Atresia In Pig Ovaries Research Articles

METTL3-mediated m6A methylation regulates granulosa cells autophagy during follicular atresia in pig ovaries

Follicular atresia is a normal physiological event in mammals, yet its mechanism remains to be studied. Granulosa cell (GC) autophagy is closely associated with follicular atresia. The N6-methyladenosine (m6A) modification is the most common post-transcriptional modification in eukaryotes, but its role in follicular atresia is still unknown. In this study, the possible relationship amongst follicular atresia, GC autophagy and m6A modification was studied. Our results showed that the level of autophagy in GCs increased with the degree of follicle atresia, whereas the overall m6A level decreased. Rapamycin treatment induced atresia in vitro cultured follicles, whereas 3-Methyladenine inhibited follicular atresia. Progressed atretic follicle (PAF) GCs had significantly lower METTL3 levels and significantly higher FTO levels than healthy follicle (HF) GCs. Differential gene expression analysis of GCs in PAF and HF by RNA sequencing was showed that the autophagy-related genes ULK1, ULK2, ATG2A, and ATG2B were significantly elevated in the PAF. In cultured GCs, overexpression of METTL3 significantly decreased the mRNA level of ULK1, as well as the autophagy level, whereas knockdown of METTL3 by RNAi significantly increased the mRNA level of ULK1, as well as the autophagy level. Our results indicate that m6A modification can regulate autophagy in GCs and play a role in the process of porcine follicular atresia.

Mettl3-Mediated M6a Methylation Regulates Granulosa Cells Autophagy During Follicular Atresia in Pig Ovaries

Mettl3-Mediated M6a Methylation Regulates Granulosa Cells Autophagy During Follicular Atresia in Pig Ovaries

Initiation of follicular atresia: gene networks during early atresia in pig ovaries

In mammals, more than 99% of ovarian follicles undergo a degenerative process known as atresia. The molecular events involved in atresia initiation remain incompletely understood. The objective of this study was to analyze differential gene expression profiles of medium antral ovarian follicles during early atresia in pig. The transcriptome evaluation was performed on cDNA microarrays using healthy and early atretic follicle samples and was validated by quantitative PCR. Annotation analysis applying current database (Sus scrofa 11.1) revealed 450 significantly differential expressed genes between healthy and early atretic follicles. Among them, 142 were significantly upregulated in early atretic with respect to healthy group and 308 were downregulated. Similar expression trends were observed between microarray data and quantitative RT-PCR confirmation, which indicated the reliability of the microarray analysis. Further analysis of the differential expressed genes revealed the most significantly affected biological functions during early atresia including blood vessel development, regulation of DNA-templated transcription in response to stress and negative regulation of cell adhesion. The pathway and interaction analysis suggested that atresia initiation associates with (1) a crosstalk of cell apoptosis, autophagy and ferroptosis rather than change of typical apoptosis markers, (2) dramatic shift of steroidogenic enzymes, (3) deficient glutathione metabolism and (4) vascular degeneration. The novel gene candidates and pathways identified in the current study will lead to a comprehensive view of the molecular regulation of ovarian follicular atresia and a new understanding of atresia initiation.

MiR-1275 controls granulosa cell apoptosis and estradiol synthesis by impairing LRH-1/CYP19A1 axis

miR-1275 is one of the microRNAs (miRNAs) that are differentially expressed during follicular atresia in pig ovaries, as identified by a miRNA microarray assay in our previous study [1]. However, its functions in follicular atresia remain unknown. In this study, we showed that miR-1275 promotes early apoptosis of porcine granulosa cells (pGCs) and the initiation of follicular atresia, and inhibits E2 release and expression of CYP19A1, the key gene in E2 production. Bioinformatics and luciferase reporter assays revealed that liver receptor homolog (LRH)-1, not CYP19A1, is a direct functional target of miR-1275. In vitro overexpression and knockdown experiments showed that LRH-1 significantly repressed apoptosis and induced E2 secretion and CYP19A1 expression in pGCs. LRH-1, whose expression was regulated by miR-1275, prevented apoptosis in pGCs. Furthermore, luciferase and chromatin immunoprecipitation assays demonstrated that LRH-1 protein bound to the CYP19A1 promoter and increased its activity. Our findings suggest that miR-1275 attenuates LRH-1 expression by directly binding to its 3’UTR. This prevents the interaction of LRH-1 protein with the CYP19A1 promoter, represses E2 synthesis, promotes pGC apoptosis, and initiates follicular atresia in porcine ovaries.

