Quantification of P450 aromatase gene expression in cultured and in vivo grown ovarian follicles in sheep

In vitro culture of sheep early-antral follicles: Milestones, challenges and future perspectives

Early antral follicles (EAFs) represent the transitional stage between pre-antral and antral follicles, containing oocytes that have completed most of their growth phase. Therefore, they offer an easily exploitable reserve for producing mature oocytes and preserving genetic resources, given their higher abundance compared to antral follicles (AFs) and shorter culture period than other pre-antral follicles (PAFs). Despite these advantages, the culture of EAFs remains challenging, and the success rates of in vitro embryo production (IVEP) from EAF-derived oocytes are still far below the standard achieved with fully grown oocytes in ruminant species. The difficulty is related to developing suitable in vitro culture systems tailored with nutrients, growth factors, and other signaling molecules to support oocyte growth. In this review, we focus on the in vitro development of sheep EAFs to provide an informative reference to current research progress. We also summarize the basic aspect of folliculogenesis in sheep and the main achievements and limitations of the current methods for EAF isolation, in vitro culture systems, and medium supplementation. Finally, we highlight future perspectives and challenges for improving EAF culture outcomes.

Kisspeptin promotes follicular development through its effects on modulation of P450 aromatase expression and steroidogenesis in sheep ovarian follicles

The present study was conducted to ascertain whether the role of kisspeptin in promoting in vitro development of preantral follicles was through the regulation of P450 aromatase gene expression and steroidogenesis in sheep. Accordingly, the cumulus cells and oocytes were collected from different development stages of preantral follicles grown in vivo and cultured in vitro in TCM199B (Group I), TCM199B + KP (10 μg/mL) (Group II) and Standard medium + KP (10 μg/mL). To measure the steroid (Estradiol-17β; E2 and Progesterone; P4 ) synthesis through ELISA, spent culture medium was collected separately from the same in vitro groups. E2 synthesis in the spent medium collected from all the three groups showed an increasing trend from PFs' exposed to respective culture media for 3 min to 2-day culture stage but decreased thereafter till 6-day culture stage. This is followed by a sharp increase in E2 concentration in the spent medium collected after in vitro maturation. However, P4 synthesis in group III followed increased pattern as the development progressed from PFs' exposed to culture medium for 3 min to in vitro maturation stage. The steroid production was observed at all stages of in vitro development in altered supplemented conditions. The steroid synthesis in the spent medium was highest in the 6 day cultured PFs' in Standard medium + KP matured in vitro for 24 h. Therefore, supplementation of kisspeptin along with other growth factors promoted steroid production in cultured preantral follicles far better than in other media.

Influence of TCM 199B, α-MEM, Waymouth MB 752/1 culture media, VEGF, Estradiol-17β, GDF-9 and FGF on in vitro development of preantral follicles in sheep

A total of 2792 preantral follicles (PFs') isolated from 750 ovaries of sheep were cultured in four different experiments. The efficacy of three commercially available culture media viz., TCM 199B, α-MEM and Waymouth MB 752/1 on the growth of sheep PFs' was tested in experiment I. Among the three media TCM 199B supported better development of PFs' in culture. The remaining experiments established the best concentrations of vascular endothelial growth factor (VEGF), Estradiol-17β (E2), GDF-9, Fibroblast growth factor (FGF) and their best combinations for the in-vitro development of PFs'. Inclusion of VEGF at 10 ng/mL, Estradiol-17β at 5 ng/mL, GDF-9 at 10 ng/mL or FGF at 10 ng/mL individually in a standard medium (SM) (containing FSH, IGF-I, GH and T4) supported better nuclear maturation of the oocytes to MII stage. Different combinations of VEGF, Estradiol-17β, GDF-9 and FGF supplemented in the SM promoted similar overall follicular growth. However, (a) SM + VEGF(10 ng/mL) + E2(5 ng/mL) supported higher increase in the diameter, (b) SM without any supplements induced antrum formation in greater proportion of follicles, and (c) SM + VEGF(10 ng/mL) + GDF 9(10 ng/mL) or SM + E2 (5 ng/mL) + FGF(10 ng/mL) supported high proportion of oocytes to reach MII stage. To conclude, TCM 199B appeared to be a better medium for development of sheep PFs'. VEGF, Estradiol-17 β, GDF-9 and FGF have beneficial influence on the development of sheep PFs' when supplemented in TCM 199B.

