Yuechao Lu

Oocyte cryopreservation and in vitro culture affect calcium signalling during human fertilization

STUDY QUESTION What are the precise patterns of calcium oscillations during the fertilization of human oocytes matured either in vivo or in vitro or aged in vitro and what is the effect of cryopreservation? SUMMARY ANSWER Human oocytes matured in vivo exhibit a specific pattern of calcium oscillations, which is affected by in vitro maturation, in vitro ageing and cryopreservation. WHAT IS KNOWN ALREADY Oscillations in cytoplasmic calcium concentration are crucial for oocyte activation and further embryonic development. While several studies have described in detail the calcium oscillation pattern during fertilization in animal models, studies with human oocytes are scarce. STUDY DESIGN, SIZE, DURATION This was a laboratory-based study using human MII oocytes matured in vivo or in vitro either fresh or after cryopreservation with slow freezing or vitrification. Altogether, 205 human oocytes were included in the analysis. PARTICIPANTS/MATERIALS, SETTING, METHODS In vivo and in vitro matured human oocytes were used for this research either fresh or following vitrification/warming (V/W) and slow freezing/thawing (F/T). Human oocytes were obtained following written informed consent from patients undergoing ovarian hyperstimulation. For the calcium pattern analysis, oocytes were loaded with the ratiometric calcium indicator fluorescent dye Fura-2. Following ICSI using sperm from a single donor, intracellular calcium was measured for 16 h at 37°C under 6% CO(2). The calcium oscillation parameters were calculated for all intact oocytes that showed calcium oscillations and were analyzed using the Mann-Whitney U-test. MAIN RESULTS AND THE ROLE OF CHANCE Human in vivo MII oocytes display a specific pattern of calcium oscillations following ICSI. This pattern is significantly affected by in vitro ageing, with the calcium oscillations occurring over a longer period of time and with a lower frequency, shorter duration and higher amplitude (P < 0.05). In vitro matured oocytes from the GV and MI stage exhibit a different pattern of calcium oscillations with calcium transients being of lower frequency and shorter duration compared with in vivo matured MII. In MI oocytes that reached the MII stage within 3 h the calcium oscillations additionally appear over a longer period of time (P < 0.05). In vivo MII oocytes show a different calcium oscillation pattern following V/W with calcium oscillations occurring over a longer period of time, with a higher amplitude and a lower frequency (P < 0.05). In vitro matured oocytes, either from the GV or the MI stage, also display an altered pattern of calcium oscillations after V/W and the parameters that were similarly affected in all these oocyte groups are the frequency and the amplitude of the calcium transients. Slow freezing/thawing differentially affects the calcium oscillation pattern of in vitro matured and in vitro aged oocytes. LIMITATIONS, REASONS FOR CAUTION The relationship between a specific pattern of calcium oscillations and subsequent human embryonic development could not be evaluated since the calcium indicator used and the high-intensity excitation light impair development. Furthermore, all oocytes were derived from stimulated cycles and immature oocytes were denuded prior to in vitro maturation. WIDER IMPLICATIONS OF THE FINDINGS Our data show for the first time how calcium signalling during human fertilization is affected by oocyte in vitro maturation, in vitro ageing as well as V/W and slow freezing/thawing. The analysis of calcium oscillations could be used as an oocyte quality indicator to evaluate in vitro culture and cryopreservation techniques of human oocytes. STUDY FUNDING/COMPETING INTEREST(S) This work was supported by a clinical research mandate from the Flemish Foundation of Scientific Research (FWO-Vlaanderen, FWO09/ASP/063) to F.V.M, a fundamental clinical research mandate from the FWO-Vlaanderen (FWO05/FKM/001) to P.D.S and a Ghent University grant (KAN-BOF E/01321/01) to B.H. The authors have no conflict of interest to declare.