Expression and Function of Apoptosis Initiator FOXO3 in Granulosa Cells During Follicular Atresia in Pig Ovaries

In mammalian ovaries, most follicles are lost by atresia before ovulation. It has become apparent that the apoptosis of granulosa cells induces follicular atresia. Forkhead box O3 (FOXO3), also called FKHRL1 (forkhead in rhabdomyosarcoma-like 1), is a proapoptotic molecule that belongs to the FOXO subfamily of forkhead transcription factors. Foxo3-deficient female mice were reported to be infertile because of abnormal ovarian follicular development, but the precise influences of FOXO3 on follicular atresia of mature ovary have not been determined. Therefore, we examined the expression and function of FOXO3 in porcine ovarian follicles and granulosa-derived cells. FOXO3 mRNA levels in granulosa cells of porcine ovaries increased during atresia, while FOXO3 protein was abundant in granulosa cells of early atretic follicles. By immunohistochemistry, the inner surface area of the granulosa layer in early atretic follicles was strongly stained with anti-FOXO3 antibody. The granulosa cells expressing FOXO3 coincided with apoptotic cells, indicating a role of FOXO3 as a proapoptotic factor in granulosa cells of porcine ovaries. In porcine (JC-410) and human (KGN) granulosa-derived cells, cell death was induced by transfection of FOXO3 expression vectors. Expression of the proapoptotic factors Fas ligand (FASLG) and BCL2-like 11 (BCL2L11) was upregulated by FOXO3 in KGN cells. In conclusion, FOXO3 is expressed in porcine ovarian follicles and induces apoptosis in granulosa cells, suggesting that it is a candidate for the initiator of follicular atresia.

Expression of Hyaluronan Synthase 1 and Distribution of Hyaluronan During Follicular Atresia in Pig Ovaries1

CD44 on macrophages is recognized as a phagocytic receptor involved in the phagocytosis of apoptotic cells. Recently, we detected CD44 on macrophages in atretic follicles during atresia. In this study, we evaluated the distribution of the principal CD44 ligand hyaluronan (HA) and the expressions of HA synthases (HAS: HAS1, HAS2, and HAS3) during atresia in pig ovaries. We determined the 2139-bp sequence of Sus scrofa HAS1 and raised an anti-HAS1 polyclonal antibody. The S. scrofa HAS1 sequence contained six putative HA-binding motifs and conserved amino acid residues crucial for GlcNac transferase activity. HAS1 mRNA expression was upregulated during atresia; however, HAS2 and HAS3 mRNA expression levels were low and very low to undetectable, respectively. Western blotting showed that HAS1 was markedly upregulated during atresia. Immunohistochemical analyses revealed HAS1 distribution in theca cells of healthy and early atretic (stages I and II) follicles and in progressing atretic (stage III) follicles. Hyaluronan was visualized with the HA-binding protein; it accumulated in the theca layer during all stages and in stage III follicles. Hyaluronan assay showed a significantly increased HA concentration in follicular fluid at stage III. Flow cytometry showed HAS1 expression in 55.7% of SIRPA-positive macrophages in stage III follicles. Our results suggest that the HA concentration in follicular fluids increased during atresia and that HAS1 may be the dominant HAS protein in theca cells to produce HA in pig ovaries.

Role of Cell Death Ligand and Receptor System on Regulation of Follicular Atresia in Pig Ovaries

Several hundred thousand primordial follicles are present in the mammalian ovary, however, only a limited number develop to the pre-ovulatory stage, and then finally ovulate. The others, more than 99%, will be eliminated through a degenerative process called 'atresia'. The endocrinological regulatory mechanisms involved in follicular development and atresia have been characterized to a large extent, but the precise temporal and molecular mechanisms involved in the regulation of these events have remained unknown. From many recent studies, it is suggested that the apoptosis in ovarian granulosa cells plays a crucial role in follicular atresia. Notably, death ligand-receptor interaction and subsequent intracellular signalling have been demonstrated to be the key mechanisms regulating granulosa cell apoptosis. In this review, we provide an overview of granulosa cell apoptosis regulated by death ligand-receptor signalling. The roles of death ligands and receptors [Fas ligand (FasL)-Fas, tumour necrosis factor (TNF)alpha-TNF receptor (TNFR), and TNFalpha-related apoptosis-inducing ligand (TRAIL)-TRAIL receptor (TRAILR)] and intracellular death-signal mediators [Fas-associated death domain protein (FADD), TNF receptor 1-associated death domain protein (TRADD), caspases, apoptotic protease-activating factor 1 (Apaf1), TNFR-associated factor 2 (TRAF2), and cellular FLICE-like inhibitory protein (cFLIP), etc.] in granulosa cells will be discussed.

