Analysis of cat oocyte activation methods for the generation of feline disease models by nuclear transfer

Feline cumulus cells and oocytes show massive accumulation of lipid droplets and upregulation of PLIN2 expression after in vitro maturation

This study investigates lipid content and expression of genes involved in lipid metabolism in feline cumulus cells and oocytes before and after in vitro maturation (IVM). Domestic cats represent valuable models in reproductive research, yet the efficiency of in vitro embryo production remains suboptimal, with contributing factors still under investigation. We characterized lipid droplets (LDs) in oocytes collected from adult queens, both before and after IVM, using confocal microscopy with Bodipy 493/503 staining. We also quantified the expression of four genes involved in regulation of lipid metabolism. Our findings revealed a substantial accumulation of LDs and a significant upregulation of PLIN2 expression in both cumulus cells and oocytes following IVM. The number and total area of LDs and fluorescence intensity increased markedly in oocytes at the MII stage, while the average LD diameter decreased. Similarly, cumulus cells showed an increase in number and total area of LDs post-IVM, suggesting their involvement in oocyte maturation by modulating lipid homeostasis. This study presents the first detailed characterization of lipid droplet dynamics in feline oocytes and cumulus cells, providing insights into metabolic processes during IVM, which are critical for optimizing assisted reproductive technologies in felids.

The role of passage numbers of donor cells in the development of Arabian Oryx – Cow interspecific somatic cell nuclear transfer embryos

The cloning between different animals known as interspecific somatic cell nuclear transfer (iSCNT) was carried out for endangered species. The iSCNT has been characterized by a poor success rate due to several factors that influence the formation of the SCNT in various cytoplasms. The cell cycle of the transferred somatic cell, the passage number of the cultured somatic cell, the mitochondria oocytes, and their capabilities are among these factors. This study investigates the role of the passage number of the Arabian Oryx somatic cell culture when transplanted to an enucleated domestic cow oocyte and embryo development in vitro. The fibroblast somatic cell of the Arabian Oryx was cultured for several passage lanes (3–13). The optimal passage cell number was found to be 10–13 Oryx cell lines that progressed to various cell stages up to the blastula stage. There was some variation between the different passage numbers of the oryx cell line. The 3–9 cell line did not show a good developmental stage. These could be attributed to several factors that control the iSCNT as stated by several investigators. More investigation is needed to clarify the role of factors that affect the success rate for the iSCNT.

Design of novel oocyte activation methods: The role of zinc

Oocyte activation occurs at the time of fertilization and is a series of cellular events initiated by intracellular Ca2+ increases. Consequently, oocytes are alleviated from their arrested state in meiotic metaphase II (MII), allowing for the completion of meiosis. Oocyte activation is also an essential step for somatic cell nuclear transfer (SCNT) and an important tool to overcome clinical infertility. Traditional artificial activation methods aim to mimic the intracellular Ca2+ changes which occur during fertilization. Recent studies emphasize the importance of cytoplasmic Zn2+ on oocyte maturation and the completion of meiosis, thus suggesting artificial oocyte activation approaches that are centered around the concentration of available Zn2+in oocytes. Depletion of intracellular Zn2+ in oocytes with heavy metal chelators leads to successful oocyte activation in the absence of cellular Ca2+ changes, indicating that successful oocyte activation does not always depends on intracellular Ca2+ increases. Current findings lead to new approaches to artificially activate mammalian oocytes by reducing available Zn2+ contents, and the approaches improve the outcome of oocyte activation when combined with existing Ca2+ based oocyte activation methods. Here, we review the important role of Ca2+ and Zn2+ in mammalian oocyte activation and development of novel oocyte activation approaches based on Zn2+ availability.