A systematic analysis of the suitability of preimplantation genetic diagnosis for mitochondrial diseases in a heteroplasmic mitochondrial mouse model

STUDY QUESTION What is the reliability of preimplantation genetic diagnosis (PGD) based on polar body (PB), blastomere or trophectoderm (TE) analysis in a heteroplasmic mitochondrial mouse model? SUMMARY ANSWER The reliability of PGD to determine the level of mitochondrial DNA (mtDNA) heteroplasmy is questionable based on either the first or second PB analysis; however, PGD based on blastomere or TE analysis seems more reliable. WHAT IS KNOWN ALREADY PGD has been suggested as a technique to determine the level of mtDNA heteroplasmy in oocytes and embryos to avoid the transmission of heritable mtDNA disorders. A strong correlation between first PBs and oocytes and between second PBs and zygotes was reported in mice but is controversial in humans. So far, the levels of mtDNA heteroplasmy in first PBs, second PBs and their corresponding oocytes, zygotes and blastomeres, TE and blastocysts have not been analysed within the same embryo. STUDY DESIGN, SIZE AND DURATION We explored the suitability of PGD by comparing the level of mtDNA heteroplasmy between first PBs and metaphase II (MII) oocytes (n = 33), between first PBs, second PBs and zygotes (n = 30), and between first PBs, second PBs and their corresponding blastomeres of 2- (n = 10), 4- (n = 10) and 8-cell embryos (n = 11). Levels of mtDNA heteroplasmy in second PBs (n = 20), single blastomeres from 8-cell embryos (n = 20), TE (n = 20) and blastocysts (n = 20) were also compared. PARTICIPANTS/MATERIALS, SETTING, METHODS Heteroplasmic mice (BALB/cOlaHsd), containing mtDNA mixtures of BALB/cByJ and NZB/OlaHsd, were used in this study. The first PBs were biopsied from in vivo matured MII oocytes. The ooplasm was then subjected to ICSI. After fertilization, second PBs were biopsied and zygotes were cultured to recover individual blastomeres from 2-, 4- and 8-cell embryos. Similarly, second PBs were biopsied from in vivo fertilized zygotes and single blastomeres were biopsied from 8-cell stage embryos. The remaining embryo was cultured until the blastocyst stage to isolate TE cells. Polymerase chain reaction followed by restriction fragment length polymorphism was performed to measure the level of mtDNA heteroplasmy in individual samples. MAIN RESULTS AND THE ROLE OF CHANCE Modest correlations and wide prediction interval [PI at 95% confidence interval (CI)] were observed in the level of mtDNA heteroplasmy between first PBs and their corresponding MII oocytes (r(2) = 0.56; PI = 45.96%) and zygotes (r(2) = 0.69; PI = 37.07%). The modest correlations and wide PI were observed between second PBs and their corresponding zygotes (r(2) = 0.65; PI = 39.69%), single blastomeres (r(2) = 0.42; PI = 48.04%), TE (r(2) = 0.26; PI = 54.79%) and whole blastocysts (r(2) = 0.40; PI = 57.48%). A strong correlation with a narrow PI was observed among individual blastomeres of 2-, 4- and 8-cell stage embryos (r(2) = 0.92; PI = 11.73%, r(2) = 0.86; PI = 18.85% and r(2) = 0.85; PI = 21.42%, respectively), and also between TE and whole blastocysts (r(2) = 0.90; PI = 23.58%). Moreover, single blastomeres from 8-cell stage embryos showed a close correlation and an intermediate PI with corresponding TE cells (r(2) = 0.81; PI = 28.15%) and blastocysts (r(2) = 0.76; PI = 36.43%). LIMITATIONS, REASONS FOR CAUTION These results in a heteroplasmic mitochondrial mouse model should be further verified in patients with mtDNA disorders to explore the reliability of PGD. WIDER IMPLICATIONS OF THE FINDINGS To avoid the transmission of heritable mtDNA disorders, PGD techniques should accurately determine the level of heteroplasmy in biopsied cells faithfully representing the heteroplasmic load in oocytes and preimplantation embryos. Unlike previous PGD studies in mice, our results accord with PGD results for mitochondrial disorders in humans, and question the reliability of PGD using different stages of embryonic development. TRIAL REGISTRATION NUMBER Not applicable.

Effect of two assisted oocyte activation protocols used to overcome fertilization failure on the activation potential and calcium releasing pattern.