The Role of Interleukin-6 in the Regulation of Granulosa Cell Apoptosis During Follicular Atresia in Pig Ovaries

More than 99% of follicles in mammalian ovaries undergo a degenerative process known as atresia, and only a few follicles actually ovulate during follicular growth and development. Follicular selection mostly depends on granulosa cell apoptosis, but the molecular mechanism behind the regulation of this selective atresia is still largely unknown. In the present study, to examine whether or not interleukin-6 (IL-6), a multifunctional cytokine, is involved in apoptosis during atresia in pig ovaries, the expression of IL-6 mRNA in granulosa cells was quantified by real-time reverse transcription-polymerase chain reaction (RT-PCR). The level of mRNA decreased during atresia. Enzyme-linked immunosorbent assay (ELISA) showed that the level of IL-6 protein in follicular fluid also decreased during atresia. Moreover, recombinant human IL-6 upregulated the expression of an intracellular apoptosis inhibitor, cellular FLICE-like inhibitory protein long form (cFLIP(L)), in cultured cells derived from human granulosa cells. It is possible that IL-6 is produced in the granulosa cells of healthy follicles, that it increases the cFLIP(L) level, and cFLIP(L) then prevents apoptotic cell death.

Changes in Expression of Interleukin-6 Receptors in Granulosa Cells During Follicular Atresia in Pig Ovaries

More than 99% of follicles undergo a degenerative process known as "atresia" in mammalian ovaries, and only a few follicles ovulate during follicular growth and development. Follicular selection predominantly depends on granulosa cell apoptosis. To reveal the molecular mechanisms of selective follicular atresia, we examined the changes in the levels of interleukin-6 (IL-6) receptors expressed in the granulosa cells of pig ovaries. The levels of IL-6 receptor (IL-6R)-alpha mRNA and protein in granulosa cells were quantified by reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting, respectively. IL-6R alpha mRNA and protein were highly expressed in the granulosa cells of progressed atretic follicles. Enzyme-linked immunosorbent assay showed that the expression of IL-6 soluble receptor (IL-6sR) protein in follicular fluid decreased during atresia. Moreover, we isolated porcine cDNA encoding an IL-6 signal transducer, gp130. Porcine gp130 (2,754 bp and 917 amino acids) was identified from a cDNA library prepared using follicular granulosa cells of pig ovaries. Porcine gp130 was highly homologous with human and murine gp130. RT-PCR analysis revealed that the level of gp130 mRNA also decreased during atresia. We presume that IL-6sR and gp130, but not IL-6R alpha, play important roles in regulation of granulosa cell survival.

222THE ROLE OF TUMOR NECROSIS FACTOR-RELATED APOPTOSIS INDUCING LIGAND DURING FOLLICULAR ATRESIA IN PIG OVARIES

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) can induce cell death by binding to its receptors (DR4 and DR5). However, binding to DcR1 or DcR2 cannot induce apoptosis. DcRs compete with DRs. TRAIL has been reported to induce apoptosis in various tumor cells but not in normal cells. However, a recent study revealed that TRAIL induces apoptosis in normal hepatocytes of human but not in those of rat, mouse, or rhesus monkey, indicating that there are species-specific differences in TRAIL and receptor systems. In the present study, we demonstrated Immunohistochemical, Western immunoblotting, and reverse transcription-polymerase chain reaction analyses (RT-PCR) of TRAIL and DR4 in granulosa cells during follicular atresia in pig ovaries. For immunohistochemistry, pig ovaries obtained at a local slaughterhouse were fixed with 20% buffered formalin. For Western blotting and RT-PCR analysis, individual preovulatory antral follicles were dissected from the ovaries. Based on morphological and endocrinological criteria, the antral follicles were divided into three categories as follows: healthy, early stage of atresia, progressed stage of atresia. Significant increases were demonstrated in TRAIL protein and mRNA levels during atresia, but not in DR4 protein. Moreover in an in vitro apoptosis-inducing assay using cultured granulosa cells prepared from healthy follicles, we showed that more than 200ng/mL TRAIL could activate caspase-3 and induce apoptotic cell death in a dose-and time-dependent manner, but less than 100ng/mL of TRAIL could not induce apoptosis. When DcR1 was removed from the cell membrane of granulosa cells, a lower dose of TRAIL could induce apoptosis. The present findings suggested that the TRAIL can induce granulosa cell apoptosis, and that DcR1 blocks TRAIL-induced apoptosis in granulosa cells of healthy follicles in porcine ovaries.

