Calcium release of heat-shocked porcine oocytes induced by thimerosal or inositol 1,4,5-trisphosphate (IP3)

Time-dependent effects of heat shock on the zona pellucida ultrastructure and in vitro developmental competence of bovine oocytes

The aim of this study was to determine the effects of different exposure lenght to heat shock (HS) during in vitro maturation (IVM) on zona pellucida (ZP) ultrastructure and developmental competence of bovine oocytes. Cumulus-oocyte complexes (COCs) were matured in vitro (IVM) at 38.5 °C for 24 h (control group, CG), or incubated at 41 °C (HS) for 6 h (HS-6h), 12 h (HS-12h), 18 h (HS-18h), and 22h (HS-22h) followed by incubation at 38.5 °C to complete a full 24-h period of maturation. After IVM, oocytes were subjected to scanning electron microscopy (SEM) or in vitro fertilization and culture until the blastocyst stage. For heat-shocked oocytes, with exception of those in the HS-6h group, SEM examinations revealed that ZP surfaces were rough and characterized by a presence of spongy network. Oocytes from the HS-22h group displayed an increase in the number of pores, as well as a higher proportion of oocytes with amorphous ZPs. The proportion of oocytes that reached metaphase II (MII) stage decreased in all HS groups, regardless of the duration of exposure to 41 °C. These results provide evidence that HS during IVM for 12-22 h reduces the developmental competence of bovine oocytes, increasing the percentage of oocytes with abnormal chromosomal organization, and reducing fertilization and blastocysts formation rate. The effects of HS were more pronounced for the 22-h exposure group. The damage induced by HS on oocyte function clearly increased upon exposure to elevated temperature.

Expression and activation of mitogen-activated protein kinases in matured porcine oocytes under thermal stress

In this study, we determined the expression and activation of p38 MAPK in matured porcine oocytes subjected to heat shock (HS). When MII oocytes were heated, only the phosphorylated p38 levels relative to the total p38 levels decreased (P < 0.01) after HS, but no clear relationship with HS treatments was observed in the ERK, JNK and p90^rsk expressions of matured oocytes. To confirm p38 activation in matured oocytes, immunocytochemical staining was performed to localize its expression and distribution in the ooplasm, and the results were largely consistent with previous Western blot analyses. Moreover, when matured oocytes were co-cultured with a P38 MAPK inhibitor, SB203580, for 4 h at 41.5 C, the activation of its immediate downstream substrate MAPKAPK-2 was not inhibited within any of the treatment groups. It appears that the MAPKAPK2 levels increased only under prolonged culture (HS4h and C4h) compared with the control group. In conclusion, p38 activity in porcine oocytes was decreased after exposure to HS and prolonged culture. These alterations of p38 and activation of MAPKAPK2 may be associated with porcine oocyte viability under HS conditions, and a potential cross-talk between p38 MAPK and other signaling cascades may exist, which warrants additional investigation.

Effects of heat shock during the early stage of oocyte maturation on the meiotic progression, subsequent embryonic development and gene expression in ovine

Heat shock may affect different aspects of oocyte maturation and its subsequent development to the blastocyst stage. A series of in vitro experiments was performed to determine whether physiologically heat shock (41°C) disrupts the progression of the ovine oocytes through meiosis, activation and blastocyst formation. Cumulus–oocyte complexes (COCs) were aspirated from 2–6-mm follicles and cultured at 38.5°C (control) or 41°C (heat shock) for the first 12 h of maturation. The oocytes were incubated at 38.5°C during the last 10 h of maturation and 8 days after activation. Results showed that most of the oocytes matured under heat-shock conditions remained at the germinal vesicle breakdown (GVBD) stage and they showed an aberrant chromatin configuration. After heat shock, oocyte diameter and time spent for zona pellucida dissolution increased (P < 0.05). The heat-shocked group had a higher percentage of oocytes with incomplete migration of cortical granules (P < 0.05). The heat-shock condition decreased (P < 0.05) cleavage rates (56.19 versus 89.28%) and morula formation (26.85 versus 37.81%). However, there was no significant difference in blastocyst formation and percentage of hatched blastocysts. At 12 h, heat shock had an adverse effect on embryo quality and reduced inner cell mass number (P < 0.05). Quantitative gene expression analysis showed greater transcripts (P < 0.05) for Na/K-ATPase mRNA in heat-shocked oocytes. To sum up, heat shock has disruptive effects on ovine oocyte maturation and can impair cellular and molecular factors that are important for embryo development.

