Evaluation of ribonucleic acid amplification protocols for human oocyte transcriptome analysis

Comparative study of preimplantation development following distinct assisted oocyte activation protocols in a PLC-zeta knockout mouse model

Mammalian fertilization encompasses a series of Ca2+ oscillations initiated by the sperm factor phospholipase C zeta (PLCζ). Some studies have shown that altering the Ca2+ oscillatory regime at fertilization affects preimplantation blastocyst development. However, assisted oocyte activation (AOA) protocols can induce oocyte activation in a manner that diverges profoundly from the physiological Ca2+ profiling. In our study, we used the newly developed PLCζ-null sperm to investigate the independent effect of AOA on mouse preimplantation embryogenesis. Based on previous findings, we hypothesized that AOA protocols with Ca2+ oscillatory responses might improve blastocyst formation rates and differing Ca2+ profiles might alter blastocyst transcriptomes. A total of 326 MII B6D2F1-oocytes were used to describe Ca2+ profiles and to compare embryonic development and individual blastocyst transcriptomes between four control conditions: C1 (in-vivo fertilization), C2 (ICSI control sperm), C3 (parthenogenesis) and C4 (ICSI-PLCζ-KO sperm) and four AOA groups: AOA1 (human recombinant PLCζ), AOA2 (Sr2+), AOA3 (ionomycin) and AOA4 (TPEN). All groups revealed remarkable variations in their Ca2+ profiles; however, oocyte activation rates were comparable between the controls (91.1% ± 13.8%) and AOA (86.9% ± 11.1%) groups. AOA methods which enable Ca2+ oscillatory responses (AOA1: 41% and AOA2: 75%) or single Ca2+ transients (AOA3: 50%) showed no significantly different blastocyst rates compared to ICSI control group (C2: 70%). In contrast, we observed a significant decrease in compaction (53% vs. 83%) and blastocyst rates (41% vs. 70%) in the absence of an initial Ca2+ trigger (AOA4) compared with the C2 group. Transcription profiles did not identify significant differences in gene expression levels between the ICSI control group (C2) and the four AOA groups.

Factors affecting the gene expression of in vitro cultured human preimplantation embryos

STUDY QUESTION

What is the relative effect of common environmental and biological factors on transcriptome changes during human preimplantation development?

SUMMARY ANSWER

Developmental stage and maternal age had a larger effect on the global gene expression profile of human preimplantation embryos than the culture medium or oxygen concentration used in in vitro culture.

WHAT IS KNOWN ALREADY

Studies on mouse and bovine embryos have shown that different conditions in the in vitro culture of embryos can lead to changes in transcriptome profiles. For humans, an effect of developmental stage on the transcriptome profile of embryos has been demonstrated, but studies on the effect of maternal age or culture conditions are lacking.

STUDY DESIGN, SIZE, DURATION

Donated, good quality, day 4 cryopreserved human preimplantation embryos (N = 89) were randomized to be cultured in one of two culture media (G5 medium or HTF medium) and one of two oxygen concentrations (5% or 20%), with stratification for maternal age. Next to these variables, developmental stage after culture was taken into account in the analysis.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Embryos that developed to morula or blastocyst stage during these 2 days whose amplified mRNA passed our quality control criteria for microarray hybridization were individually examined for genome-wide gene expression (N = 37).

MAIN RESULTS AND THE ROLE OF CHANCE

Based on the number of differentially expressed genes (DEGs), developmental stage (3519 DEGs) and maternal age (1258 DEGs) had a larger effect on the global gene expression profile of human preimplantation embryos than either tested culture medium (596 DEGs) or oxygen concentration (492 DEGs) used during in vitro culture. Interactions between the factors were found, indicating that culture conditions might have a different effect depending on the developmental stage or the maternal age of the embryos. Affected pathways included metabolism, cell cycle processes and oxidative phosphorylation.

LIMITATIONS, REASONS FOR CAUTION

Culture of embryos for only 2 days might have limited the effect on global gene expression by the investigated culture conditions. Earlier stages of development (Day 0 until Day 4) were not analyzed and these embryos might respond differently to the experimental conditions. The freezing and thawing procedures might have had an effect on gene expression. RT-PCR validation was not performed due to scarcity of the material.

WIDER IMPLICATIONS OF THE FINDINGS

Our results show that when studying gene expression in single human preimplantation embryos under various experimental conditions, one should take into account the confounding effect of biological variables, such as developmental stage and maternal age. This makes these experiments different from gene expression experiments where these variables can be tightly controlled, for example when using cell lines.

STUDY FUNDING/COMPETING INTERESTS

This study received no external funding and there were no competing interests.