Developmental competence of cat (Felis domesticus) oocytes and embryos after parthenogenetic stimulation using different methods

The aim of this study was to compare the effects of various activating factors on feline oocytes. The study included activation within the ovary (natural), activation during in vitro maturation (spontaneous activation), chemical activation (ionomycin + 6-DMAP), activation by spermatozoa and injection (ICSI) and mechanical activation (sham ICSI). According to our results, parthenogenetic embryos could emerge at every step of in vitro embryo production (IVP) procedures. After oocyte collection, 6% of parthenogenetic embryos were observed, mainly at the 2-4-blastomere stages. After 24 h of in vitro maturation, parthenogenetic activation was observed in 7% of oocytes. Using ionomycin and 6-DMAP to artificially activate oocytes, 53% of cleaved embryos were obtained. The results after ICSI (54% cleaved embryos) were not significantly different from the results in Group III using chemical activation (53% cleaved embryos). But only after ICSI were blastocysts obtained (5/73.7%) as a result of in vitro culture. Moreover, embryos after ICSI were of the best morphological quality with minor levels of fragmentation evident in the embryos. After sham mechanical activation, 'sham ICSI', 8% of cleaved embryos were noted. Therefore, it is advised to maintain a negative control in parallel with each step of IVP techniques, to avoid misleading results. Chemical methods for artificial activation of feline oocytes are the most promising for application to the cloning and production of parthenogenetic embryos for experimental studies.

The effects of permeating cryoprotectants on intracellular free-calcium concentrations and developmental potential of in vitro-matured feline oocytes

Embryos produced from vitrified feline oocytes have resulted in pregnancies, but the efficiency of oocyte vitrification in cats is still low. Our objectives were to evaluate the effects of exposing feline oocytes to ethylene glycol (EG), propanediol (PrOH) and dimethyl sulfoxide (DMSO) on changes in intracellular free-calcium concentrations ([Ca2+]i), the time needed for enzymatic digestion of the zona pellucida (ZP), the incidence of parthenogenetic activation and degeneration and embryonic development following in vitro fertilisation (IVF). All of the chemicals tested altered [Ca2+]i, but changes in [Ca2+]i, resistance of the ZP to enzymatic digestion and the incidence of parthenogenetic activation (<5% for all treatments) were not affected (P > 0.05) by extracellular Ca2+. Exposure to EG (>44.1%) and DMSO (19.7%) increased (P < 0.05) oocyte degeneration compared with control oocytes and oocytes exposed to PrOH (≤2.5%). Following exposure to a combination of PrOH and DMSO (10% v/v each), blastocyst development (per cleaved embryo; 52.1%) was similar (P > 0.05) to control oocytes (64.4%). When oocytes were vitrified with PrOH and DMSO, 28.3% of surviving (intact plasma membrane) oocytes cleaved following IVF, but no blastocyst developed. When a non-permeating cryoprotectant (galactose, 0.25 M) was added to the vitrification medium, 47.7% of surviving oocytes cleaved and 14.3% developed to the blastocyst stage.

Cheetah interspecific SCNT followed by embryo aggregation improves in vitro development but not pluripotent gene expression

The aim of this study was to evaluate the capacity of domestic cat (Dc,Felis silvestris) oocytes to reprogram the nucleus of cheetah (Ch,Acinonyx jubatus) cells by interspecies SCNT (iSCNT), by using embryo aggregation. Dc oocytes werein vitromatured and subjected to zona pellucida free (ZP-free) SCNT or iSCNT, depending on whether the nucleus donor cell was of Dc or Ch respectively. ZP-free reconstructed embryos were then cultured in microwells individually (Dc1X and Ch1X groups) or in couples (Dc2X and Ch2X groups). Embryo aggregation improvedin vitrodevelopment obtaining 27.4, 47.7, 16.7 and 28.3% of blastocyst rates in the Dc1X, Dc2X, Ch1X and Ch2X groups, respectively (P<0.05). Moreover, aggregation improved the morphological quality of blastocysts from the Dc2X over the Dc1X group. Gene expression analysis revealed that Ch1X and Ch2X blastocysts had significantly lower relative expression of OCT4, CDX2 and NANOG than the Dc1X, Dc2X and IVF control groups. The OCT4, NANOG, SOX2 and CDX2 genes were overexpressed in Dc1X blastocysts, but the relative expression of these four genes decreased in the Dc2X, reaching similar relative levels to those of Dc IVF blastocysts. In conclusion, Ch blastocysts were produced using Dc oocytes, but with lower relative expression of pluripotent and trophoblastic genes, indicating that nuclear reprogramming could be still incomplete. Despite this, embryo aggregation improved the development of Ch and Dc embryos, and normalized Dc gene expression, which suggests that this strategy could improve full-term developmental efficiency of cat and feline iSCNT embryos.