Leukemia inhibitory factor enhances bovine oocyte maturation and early embryo development

The present study was conducted to examine the effects of leukemia inhibitory factor (LIF) on bovine oocyte maturation and early embryo development in vitro. Results showed that LIF supplementation (25 ng/ml) enhanced nuclear maturation of intact cumulus-oocyte complexes (COCs) compared to the vehicle control. Similar results were observed in denuded oocytes, indicating that LIF directly influences oocyte development. LIF-treated oocytes showed a higher cortical-granule-migration rate and increased expression of CD9, a tetraspanin transmembrane protein essential for fertilization. After in vitro fertilization, oocytes receiving LIF supplementation exhibited a higher cleavage rate and yielded a significantly higher number of blastocysts. To further dissect the molecular mechanism underlying this LIF-induced bovine oocyte maturation phenotype, we examined the involvement of two signaling cascades, mitogen-activated protein kinases (MAPK3/1)- and the signal transducer and activator of transcription 3 (STAT3)-dependent pathways. Western blot results revealed that LIF phosphorylated MAPK3/1 and STAT3. Inhibition of MAPK3/1 activation with MEK inhibitor U0126 only partially blocked LIF-induced nuclear maturation, although it attenuated oocyte cytoplasmic maturation. Inhibition of JAK/STAT3 activation with a specific pharmacological inhibitor completely abolished the LIF-response in bovine oocyte. In summary, these data revealed a novel role for LIF in bovine oocyte maturation subsequent embryonic development.

Disparate consequences of heat stress exposure during meiotic maturation: embryo development after chemical activation vs fertilization of bovine oocytes

Consequences of heat stress exposure during the first 12 h of meiotic maturation differed depending on how and when bovine oocytes were activated. If heat-stressed oocytes underwent IVF at ~24 h, blastocyst development was less than for respective controls and similar to that obtained for nonheat-stressed oocytes undergoing IVF at 30 h (i.e. slightly aged). In contrast, if heat-stressed oocytes underwent chemical activation with ionomycin/6-dimethylaminopurine at 24 h, blastocyst development was not only higher than respective controls, but also equivalent to development obtained after activation of nonheat-stressed oocytes at 30 h. Developmental differences in chemically activated vs IVF-derived embryos were not related to fertilization failure or gross alterations in cytoskeletal components. Rather, ionomycin-induced calcium release and MAP kinase activity were less in heat-stressed oocytes. While underlying mechanisms are multifactorial, ability to obtain equivalent or higher development after parthenogenetic activation demonstrates that oocytes experiencing heat stress during the first 12 h of meiotic maturation have the necessary components to develop to the blastocyst stage, but fail to do so after fertilization.

Impact of heat stress exposure during meiotic maturation on oocyte, surrounding cumulus cell, and embryo RNA populations

To determine if reductions in developmental competence related to heat stress exposure were correlated with perturbations in certain RNA populations, poly(A) RNA, total RNA, RNA size distribution, and the abundance of transcripts (cyclin B1, GDF9, BMP15, poly(A) polymerase, HSP70, 18S & 28S rRNA) were examined in oocytes matured at 38.5 or 41 C. Performing in vitro fertilization resulted in embryos for examining RNA. Relative to germinal vesicle-stage oocytes, total amount of poly(A) RNA decreased similarly in oocytes matured at 38.5 or 41 C. Total RNA did not change during meiotic maturation or up through the 4 to 8-cell stage of embryonic development. Blastocyst-stage embryos had more total RNA; those originating from heat-stressed oocytes had more than those from nonheat-stressed oocytes. Oocytes and 4 to 8-cell embryos had similar RIN values and ratios for rRNA, 18S/fast region, and 18S/inter region. Values obtained for blastocyst-stage embryos were similar to those obtained for cumulus cell RNA, which did not change during maturation. Culture at 41 C for the first 12 h of meiotic maturation had no impact on RNA size distribution or transcripts examined from oocytes, surrounding cumulus or resultant 4 to 8-cell embryos. Interestingly, however, RNA from blastocysts originating from heat-stressed oocytes had lower 18S/fast region and 18S/inter region ratios compared to other developmental stages and cumulus cells. Although biological significance of these RNA changes is unclear, differences at the molecular level in embryos from heat-stressed oocytes emphasize the importance of minimizing stress exposure during meiotic maturation, if the intent is to obtain developmentally-competent embryos.