OBJECTIVE To assess the effect of two assisted oocyte activation (AOA) protocols with the use of two calcium (Ca(2+)) ionophores, ionomycin and A23187 (calcimycin), on the intracellular Ca(2+) level in mouse and human oocytes and the fertilization rates. DESIGN Comparison of two Ca(2+) ionophores, ionomycin and A23187, regarding their capacity to increase the intracellular Ca(2+) level and to support subsequent oocyte activation and development. SETTING University hospital research laboratory. PATIENT(S)/ANIMAL(S) Patients undergoing intracytoplasmic sperm injection (ICSI) treatment and B6D2F1 mice. INTERVENTION(S) Assisted oocyte activation and microinjection of mouse and human oocytes with sperm. MAIN OUTCOME MEASURE(S) Measurement of the fertilizing and Ca(2+)-releasing ability of human sperm. RESULT(S) Ionomycin was more potent than A23187 in provoking Ca(2+) increases in both mouse and human oocytes with significantly higher amplitude and area under the receiver operating characteristic curve. The oocyte activation rate was significantly higher when mouse oocytes were activated with the use of the ionomycin- rather than the A23187-based AOA protocol. Furthermore, oocyte activation rate was higher when human in vitro matured oocytes were activated with the ionomycin-based AOA protocol, but the difference did not reach statistical significance. CONCLUSION(S) In both mouse and human oocytes, the AOA protocol that used ionomycin was more efficient than the one that used A23187. Bearing in mind that mammalian fertilization is successful when the total dose of Ca(2+) released reaches a minimal threshold, the use of ionomycin for human AOA might be justified instead of the use of A23187.

Assessment of nuclear transfer techniques to prevent the transmission of heritable mitochondrial disorders without compromising embryonic development competence in mice

To evaluate and compare mitochondrial DNA (mtDNA) carry-over and embryonic development potential between different nuclear transfer techniques we performed germinal vesicle nuclear transfer (GV NT), metaphase-II spindle-chromosome-complex (MII-SCC) transfer and pronuclear transfer (PNT) in mice. No detectable mtDNA carry-over was seen in most of the reconstructed oocytes and embryos. No significant differences were seen in mtDNA carry-over rate between GV NT (n=20), MII-SCC transfer (0.29 ± 0.63; n=21) and PNT (0.29 ± 0.75; n=25). Blastocyst formation was not compromised after either PNT (88%; n=18) or MII-SCC transfer (86%; n=27). Further analysis of blastomeres from cleaving embryos (n=8) demonstrated undetectable mtDNA carry-over in all but one blastomere. We show that NT in the germ line is potent to prevent transmission of heritable mtDNA disorders with the applicability for patients attempting reproduction.

Culture conditions affect Ca2+ release in artificially activated mouse and human oocytes

Inconsistent fertilisation and pregnancy rates have been reported by different laboratories after application of ionomycin as a clinical method of assisted oocyte activation (AOA) to overcome fertilisation failure. Using both mouse and human oocytes, in the present study we investigated the effects of ionomycin and Ca2+ concentrations on the pattern of Ca2+ release and embryonic developmental potential. In the mouse, application of 5μM ionomycin in potassium simplex optimisation medium (KSOM) or 10µM ionomycin in Ca2+-free KSOM significantly reduced the Ca2+ flux and resulted in failure of blastocyst formation compared with 10μM ionomycin in KSOM. Increasing the Ca2+ concentration up to three- or sixfold did not benefit mouse embryonic developmental potential. Similarly, 10μM ionomycin-induced rise in Ca2+ in human oocytes increased with increasing total calcium concentrations in the commercial medium. Remarkably, we observed significantly reduced mouse embryo development when performing AOA over a period of 10min in Quinns AdvantageTM Fertilisation medium (Cooper Surgical) and IVFTM medium (Vitrolife) compared with Sydney IVF COOK cleavage medium (Cook Ireland), using the same sequential culture system from the post-activation stage to blastocyst formation stage in different AOA groups. In conclusion, concentrations of both ionomycin and Ca2+ in culture media used during AOA can have significant effects on Ca2+ release and further embryonic developmental potential.

