Kinetics of GDF9 expression in buffalo oocytes during in vitro maturation and their associated development ability

Transient suppression of Wnt signaling in poor-quality buffalo oocytes improves their developmental competence

Introduction

One of the most evolutionary conserved communication systems, the Wnt signaling pathway is a major gene regulatory pathway that affects the developmental competence of oocytes and regulates most embryonic developmental processes. The present study was undertaken to modulate the canonical Wnt (Wingless/integration) signaling pathway in the poor-quality (colorless cytoplasm after Brilliant Cresyl Blue staining, BCB-) buffalo cumulus-oocyte complexes (COCs) to improve their in vitro maturation (IVM) and embryo production (IVEP) rates.

Methods

The expression of key Wnt pathway genes was initially assessed in the good (blue cytoplasm after Brilliant Cresyl Blue staining, BCB+) and poor quality (BCB-) buffalo COCs to establish a differential activity of the Wnt pathway. The BCB- COCs were supplemented with the Wnt pathway inhibitor, Dickkopf-related protein 1 (DKK1) and later subjected to IVM and IVEP along with the BCB+ and BCB- controls. The cumulus expansion index (CEI), rate of nuclear maturation (mean percentage of oocytes in the MII stage) and embryo production, and the expression of developmentally important genes were evaluated to assess the effect of Wnt pathway inhibition on the development competence of these poor-quality oocytes.

Results

The Wnt pathway genes exhibited a significantly higher expression (p < 0.05) in the poor-quality BCB- oocytes compared to the good-quality BCB+ oocytes during the early maturation stages. The supplementation of BCB- COCs with 100 ng/mL DKK1 effectively inhibited the expression of the key mediators of the Wnt pathway (β-catenin and dishevelled homolog 1, DVL1). DKK1 supplemented BCB- COCs exhibited significantly improved cytoplasmic and nuclear maturation indices, development rates and significantly elevated expression (p < 0.05) of genes implicated in germinal vesicle breakdown (GVBD) and embryonic genome activation (EGA) vis-à-vis BCB- control COCs.

Conclusion

These data indicate that inhibition of the Wnt pathway during the initial course of oocyte maturation can improve the development competence of poor-quality buffalo oocytes.

A novel lincRNA identified in buffalo oocytes with protein binding characteristics could hold the key for oocyte competence

Studying the maternal oocyte-specific genes, in farm animals is a significant step towards delineating the underlying mechanisms that regulate oocyte quality, early embryonic development and survival. With the creation of buffalo oocyte-specific subtracted cDNA library, it has raised new questions which need to be answered. The present study has characterized one of the ESTs selected from the library and highlighted its importance in the oocyte quality. The selected EST was made full length by RLM-RACE and four transcript variants were identified. Bioinformatics analysis indicated the novelty of full-length transcript along with conserved intergenic nature. The largest transcript was identified as long intergenic noncoding RNA based upon coding potential calculator output. The expression analysis at different hours of oocyte maturation showed a significant variation in developmentally competent oocytes to that of incompetent ones. Along with this, the transcript was also found to have protein binding ability which was confirmed by RNA electrophoretic mobility shift assay. The protein used in the experiment was isolated from oocyte and cumulus cells via sonication. A novel lincRNA has been reported here that might have an important role in maturation of oocytes, inferred from its relative gene expression study and protein binding characteristics.

Transient Arrest of Germinal Vesicle Breakdown Improved In Vitro Development Potential of Buffalo (Bubalus Bubalis) Oocytes