Effect of thimerosal on Ca2+ movement and viability in human oral cancer cells

The effect of thimerosal on cytosolic free Ca2+ concentrations ([Ca2+]i ) in human oral cancer cells (OC2) is unclear. This study explored whether thimerosal changed basal [Ca2+]i levels in suspended OC2 cells using fura-2. Thimerosal at concentrations between 1and 50 μM increased [Ca2+]i in a concentration-dependent manner. The Ca2+ signal was reduced partly by removing extracellular Ca 2+. Thimerosal-induced Ca2+ influx was not blocked by L-type Ca2+ entry inhibitors and protein kinase C modulators (phorbol 12-myristate 13-acetate [PMA] and GF109203X). In Ca2+-free medium, 50 μM thimerosal failed to induce a [Ca2+]i rise after pretreatment with thapsigargin (an endoplasmic reticulum Ca2+ pump inhibitor). Inhibition of phospholipase C with U73122 did not change thimerosal-induced [Ca2+]i rises. At concentrations between 5 and 10 μM, thimerosal killed cells in a concentration-dependent manner. The cytotoxic effect of 8 μM thimerosal was potentiated by prechelating cytosolic Ca2+ with the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetate/acetomethyl (BAPTA/ AM). Flow cytometry data suggested that 1—7 μM thimerosal-induced apoptosis in a concentration-dependent manner. Collectively, in OC2 cells, thimerosal-induced [Ca2+]i rises by causing phospholipase C-independent Ca2+ release from the endoplasmic reticulum and Ca2+ influx through non—L-type Ca2+ channels. Thimerosal killed cells in a concentration-dependent manner through apoptosis.

Sonic hedgehog promotes porcine oocyte maturation and early embryo development

In the present study, we investigated the effects of the Sonic hedgehog (Shh) protein on porcine oocyte maturation and early embryo development. Immunohistochemistry showed activation of Shh signalling in cumulus–oocyte complexes (COCs), as reflected by Patched (Ptc), Smoothened (Smo) and Gli1 expression in oocytes, cumulus cells and granulosa cells, particularly those of small follicles (<2 mm in diameter). Western blot analysis showed Smo expression in COCs and in denuded oocytes derived from small and medium (3–7 mm)-sized follicles. Small follicles contained the highest concentration of Shh in follicular fluid compared with medium-sized and large (>7 mm in diameter) follicles. Supplementation with Shh (0.5 or 1 μg mL–1) enhanced oocyte maturation compared with the control group (92.4% and 90.4% v. 81.9%, respectively; P < 0.05). This effect was reversed by the simultaneous addition of cyclopamine (1–2 μm), an Shh inhibitor. Similar to intact COCs, denuded COCs showed enhanced maturation following Shh supplementation. Furthermore, cyclin B1 content, extracellular signal-regulated kinase 1/2 phosphorylation, intracellular calcium release, blastocyst rate and total cell numbers were greater (P < 0.05) in oocytes matured in the presence of 0.5 and 1 μg mL–1 Shh compared with control oocytes. The findings of the present study provide the first evidence that the Shh signalling pathway is active, or at least partially activated, in the porcine ovary and is likely to promote oocyte cytoplasmic and nuclear maturation, as well as subsequent in vitro development, although the underlying mechanisms remain to be elucidated.