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Sex chromosome transcripts can lead to a broad transcriptional sexual dimorphism in the absence of concomitant or previous exposure to sex hormones, especially when X-chromosome inactivation (XCI) is not complete. XCI timing has been suggested to differ greatly among species, and in bovine, most of the X-linked transcripts are upregulated in female blastocysts. To determine the timing of XCI, we analyzed in day 14 bovine embryos the sexual dimorphic transcription of seven X-linked genes known to be upregulated in female blastocysts (X24112, brain-expressed X-linked 2 (BEX2), ubiquitin-conjugating enzyme E2A (UBE2A), glucose-6-phosphate dehydrogenase (G6PD), brain-expressed X-linked 1 (BEX1), calpain 6 (CAPN6), and spermidine/spermine N-acetyltransferase 1 (SAT1)). The transcription of five genes whose expression differs between sexes at the blastocyst stage (DNMT3A, interferon tau (IFNT2), glutathione S-transferase mu 3 (GSTM3), progesterone receptor membrane component 1 (PGRMC1), and laminin alpha 1 (LAMA1)) and four genes related with sex determination (Wilms tumor 1 (WT1), gata binding protein 4 (GATA4), zinc finger protein multitype 2 (ZFPM2), and DMRT1) was also analyzed to determine the evolution of transcriptional sexual dimorphism. The expression level of five X-linked transcripts was effectively equalized among sexes suggesting that, in cattle, a substantial XCI occurs during the period between blastocyst hatching and initiation of elongation, although UBE2A and SAT1 displayed significant transcriptional differences. Similarly, sexual dimorphism was also reduced for autosomal genes with only DNMT3A and IFNT2 exhibiting sex-related differences. Among the genes potentially involved in sex determination, Wilms tumor 1 (WT1) was significantly upregulated in males and GATA4 in females, whereas no differences were observed for ZFPM2 and DMRT1. In conclusion, a major XCI occurred between the blastocyst and early elongation stages leading to a reduction in the transcriptional sexual dimorphism of autosomal genes, which makes the period the most susceptible to sex-specific embryo loss.

In adult tissues, sexual dimorphism is largely attributed to sex hormone effects, although there is increasing evidence for a major role of sex chromosome dosage. During preimplantation development, male and female embryos can display phenotypic differences that can only be attributed to the transcriptional differences resulting from their different sex chromosome complements. Thus, all expressed Y-linked genes and those X-linked genes that totally or partially escape X-chromosome inactivation at each specific developmental stage display transcriptional sexual dimorphism. Furthermore, these differentially expressed sex chromosome transcripts can regulate the transcription of autosomal genes, leading to a large transcriptional sexual dimorphism. The sex-dependent transcriptional differences may affect several molecular pathways such as glucose metabolism, DNA methylation and epigenetic regulation, and protein metabolism. These molecular differences may have developmental consequences, including sex-selective embryo loss and sex-specific epigenetic responses to environmental hazards, leading to long-term effects. This review discusses transcriptional sexual dimorphism in preimplantation embryos, its consequences on sex ratio biases and on the developmental origin of health and disease, and its significance for transcriptional studies and adult sexual dimorphism.

The aim of this study was to investigate, in unstimulated and superstimulated heifers, the effect of follicle aspiration just before ovulation on corpus luteum (CL) development, circulating progesterone (P₄) concentrations and the ability of the uterus to support embryo development. Following follicle aspiration or ovulation timed from GNRH administration, CL development was assessed by daily ultrasonography, and CL function was assessed in terms of the capacity to produce P₄ and the expression of genes involved in steroidogenesis in luteal tissue. The capacity of the uterine environment to support conceptus development was assessed following transfer and recovery of in vitro-produced embryos. Follicular aspiration just before the expected time of ovulation leads to a significant reduction in CL diameter, CL area and area of luteal tissue. This was associated with a decrease in circulating P₄ in both unstimulated and superstimulated heifers. Follicle aspiration leads to a reduction in conceptus length and area on day 14 in unstimulated heifers only. Follicle aspiration leads to a reduction in the expression of LHCGR in luteal tissue from unstimulated heifers compared with those in which the CL formed after ovulation. Superstimulation significantly reduced the expression of STAR in luteal tissue in both ovulated and follicle-aspirated heifers. In conclusion, in stimulated and unstimulated heifers, aspiration of the preovulatory dominant follicle(s) just before expected ovulation interferes with the subsequent formation and function of the CL, in terms of size and P₄ output and this, in turn, is associated with a reduced capacity of the uterus to support conceptus elongation in unstimulated heifers.