Germinal vesicle breakdown (GVBD) is the first milestone that an oocyte needs to achieve toward completing the maturation and gaining potential to fertilize. Significantly lower in vitro embryo production rate in buffaloes can be attributed to heterogeneity of GVBD occurrence among oocytes obtained from abattoir derived ovaries. Evidence from our earlier work had suggested that different qualities of buffalo oocytes differ significantly in their timing of GVBD. Besides, these oocytes also differ in terms of volume of Akt phosphorylation, which initiates the process of GVBD. With objective of synchronizing the oocytes for GVBD, immature buffalo oocytes were subjected to a two-step culture protocol, initially in the presence of GVBD inhibitors and subsequently, in vitro maturation (IVM) with added SC79 (activates Akt). Expression of developmentally important genes was assessed along with embryo development rate and blastocyst health to interpret the consequences. Oocytes subjected to a short GVBD inhibition period of 6 h followed by IVM with SC79 resulted in improved cleavage and blastocyst rates. Resultant blastocysts also possessed higher ICM: TE ratio. Further, GVBD inhibited oocytes displayed a sustained cytoplasmic maturation status in terms of reorganization of cortical granules (CGs), mitochondrial membrane potential, and glutathione levels during the period of inhibition. We conclude that a temporary GVBD arrest of buffalo oocytes and modulation of Akt improves the in vitro embryo development rate as well as quality of resultant embryos. Besides, our meiotic arrest protocol does not affect the cytoplasmic maturation. J. Cell. Biochem. 119: 278-289, 2018. © 2017 Wiley Periodicals, Inc.

Expression of growth differentiation factor 9 (GDF-9) during in vitro maturation in canine oocytes

The aim of this study was to characterize in canine oocytes and cumulus cells the dynamic expression of growth differentiation factor 9 (GDF-9) in relation to meiotic development and cumulus expansion throughout in vitro maturation (IVM). Cumulus oocytes complexes (COCs) from ovaries of adult bitches were cultured intact for IVM during 0, 48, 72, and 96 hours. At 0 hours or after IVM, COCs were divided into two groups: one group remained with their cumulus cells and in the other group the cumulus cells were extracted. The expression levels of GDF-9 were determined in both groups using indirect immunofluorescence and Western blot analysis. For immunofluorescence assay, in vivo-matured oocytes collected from oviducts were also used as a positive control. The nuclear stage was analyzed in parallel with 4'-6-diamidino-2-phenylindole staining in denuded oocytes from all maturing groups. The intensity of fluorescence, indicative of GDF-9 expression level, decreased with time (P < 0.05). High expression was observed only in germinal vesicle nonmature oocytes; in contrast, second metaphase oocytes showed only low expression. Western blot analysis showed bands of approximately 56 kd and a split band of approximately 20 kd representing the proprotein and possibly two mature protein forms of GDF-9, respectively. The proprotein was detected in all samples, and it was highly expressed before IVM and in a lesser degree, during the first 48 hours, declining thereafter in coincidence with the expansion of the cumulus cell (P < 0.05). There was a negative correlation (r = -0.97; P < 0.05) between the expression level of GDF-9 and mucification. Mature forms were evident only in COCs, before culture and up to 48 hours of IVM. It was concluded that GDF-9 is expressed in canine oocytes and cumulus cells, mainly in the early developmental states, with low levels in mature oocytes in vitro and in vivo, representing the first approach of GDF-9 dynamic in dog oocyte maturation.

A reporter promoter assay confirmed the role of a distal promoter NOBOX binding element in enhancing expression of GDF9 gene in buffalo oocytes

Growth differentiation factor 9 is primarily expressed in oocytes and plays a vital role in oocyte cumulus crosstalk. Earlier studies with buffalo oocytes revealed differential expression of this gene under different media stimulation conditions which, in turn, are correlated with the blastocyst yield. In this study, different germ cell specific cis elements including a NOBOX binding elements (NBE) and several E-boxes were identified at the 5' upstream region of buffalo GDF9 gene and their potential role in GDF9 expression was investigated. Transfecting oocytes with GDF9 promoter deletion constructs harbouring the NBE reporter gene revealed a 33% increase in GFP as well as the luciferase signal signifying its role in stimulating the minimal promoter activity of GDF9 in buffalo oocytes. Site directed mutation of core binding nucleotides at NBE at 1.8 kb upstream to TSS further confirmed its role for enhancing the basal transcriptional activity of GDF9 promoter in buffalo oocytes. Current work will provide important leads for understanding the role of GDF9 in oocytes competence and designing a more physiological IVF protocol in case of buffalo.