Expression and activity of Apaf1 and caspase-9 in granulosa cells during follicular atresia in pig ovaries.

Apoptosis in granulosa cells plays a crucial role in ovarian follicular atresia, but the intracellular regulating mechanism, especially the mitochondrion-dependent apoptosis signalling pathway, is still largely unknown. This study examined whether the mitochondrial pathway is associated with granulosa cell apoptosis during atresia in pig ovaries. Both mRNAs of caspase-9 and apoptotic protease-activating factor 1 (Apaf1), which are major signal transducing components in the mitochondrial pathway, were detected in granulosa cells in healthy, early atretic and progressed atretic follicles by RT-PCR. No changes in the expression of Apaf1 mRNA were seen during follicular atresia, but the expression of caspase-9 mRNA increased during atresia. Apaf1 protein was steadily detected in granulosa cells prepared from healthy, early atretic and progressed atretic follicles by western blot analysis, but high expression of the precursor of caspase-9 (procaspase-9) was detected only in granulosa cells of healthy follicles. Decreased procaspase-9 protein was demonstrated during follicular atresia. Proteolytic activity of caspase-9 increased during atresia, in agreement with the diminution of procaspase-9 protein. Intensive expression of caspase-9 mRNA was demonstrated in the granulosa cells of early atretic and progressed atretic follicles but not in those of healthy follicles. These results indicate that the mitochondrial signalling pathway, which is mediated by Apaf1 and caspase-9, plays a crucial role in determining the fate of granulosa cells during atresia in pig ovaries.

Expression and activity of Apaf1 and caspase-9 in granulosa cells during follicular atresia in pig ovaries

Apoptosis in granulosa cells plays a crucial role in ovarian follicular atresia, but the intracellular regulating mechanism, especially the mitochondrion-dependent apoptosis signalling pathway, is still largely unknown. This study examined whether the mitochondrial pathway is associated with granulosa cell apoptosis during atresia in pig ovaries. Both mRNAs of caspase-9 and apoptotic protease-activating factor 1 (Apaf1), which are major signal transducing components in the mitochondrial pathway, were detected in granulosa cells in healthy, early atretic and progressed atretic follicles by RT-PCR. No changes in the expression of Apaf1 mRNA were seen during follicular atresia, but the expression of caspase-9 mRNA increased during atresia. Apaf1 protein was steadily detected in granulosa cells prepared from healthy, early atretic and progressed atretic follicles by western blot analysis, but high expression of the precursor of caspase-9 (procaspase-9) was detected only in granulosa cells of healthy follicles. Decreased procaspase-9 protein was demonstrated during follicular atresia. Proteolytic activity of caspase-9 increased during atresia, in agreement with the diminution of procaspase-9 protein. Intensive expression of caspase-9 mRNA was demonstrated in the granulosa cells of early atretic and progressed atretic follicles but not in those of healthy follicles. These results indicate that the mitochondrial signalling pathway, which is mediated by Apaf1 and caspase-9, plays a crucial role in determining the fate of granulosa cells during atresia in pig ovaries.

Changes in the expression of tumor necrosis factor (TNF) alpha, TNFalpha receptor (TNFR) 2, and TNFR-associated factor 2 in granulosa cells during atresia in pig ovaries.

Tumor necrosis factor (TNF) alpha can induce both cell death and cell proliferation and exerts its effects by binding to either TNF receptor (TNFR) 1 or 2. When TNFalpha-bound TNFR2 interacts with TNFR-associated factor 2 (TRAF2), expression of survival/antiapoptotic genes is up-regulated. In the present study we determined the changes in localization of TNFalpha and TRAF2 and their mRNAs and the expression of TNFR2 in granulosa cells during follicular atresia in pig ovaries. In healthy follicles, intense signals for TNFalpha and TRAF2 and their mRNAs were demonstrated in the outer zone of the granulosa layer, where many proliferating cells and no apoptotic cells were observed. In atretic follicles, decreased or trace staining for TRAF2 and its mRNA and decreased expression of TNFR2 were observed in the granulosa layer, where many apoptotic cells were seen. These findings suggested that TNFalpha acts as a survival factor in granulosa cells during follicular atresia in pig ovaries.

