Age-related alterations in fertilization-induced Ca2+ oscillations depend on the genetic background of mouse oocytes†

Mitochondrial activity and redox status in oocytes from old mice: The interplay between maternal and postovulatory aging

Maternal aging has been reported to reduce oocyte quality and, in turn, lower the developmental potential of the resulting embryos. Here, we show that maternally aged oocytes display two strikingly different phenotypes: some have normal morphology, whereas others have significantly shrunk cytoplasm. The latter phenotype usually prevails in aged females. Our objective was to characterize both types of maternally aged oocytes and investigate the origins of this diversity. Importantly, our experiments indicate that shrunk maternally aged oocytes are severely compromised in terms of mitochondrial functionality as compared to their young or morphologically normal maternally aged counterparts: they display significantly decreased mitochondrial activity and lower amounts of ROS. In contrast, morphologically normal maternally aged oocytes had the same mitochondrial activity as young ones, while their ROS levels were higher. Surprisingly, the shrunk phenotype was completely absent in maternally aged oocytes that matured in vitro, suggesting that it is not caused inherently by maternal aging, but may be related to other factors, like postovulatory aging. Indeed, an additional culture of in vitro matured young and old oocytes (i.e., in vitro postovulatory aging) significantly decreased their mitochondrial activity and led to cytoplasm shrinkage. In vivo postovulatory aging had a similar effect on oocytes from both young and old females. Finally, we examined the developmental potential of oocytes obtained from aged females. Shrunk (i.e., most likely postovulatory aged) oocytes failed to become fertilized, whereas morphologically normal ones (i.e., most likely not subjected to postovulatory aging) underwent fertilization and subsequent cleavage divisions, although they achieved the 2-cell stage less frequently than morphologically normal oocytes from young females. Importantly, the quality of blastocysts as well as the live birth rate for morphologically normal oocytes from old and young females were similar. In summary, our data clearly indicate that two pools of oocytes present in oviducts of aged females differ significantly in their quality and developmental potential and that the more severely affected phenotype results most likely from a synergistic action of maternal and postovulatory aging.

Effects of an obesogenic diet on the oviduct depend on the duration of feeding

Research question

How long does it take for an obesogenic (high-fat/high-sugar, HF/HS) diet to influence the oviductal microenvironment? What are the affected cellular pathways and are they dependent on the genetic background of the mouse model?

Design

Female Swiss (outbred) and C57BL/6N (B6, inbred) mice were fed either a control (10% fat) or HF/HS (60% fat, 20% fructose) diet. Body weight was measured weekly. Mice were sacrificed at 3 days (3d), 1 week (1w), 4w, 8w, 12w and 16w on the diet (n = 5 per treatment per time point). Total cholesterol concentrations and inflammatory cytokines were measured in serum. Oviductal epithelial cells (OECs) were used to study the expression of genes involved in (mitochondrial) oxidative stress (OS), endoplasmic reticulum (ER) stress and inflammation using qPCR.

Results

Body weight and blood cholesterol increased significantly in the HF/HS mice in both strains compared to controls. In Swiss mice, HF/HS diet acutely increased ER-stress and OS-related genes in the OECs already after 3d. Subsequently, mitochondrial and cytoplasmic antioxidants were upregulated and ER-stress was alleviated at 1w. After 4-8w (mid-phase), the expression of ER-stress and OS-related genes was increased again and persisted throughout the late-phase (12-16w). Serum inflammatory cytokines and inflammatory marker-gene expression in the OECs were increased only in the late-phase. Some of the OEC stress responses were stronger or earlier in the B6.

Conclusions

OECs are sensitive to an obesogenic diet and may exhibit acute stress responses already after a few days of feeding. This may impact the oviductal microenvironment and contribute to diet-induced subfertility.

Dual spindles assemble in bovine zygotes despite the presence of paternal centrosomes

The first mitosis of the mammalian embryo must partition the parental genomes contained in two pronuclei. In rodent zygotes, sperm centrosomes are degraded, and instead, acentriolar microtubule organizing centers and microtubule self-organization guide the assembly of two separate spindles around the genomes. In nonrodent mammals, including human or bovine, centrosomes are inherited from the sperm and have been widely assumed to be active. Whether nonrodent zygotes assemble a single centrosomal spindle around both genomes or follow the dual spindle self-assembly pathway is unclear. To address this, we investigated spindle assembly in bovine zygotes by systematic immunofluorescence and real-time light-sheet microscopy. We show that two independent spindles form despite the presence of centrosomes, which had little effect on spindle structure and were only loosely connected to the two spindles. We conclude that the dual spindle assembly pathway is conserved in nonrodent mammals. This could explain whole parental genome loss frequently observed in blastomeres of human IVF embryos.