Felis catus ovary as a model to study follicle biology in vitro

PURPOSE

The current study was designed to evaluate the response of individual intact antral follicles from adult female domestic cats to a luteinizing hormone (LH) stimulus in vitro by assessing cumulus-oocyte expansion (C-OE) and steroid production.

METHODS

C-OE and steroid levels (estradiol [E2] and progesterone [P4]) obtained from individual antral feline follicles (n = 366 follicles; n = 56 cats) were analyzed after 12 or 24 h of culture in the presence or absence of LH (low [3.4 ng/ml] or high [100 ng/ml]).

RESULTS

At the end of the culture, the highest percentage of expanded cumulus-oocyte complexes (COCs) was observed in the LH groups at 12 or 24 h in comparison to their controls (p < 0.001). There was a significant increase in expanded COCs when comparing LH concentrations (high vs. low) at 12 or 24 h. Higher levels of both E2 and P4 were observed in the media from antral follicles after 12 and 24 h of culture in the presence of LH (both concentration, p < 0.05). There was no association between hormone levels and follicle diameter; high variability was observed in the steroid levels produced by antral follicles within all treatment groups.

CONCLUSIONS

These data indicate, for the first time, that feline antral follicles (0.5-2 mm) from different stages of the natural estrous cycle can be cultured and will respond to an LH stimulus, based on an increase in steroid levels as well as C-OE after 12 or 24 h in culture.

Effects of repetitive ionomycin treatment on in vitro development of bovine somatic cell nuclear transfer embryos

To artificially activate embryos in somatic cell nuclear transfer (SCNT), chemical treatment with ionomycin has been used to induce transient levels of Ca(2+) and initiate reprogramming of embryos. Ca(2+) oscillation occurs naturally several times after fertilization (several times with 15- to 30-min intervals). This indicates how essential additional Ca(2+) influx is for successful reprogramming of embryos. Hence, in this report, the experimental design was aimed at improving the developmental efficiency of cloned embryos by repetitive Ca(2+) transients rather than the commonly used ionomycin treatment (4 min). To determine optimal Ca(2+) inflow conditions, we performed three different repetitive ionomycin (10 µM) treatments in reconstructed embryos: Group 1 (4-min ionomycin treatment, once), Group 2 (30-sec treatment, 4 times, 15-min intervals) and Group 3 (1-min treatment, 4 times, 15-min intervals). Pronuclear formation rates were checked to assess the effects of repetitive ionomycin treatment on reprogramming of cloned embryos. Cleavage rates were investigated on day 2, and the formation rates of blastocysts (BLs) were examined on day 7 to demonstrate the positive effect of repeated ionomycin treatment. In Group 3, a significant increase in BL formation was observed [47/200 (23.50%), 44/197 (22.33%) and 69/195 (35.38%) in Groups 1, 2 and 3, respectively]. Culturing embryos with different ionomycin treatments caused no significant difference among the groups in terms of the total cell number of BLs (164.3, 158.5 and 145.1, respectively). Additionally, expression of the anti-apoptotic Bcl-2 gene and MnSOD increased significantly in Group 3, whereas the expression of the pro-apoptotic Bax decreased statistically. In conclusion, the present study demonstrated that repeated ionomycin treatment is an improved activation method that can increase the developmental competence of SCNT embryos by decreasing the incidence of apoptosis.