The aim of this study was to examine the temporal sensitivity of bovine embryos to culture environment after fertilization to determine which period, if any, is most critical in determining blastocyst quality. Bovine zygotes produced in vitro were divided into six groups and cultured either in vitro (in synthetic oviductal fluid, SOF), in vivo (in the ewe oviduct) or in a combination of both systems. Development to the blastocyst stage, the ability of the blastocysts to withstand cryopreservation and the relative abundance of several gene transcripts were examined. Culture in SOF for either 2 or 4 days, followed by subsequent culture in the ewe oviduct, resulted in a significantly lower yield of blastocysts than did all other methods, the effect being most marked in embryos that were cultured in SOF for 4 days. In contrast, culture in vivo for the first 2 or 4 days after fertilization followed by culture in vitro did not have such a marked effect on blastocyst development. Blastocysts produced after culture in the oviduct for 6 days had the highest rates of survival over 72 h after warming (100% survival at 24 h; >95% survival at 72 h). The embryos that spent the last 4 days of culture in vivo also had relatively high rates of survival (100% at 24 h, 73.7% at 72 h). Blastocysts produced entirely in SOF had very low rates of survival after vitrification, with <40% viable at 24 h and <20% survival at 72 h. Blastocysts derived from embryos that spent the first 2 days in vivo and the last 4 days in vitro had the lowest rates of survival (6.7%), whereas those that spent the last 2 days only in SOF had intermediate rates of survival (40.6%). These differences were reflected in the relative abundance of transcripts for the Bax gene.

The steroid hormone progesterone (P₄) plays a key role in the reproductive events associated with pregnancy establishment and maintenance. High concentrations of circulating P₄ in the immediate post-conception period have been associated with an advancement of conceptus elongation, an associated increase in interferon-τ production and higher pregnancy rates in cattle. Using in vitro and in vivo models and ∼8500 bovine oocytes across six experiments, the aim of this study was to establish the route through which P₄ affects bovine embryo development in vitro and in vivo. mRNA for P₄ receptors was present at all stages of embryo development raising the possibility of a direct effect of P₄ on the embryo. Exposure to P₄ in vitro in the absence or presence of oviduct epithelial cells did not affect the proportion of embryos developing to the blastocyst stage, blastocyst cell number or the relative abundance of selected transcripts in the blastocyst. Furthermore, exposure to P₄ in vitro did not affect post-hatching elongation of the embryo following transfer to synchronized recipients and recovery on Day 14. By contrast, transfer of in vitro derived blastocysts to a uterine environment previously primed by elevated P₄ resulted in a fourfold increase in conceptus length on Day 14. These data provide clear evidence to support the hypothesis that P₄-induced changes in the uterine environment are responsible for the advancement in conceptus elongation reported previously in cattle and that, interestingly, the embryo does not need to be present during the period of high P₄ in order to exhibit advanced elongation.

Endocannabinoids are known to mediate practically all reproductive events in mammals; however, little is known about their role in oocyte maturation. Through RT-PCR and immunocytochemistry, this study confirms the presence of CB1 and CB2 cannabinoid receptors in bovine oocytes and shows how exposure to the exogenous cannabinoids HU-210 and THC during their in vitro maturation (IVM) activates the phosphorylation of AKT and ERK1/2 proteins associated with the resumption of meiosis. Although supplementation with HU-210 or THC during IVM did not increase blastocyst yields, the expression of interferon tau (IFNτ) and gap junction alpha-1 protein (GJA1) was enhanced at the blastocyst stage. Our data suggest that cannabinoid agonists may be useful IVM supplements as their presence during oocyte maturation upregulates the expression in blastocysts of key genes for embryo quality.

Signaling components of bone morphogenetic proteins (BMPs) are expressed in an anatomically and temporally regulated fashion in bovine oviduct. However, a local response of this signaling to the presence of the embryo has yet to be elucidated. The aim of the present study was to evaluate if early embryo-oviduct interaction induces changes in the gene expression of BMP signaling components. For this purpose, we used an in vitro co-culture system to investigate the local interaction between bovine oviductal epithelial cells (BOEC) from the isthmus region with early embryos during two developmental periods: before (from the 2-cell to 8-cell stage) or during (from the 8-cell to 16-cell stage) the main phase of embryonic genome activation (EGA). Exposure to embryos, irrespective of the period, significantly reduced the relative abundance of BMPR1B, BMPR2, SMAD1, SMAD6 and ID2 mRNAs in BOEC. In contrast, embryos that interacted with BOEC before EGA showed a significant increase in the relative abundance of SMAD1 mRNA at the 8-cell stage compared to embryos cultured without BOEC. Moreover, embryos at the 16-cell stage that interacted with BOEC during EGA showed a significant increase in BMPR1B, BMPR2 and ID2 mRNA. These results demonstrate that embryo-oviduct interaction in vitro induces specific changes in the transcriptional levels of BMP signaling, causing a bidirectional response that reduces the expression levels of this signaling in the oviductal cells while increases them in the early embryo. This suggests that BMP signaling pathway could be involved in an early cross talk between the bovine embryo and the oviduct during the first stages of development.