Human oocyte calcium analysis predicts the response to assisted oocyte activation in patients experiencing fertilization failure after ICSI

STUDY QUESTION Can human oocyte calcium analysis predict fertilization success after assisted oocyte activation (AOA) in patients experiencing fertilization failure after ICSI? SUMMARY ANSWER ICSI-AOA restores the fertilization rate only in patients displaying abnormal Ca2+ oscillations during human oocyte activation. WHAT IS KNOWN ALREADY Patients capable of activating mouse oocytes and who showed abnormal Ca2+ profiles after mouse oocyte Ca2+ analysis (M-OCA), have variable responses to ICSI-AOA. It remains unsettled whether human oocyte Ca2+ analysis (H-OCA) would yield an improved accuracy to predict fertilization success after ICSI-AOA. STUDY DESIGN, SIZE, DURATION Sperm activation potential was first evaluated by MOAT. Subsequently, Ca2+ oscillatory patterns were determined with sperm from patients showing moderate to normal activation potential based on the capacity of human sperm to generate Ca2+ responses upon microinjection in mouse and human oocytes. Altogether, this study includes a total of 255 mouse and 122 human oocytes. M-OCA was performed with 16 different sperm samples before undergoing ICSI-AOA treatment. H-OCA was performed for 11 patients who finally underwent ICSI-AOA treatment. The diagnostic accuracy to predict fertilization success was calculated based on the response to ICSI-AOA. PARTICIPANTS/MATERIALS, SETTING, METHODS Patients experiencing low or total failed fertilization after conventional ICSI were included in the study. All participants showed moderate to high rates of activation after MOAT. Metaphase II (MII) oocytes from B6D2F1 mice were used for M-OCA. Control fertile sperm samples were used to obtain a reference Ca2+ oscillation profile elicited in human oocytes. Donated human oocytes, non-suitable for IVF treatments, were collected and vitrified at MII stage for further analysis by H-OCA. MAIN RESULTS AND THE ROLE OF CHANCE M-OCA and H-OCA predicted the response to ICSI-AOA in 8 out of 11 (73%) patients. Compared to M-OCA, H-OCA detected the presence of sperm activation deficiencies with greater sensitivity (75 vs 100%, respectively). ICSI-AOA never showed benefit to overcome fertilization failure in patients showing normal capacity to generate Ca2+ oscillations in H-OCA and was likely to be beneficial in cases displaying abnormal H-OCA Ca2+ oscillations patterns. LIMITATIONS, REASONS FOR CAUTION The scarce availability of human oocytes donated for research purposes is a limiting factor to perform H-OCA. Ca2+ imaging requires specific equipment to monitor fluorescence changes over time. WIDER IMPLICATIONS OF THE FINDINGS H-OCA is a sensitive test to diagnose gamete-linked fertilization failure. H-OCA allows treatment counseling for couples experiencing ICSI failures to either undergo ICSI-AOA or to participate in gamete donation programs. The present data provide an important template of the Ca2+ signature observed during human fertilization in cases with normal, low and failed fertilization after conventional ICSI. STUDY FUNDING/COMPETING INTEREST(S) This work was supported by the Flemish fund for scientific research (FWO-Vlaanderen, G060615N). The authors have no conflict of interest to declare.

Patients with a high proportion of immature and meiotically resistant oocytes experience defective nuclear oocyte maturation patterns and impaired pregnancy outcomes

Patients presenting with abnormally high numbers of immature oocytes at retrieval are more likely to exhibit maturation resistant oocytes. However, the clinical relevance of such events remains unknown. We investigated nuclear maturation competence of immature oocytes from patients showing >40% of collected immature oocytes (Study group) and Controls, in which a normal number of mature oocytes (≥60%) was retrieved. Following in-vitro culture, oocytes were classified as maturation resistant or in-vitro matured (IVM). Treatment outcomes were evaluated in Study and Control groups based on presence of maturation resistant oocytes. Overall, similarly high spindle and chromosome abnormality rates were observed in maturation resistant oocytes from both Study and Control groups. IVM oocytes from the Study group revealed significantly higher percentages of misaligned chromosomes compared with Controls (P < 0.05). Remarkably, Study group patients with at least one maturation resistant oocyte showed significantly reduced cumulative pregnancy and live birth rates compared with Control group maturation resistant patients (P < 0.05). When further investigating the aetiology, a maturation resistant mouse model revealed defective Ca2+ signalling of maturation resistant oocytes at germinal vesicular breakdown and parthenogenetic activation. In conclusion, appropriate treatment strategies, including clinical utilization of IVM oocytes from Study group patients, warrant further investigation.