Roles of Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand Signaling Pathway in Granulosa Cell Apoptosis During Atresia in Pig Ovaries

To reveal the molecular mechanism of selective follicular atresia in porcine ovaries, we investigated the changes in the expression of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and its receptor (DR4) proteins and TRAIL mRNA in granulosa cells during follicular atresia. Immunohistochemical, Western immunoblotting and reverse transcription-polymerase chain reaction analyses (RT-PCR) revealed that significant increases in TRAIL protein and mRNA levels but not DR4 protein were changed during atresia. The RT-PCR product was confirmed to be porcine TRAIL by the cDNA sequence determination. An in vitro apoptosis inducing assay using cultured granulosa cells prepared from healthy follicles showed that TRAIL could activate caspase-3 and induce apoptotic cell death in the cells. The present findings confirm that TRAIL induces apoptosis in granulosa cells during atresia in porcine ovaries.

TRADD is Involved in Apoptosis Induction in Granulosa Cells during Atresia in Pig Ovaries.

Previously, we histochemically demonstrated the expression of tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) and its receptors, death receptor-4 (DR4), death receptor-5 (DR5) and decoy receptor-1 (DcR1) in granulosa cells of porcine follicles. However, TRAIL can induce both cell death and cell proliferation. In the present study, reverse transcription polymerase chain reaction and in situ hybridization analyses revealed increased mRNA expression of TNF receptor-associated death domain protein (TRADD), which transmits the death signal from DR4 and/or DR5 to intracellular apoptosis-signal transduction components, in granulosa cells was demonstrated only in atretic follicles but not in healthy follicles. These findings indicate that TRADD is involved in induction of apoptosis in granulosa cells, and that the TRAIL-receptor system induces apoptosis in granulosa cells during atresia in porcine ovaries.

Ca2+/Mg2+-Dependent Endonuclease but not Ca22+-Dependent, Mg2+-Dependent or Cation-lndependent Endonuclease is Involved in Granulosa Cell Apoptosis of Pig Atretic Follicles.

The antral follicles, 4-6 mm in diameter, were dissected from the ovaries of mature pigs, and then granulosa and cumulus cells were isolated from each follicle. In atretic follicles, high activity of neutral Ca2+/Mg2+-dependent endonuclease by which the chromatin DNA is degraded at internucleosomal sites in the cells undergoing apoptosis was noted only in granulosa cells but not in cumulus cells. Low activity of the Ca2+/Mg2+-dependent endonuclease was observed in both types of cells obtained from healthy follicles. Low activities of neutral Ca2+-dependent endonuclease, neutral Mg2+-dependent endonuclease and acidic cation-independent endonuclease were demonstrated in all cases. A good correlation (r=0.892) between the Ca2+/Mg2+-dependent endonuclease activity of granulosa cells and the progesterone/estradiol-17β ratio of follicular fluid in each follicle, which provides an index of follicular atresia in pig ovaries, was found. These results suggest that the Ca2+/Mg2+-dependent endonuclease, and not the Ca2+-dependent endonuclease, the Mg2+-dependent endonuclease or the cation-independent endonuclease, is involved in granulosa cell apoptosis of the atretic antral follicles in pigs.

Signal Transmission of Granulosa Cell Apoptosis in the Atretic Antral Follicles in the Pig Ovaries

The porcine antral follicles, 4-6 mm in diameter, were dissected from the ovaries of mature pigs, and then granulosa and cumulus cells were isolated from each follicle. In atretic follicles, apoptotic cells were observed histochemically among granulosa cells only. In atretic follicles, high activity of neutral Ca2+/Mg2+-dependent endonuclease by which the chromatin DNA is degraded at internucleosomal sites in the cells undergoing apoptosis was noted only in granulosa cells but not in cumulus cells. Low activity of the Ca2+/Mg2+-dependent endonuclease was observed in both types of cells obtained from healthy follicles. Low activities of neutral Ca2+-dependent endonuclease, neutral Mg2+-dependent endonuclease and acidic cation-independent endonuclease were demonstrated in all cases. A good correlation between the Ca2+/Mg2+-dependent endonuclease activity of granulosa cells and the progesterone/estradiol ratio of follicular fluid in each follicle, which provides an index of follicular atresia in pig ovaries, was found. In vitro study showed that the specific inhibitors for interleukin-β converting enzyme (ICE) like cysteine proteases could stop the apoptotic process induced by specific monoclonal antibody to granulosa cell membrane protein in primary cultured granulosa cells. These results suggest that apoptosis occurs in granulosa cells but not cumulus cells in the atretic antral follicles in pigs, and ICE like protease (s) is involved in intracellular signal transmission of granulosa cell apoptosis of the atretic antral follicles in pigs.