Granulosa cell genes that regulate ovarian follicle development beyond the antral stage: The role of estrogen receptor β

Follicle development beyond the preantral stage is dependent on gonadotropins. FSH signaling is crucial for the advancement of preantral follicles to the antral stage, and LH signaling is essential for further maturation of preovulatory follicles. Estrogen is intricately tied to gonadotropin signaling during the advanced stages of folliculogenesis. We observed that Erβ^null ovarian follicles fail to develop beyond the antral stage, even after exogenous gonadotropin stimulation. As ERβ is primarily expressed in the granulosa cells (GCs), we explored the gonadotropin-regulated GC genes that induce maturation of antral follicles. Synchronized follicle development was induced by administration of exogenous gonadotropins to wildtype 4-wk-old female rats. The GC transcriptome was analyzed via RNA-sequencing before and after gonadotropin stimulation. An Erβ^null mutant model that fails to show follicle maturation was also included in order to identify the ERβ-regulated genes involved at this step. We observed that specific groups of genes were differentially expressed in response to PMSG or hCG administration in wildtype rats. While some of the PMSG or hCG-induced genes showed a similar expression pattern in Erβ^null GCs, a subset of PMSG- or hCG-induced genes showed a differential expression pattern in Erβ^null GCs. These latter ERβ-regulated genes included previously known FSH or LH target genes including Lhcgr, Cyp11a1, Cyp19a1, Pgr, Runx2, Egfr, Kiss1, and Ptgs2, which are involved in follicle development, oocyte maturation, and ovulation. We also identified novel ERβ-regulated genes including Jaml, Galnt6, Znf750, Dusp9, Wnt16, and Mageb16 that failed to respond to gonadotropin stimulation in Erβ^null GCs. Our findings indicate that the gonadotropin-induced spatiotemporal pattern of gene expression is essential for ovarian follicle maturation beyond the antral stage. However, expression of a subset of those gonadotropin-induced genes is dependent on transcriptional regulation by ERβ.

Establishment of a protocol for the isolation of ovarian preantral follicles derived from collared peccaries (Pecari tajacu)

We compare the efficiency of mechanical or enzymatic methods, and their combination, for the isolation of ovarian preantral follicles (PFs) from collared peccaries. The ovaries from six females were subjected to the different methods investigated here. For the enzymatic method, ovary fragments were exposed to collagenase type IV in TCM-HEPES medium; the mechanical procedure was based on ovarian cortex dissociation by using a scalpel blade. The residual solution obtained after the mechanical isolation was subjected to the enzymatic procedure. The number of isolated PFs was quantified and classified as primordial, primary, or secondary; their viability was assessed using trypan blue dye assay. To confirm the results, PFs derived from the most efficient method were evaluated for integrity using scanning electron microscopy (SEM) and subjected to a 24 h in vitro culture for subsequent evaluation of viability by using fluorescent probes. A higher number of PFs (P < 0.05) was obtained from the enzymatic method (961.7 ± 132.9) in comparison with the mechanical method (434.3 ± 88.9), but no difference was observed between the two methods and their combination (743.2 ± 92.8). The trypan blue assay showed that the enzymatic method (98.7 ± 0.6%) provided the highest percentage of viable follicles (P < 0.05). Furthermore, SEM confirmed the ultrastructural integrity of the surface architecture of peccary PFs isolated by the enzymatic procedure; epifluorescence microscopy was used to confirm their viability (86.0%). In conclusion, we suggest that the enzymatic method investigated here is useful for the isolation of viable ovarian PFs from collared peccaries.

Leptin induced in vitro development of ovarian follicles in sheep is related to the expression of P450 aromatase and steroidogenesis

