, these rates may not be the same if different proportions of synonymous and nonsynonymous sites overlap with CpG dinucleotides, which are known to be hypermutable in some organisms. Here, we develop the evolutionary pathway methods for comparing r(S) and r(N) at multiple codon sites (all-sites analysis) and at single codon sites (single-site analysis) that take into account the hypermutability at CpG dinucleotides in estimating the number of synonymous substitutions per synonymous site (d(S)) and nonsynonymous substitutions per nonsynonymous site (d(N)). Computer simulations show that the direction and magnitude of the bias in the estimation of d(N)/d(S) caused by the hypermutability of CpGs are determined by both the number of CpGs and the relative proportions of synonymous and nonsynonymous sites overlapping with CpGs. This bias is greatly reduced when using the methods we propose to account for the hypermutability of CpG dinucleotides. In an all-sites analysis of protamine 1 genes from primates, d(N)/d(S) > 1 was observed for many pairs if the hypermutability was ignored. However, d(N)/d(S) becomes <or=1 for most of these pairs when the CpG sites are assumed to be hypermutable. Therefore, statistical indications of positive selection in some sequences or individual codons may be caused by mutation rate differences in synonymous and nonsynonymous sites.

Pregnancy-specific {beta}-1-glycoprotein 1 and human leukocyte antigen-E mRNA in human sperm: differential expression in fertile and infertile men and evidence of a possible functional role during early development

BACKGROUND

Mature spermatozoa contain thousands of mRNA transcripts. It has been recently shown that human sperm can deliver RNA into the oocyte, suggesting that mRNAs might have a role before or after fertilization. Human embryos express PSG1 (pregnancy-specific beta-1-glycoprotein 1) and HLA-E (human leukocyte antigen-E), molecules playing a role in implantation and early development. We compared PSG1 and HLA-E sperm mRNA levels in fertile and infertile men and we tested the hypothesis that these transcripts are selectively retained in the newly formed zygote.

METHODS

Real-time RT-PCR was used to analyze sperm mRNA levels (n = 11 fertile, n = 31 infertile patients) of PSG1, HLA-E and PRM2 (protamine 2). The presence of PSG1 and HLA-E proteins was evaluated by western blot in sperm protein extracts (n = 3). Using ICSI of human sperm into hamster oocytes we evaluated the permanence of these mRNAs at different time points (n = 10 for each time) after fertilization.

RESULTS

PSG1, HLA-E and PRM2 transcripts were demonstrated in ejaculated sperm. The fertile group showed significantly higher levels of PSG1 and HLA-E mRNA (both P < 0.05) than the infertile group, whereas PRM2 levels were not significantly different. However, PSG1 and HLA-E proteins were not found in ejaculated sperm. Following ICSI, PRM2 was undetectable after fertilization; conversely, PSG1 and HLA-E transcripts remained detectable for at least 24 h of zygotic development.

CONCLUSIONS

We provide new evidence that indicates that human sperm deliver transcripts that may have a role in early embryo development and decreased levels of these transcripts may be associated with infertility.