Elevated plasma nonesterified fatty acid (NEFA) concentrations are associated with negative energy balance and metabolic disorders such as obesity and type II diabetes. Such increased plasma NEFA concentrations induce changes in the microenvironment of the ovarian follicle, which can compromise oocyte competence. Exposing oocytes to elevated NEFA concentrations during maturation affects the gene expression and phenotype of the subsequent embryo, notably prompting a disrupted oxidative metabolism. We hypothesized that these changes in the embryo are a consequence of modified energy metabolism in the oocyte. To investigate this, bovine cumulus oocyte complexes were matured under elevated NEFA conditions, and energy metabolism-related gene expression, mitochondrial function, and ultrastructure evaluated. It was found that expression of genes related to REDOX maintenance was modified in NEFA-exposed oocytes, cumulus cells, and resultant blastocysts. Moreover, the expression of genes related to fatty acid synthesis in embryos that developed from NEFA-exposed oocytes was upregulated. From a functional perspective, inhibition of fatty acid β-oxidation in maturing oocytes exposed to elevated NEFA concentrations restored developmental competence. There were no clear differences in mitochondrial morphology or oxygen consumption between treatments, although there was a trend for a higher mitochondrial membrane potential in zygotes derived from NEFA-exposed oocytes. These data show that the degree of mitochondrial fatty acid β-oxidation has a decisive impact on the development of NEFA-exposed oocytes. Furthermore, the gene expression data suggest that the resulting embryos adapt through altered metabolic strategies, which might explain the aberrant energy metabolism previously observed in these embryos originating from NEFA-exposed maturing oocytes.

Offspring telomere length (TL) has been correlated with paternal TL, but the mechanism for this parent of origin-specific inheritance remains unclear. The objective of this study has been to determine the role of spermatozoa TL in embryonic telomere lengthening by using two mouse models showing dimorphism in their spermatozoa TL: Mus musculus vs Mus spretus and old vs young Mus musculus. Mus spretu s spermatozoa displayed a shorter TL than Mus musculus. Hybrid offspring exhibited lower TL compared with Mus musculus starting at the two-cell stage, before the onset of telomerase expression. To analyze the role of spermatozoa telomeres in early telomere lengthening, we compared the TL in oocytes, zygotes, two-cell embryos and blastocysts produced by parthenogenesis or by fertilization with Mus musculus or Mus spretus spermatozoa. TL was significantly higher in spermatozoa compared with oocytes, and it increased significantly from the oocyte to the zygote stage in those embryos fertilized with Mus musculus spermatozoa, but not in those fertilized with Mus spretus spermatozoa or produced by parthenogenesis. A further increase was noted from the zygote to the two-cell stage in fertilized Mus musculus embryos, whereas hybrid embryos maintained the oocyte TL. Spermatozoa TL shortened with age in Mus musculus and the offspring from young males showed a significantly higher TL compared with that fathered by old males. These significant differences were already noticeable at the two-cell stage. These results suggest that spermatozoa telomeres act as a guide for telomerase-independent telomere lengthening resulting in differences in TL that persist after birth.

Free Spanish abstract: A Spanish translation of this abstract is freely available at http://www.reproduction-online.org/content/151/1/1/suppl/DC1.

In vivo-derived bovine embryos are of higher quality than those derived in vitro. Many of the differences in quality can be related to culture environment-induced changes in mRNA abundance. The aim of this study was to identify a range of mRNA transcripts that are differentially expressed between bovine blastocysts derived from in vitro versus in vivo culture. Microarray (BOTL5) comparison between in vivo- and in vitro-cultured bovine blastocysts identified 384 genes and expressed sequence tags (ESTs) that were differentially expressed; 85% of these were down-regulated in in vitro cultured blastocysts, showing a much reduced overall level of mRNA expression in in vitro- compared with in vivo-cultured blastocysts. Relative expression of 16 out of 23 (70%) differentially expressed genes (according to P value) were verified in new pools of in vivo- and in vitro-cultured blastocysts, using quantitative real-time PCR. Most (10 out of 16) are involved in transcription and translation events, suggesting that the reason why in vitro-derived embryos are of inferior quality compared with in vivo-derived embryos is due to a deficiency of the machinery associated with transcription and translation.