The objective of this study was to test the hypothesis that Leptin induced in vitro growth in preantral follicles in sheep involves modulation of P450 aromatase expression and steroidogenesis. Accordingly, the expression of P450 aromatase gene was studied in the cumulus cells and oocytes isolated from different stages of preantral follicles (PFs') grown in vivo, cultured in TCM 199B, TCM 199B + Leptin (10 ng/ml) (TCM199BL) or a standard PF culture medium supplemented with Leptin (10 ng/ml) (SML). Ovarian follicles grown in vivo or in SML expressed P450 aromatase both in cumulus cells and oocytes at all the development stages. In the oocytes from PFs' grown in vitro, P450 expression was consistently lower than in those from in vivo grown follicles at all except the preantral stage. The patterns of expression of aromatase gene in the cumulus cells from in vivo grown and the PFs' cultured in TCM 199BL were similar. Significantly higher levels of progesterone production were supported by SML at all the development stages than the other two media. Oestradiol concentration in the spent TCM 199B and SML showed a significant increase as the development progressed from preantral to large antral stage. However, such increase was not sustained beyond early antral stage in the PFs' cultured in TCM199BL. It is concluded that Leptin modulates the expression P450 aromatase while supporting the in vitro development of the ovarian follicles in sheep.

RNA-seq Transcriptome Analysis in Ovarian Tissue of Pelibuey Breed to Explore the Regulation of Prolificacy

The Pelibuey sheep (Ovis aries) is an indigenous breed distributed in the tropical regions of Mexico. The prolificacy of this sheep is on average from 1 to 1.5 lambs, being an important breeding characteristic that owners seek to increase with the purpose of economic improvements. New-generation RNA sequencing technology has been used to identify the genes that are expressed in the ovarian tissue of sheep that have two or more lambs per parturition, as well as to elucidate the metabolic pathways that are affected by the expression of these genes, with the purpose of better understanding the prolificacy in the sheep. In the present study, the transcriptional expression of multiparous and uniparous sheep was compared using RNA sequencing. Multiparous (M group) and uniparous (U group) sheep that had a genealogical record for three generations (M, n = 5 and U, n = 5) were selected. RNA was extracted from ovarian tissue and subsequently used to prepare the libraries that were sequenced using the Illumina NextSeq500 platform. A total of 31,575 genes were detected from the transcriptomic analysis of which 4908 were significantly expressed (p-value ≤ 0.001) in the ovary of sheep. Subsequently, a second filter was carried out to evaluate the false discovery rate (FDR) and select those genes with p-values ≤ 0.05 and values of expression ≥ 1 (log2), obtaining 354 differential expressed genes (DEG): 120 genes up-regulated and 234 genes down-regulated in the group M with respect to the group U. Through Gene Ontology (GO) and metabolic analysis, we obtained information on the function of differentially expressed genes, and its importance in the reproduction of multiparous sheep. This result suggest that genes identified in the present study participate in the development of the final stages of follicles.

Molecular evidence that follicle development is accelerated in vitro compared to in vivo

In this study, we systematically compared the morphological, functional and molecular characteristics of granulosa cells and oocytes obtained by a three-dimensional in vitro model of ovine ovarian follicular growth with those of follicles recovered in vivo. Preantral follicles of 200 µm diameter were recovered and cultured up to 950 µm over a 20-day period. Compared with in vivo follicles, the in vitro culture conditions maintained follicle survival, with no difference in the rate of atresia. However, the in vitro conditions induced a slight decrease in oocyte growth rate, delayed antrum formation and increased granulosa cell proliferation rate, accompanied by an increase and decrease in CCND2 and CDKN1A mRNA expression respectively. These changes were associated with advanced granulosa cell differentiation in early antral follicles larger than 400 µm diameter, regardless of the presence or absence of FSH, as indicated by an increase in estradiol secretion, together with decreased AMH secretion and expression, as well as increased expression of GJA1, CYP19A1, ESR1, ESR2, FSHR, INHA, INHBA, INHBB and FST. There was a decrease in the expression of oocyte-specific molecular markers GJA4, KIT, ZP3, WEE2 and BMP15 in vitro compared to that in vivo. Moreover, a higher percentage of the oocytes recovered from cultured follicles 550 to 950 µm in diameter was able to reach the metaphase II meiosis stage. Overall, this in vitro model of ovarian follicle development is characterized by accelerated follicular maturation, associated with improved developmental competence of the oocyte, compared to follicles recovered in vivo.

Expression of kit ligand and insulin-like growth factor binding protein 3 during in vivo or in vitro development of ovarian follicles in sheep

Expression of Kit ligand (KL) and insulin-like growth factor binding protein (IGFBP3) genes was studied at different in vivo and corresponding in vitro stages of development of the ovarian follicles in sheep. The expression of both KL and IGFBP3 was significantly higher in the primordial follicles relative to any other stage of development. Compared to the other stages, the KL expression in the cumulus cells from in vivo grown large antral follicles and that of IGFBP3 in COCs' isolated from large antral follicles matured in vitro for 24 hr were significantly higher. In the oocytes from in vivo grown ovarian follicles, the expression of KL was the same at all the stages of development. Insulin-like growth factor binding protein 3 expression was also the same in the oocytes at all the stages of the development except for a significantly lower expression in those from antral follicles. The expression of KL in the cumulus cells decreased significantly in the in vitro grown early antral follicles but did not change further as the development progressed. The expression of IGFBP3 in the cumulus cells from in vitro grown ovarian follicles appeared to increase as the development progressed although the increase was not significant between any two consecutive stages of development. In the oocytes in in vitro grown ovarian follicles, the expression levels of KL and IGFBP3 genes did not change with development. It is concluded that (i) KL and IGFBP3 genes follow specific patterns of expression during ovarian folliculogenesis and (ii) in vitro culture of preantral follicles compromises the development potential through alterations in the stage-specific patterns of expression of these and other developmentally important genes.

Expression of genes involved in BMP and estrogen signaling and AMPK production can be important factors affecting total number of antral follicles in ewes

Follicular growth and ovulation of healthy oocytes is a complicated process which is regulated by several endocrine and paracrine factors as well as cross-talk between the oocyte and its surrounding somatic cells. This study compared the expression profile of some candidate genes involved in BMP signaling as well as estrogen and AMPK production in cumulus-oocyte complex (COC) of small and large antral follicles and their associated somatic cell layers in ovaries from ewes with high- and low-antral follicle count (AFC). Expression of GDF9 was increased by increasing the size of antral follicles, while BMP15 expression was decreased by follicular size. It should be noteworthy that transcription of both GDF9 and BMP15 was also detected in the adjacent cellular layers under the follicles. There was a very strong positive correlation between BMP15 and BMPR2 in ovary tissues. Expression of GDF9 was highly correlated with BMP15, BMPR1B, and BMPR2 in large antral follicles. Expression of BMP7 in small antral follicles and BMPR2 in ovary tissues was significantly increased in the high-AFC group. Expression of ESR1 and ESR2 involved in estrogen production as well as PRKAA1 which involved in AMPK production were significantly greater in large antral follicles of high-AFC. There was a very high correlation between Cyp19 and ESR1 in large antral follicles and ovary tissues. Expression of Cyp19 and PRKAA1 were positively correlated with GDF9, BMP15, and BMP7 in large follicles. In conclusion, this study suggests that apart from the BMP signaling, genes involved in AMPK and estrogen production can be pivotal players in ewe's follicular development process. In addition, a strong cross-talk can exist among AMPK, BMP signaling, and estrogen synthesis systems in ewe ovary.

Analysis of genes that influence sheep follicular development by different nutrition levels during the luteal phase using expression profiling

Nutrition is an important factor that regulates reproductive performance of sheep and affects follicle development. However, the correlation between nutrition and follicle development is poorly understood at the molecular level. To study its possible molecular mechanisms, we performed expression profiling of granulosa cells isolated from sheep that were fed different levels of nutrition levels during the luteal phase. To do this, ewes received a maintenance diet (M), and their estrus was synchronized by intravaginal progestogen sponges for 12 days. Ewes were randomly divided into the short-term dietary-restricted group (R; 0.5 × M) and the nutrient-supplemented group (S; 1.5 × M). RNA samples were extracted from granulosa cells. Transcriptome libraries from each group were constructed by Illumina sequencing. Among 18 468 detected genes, 170 genes were significantly differentially expressed, of which 140 genes were upregulated and 30 genes were downregulated in group S relative to group R. These genes could be candidates regulating follicular development in sheep. Gene Ontology, KEGG and clustering analyses were performed. Genes related to oocyte meiosis, such as ADCY7, were upregulated. We identified two important groups of related genes that were upregulated with improved nutrition: one group comprising the genes PTGS2, UCP2 and steroidogenic acute regulatory protein and the other group comprising interleukin-1A and interleukin-1B. The genes within each group showed similar expression patterns. Additionally, all five genes are involved in the reproduction process. Quantitative real-time PCR was performed to validate the results of expression profiling. These data in our study are an abundant genomic resource to expand the understanding of the molecular and cellular events underlying follicle development.