The ovary is the female gonad that is present as a pair in most animals and is responsible for the generation of female gametes and the production of hormones that regulate reproductive functions. The growth and development of mammalian oocytes occurs in a specialized compartment of the ovary called the follicle. Follicle development in mammals mainly includes primordial, primary, secondary, tertiary, and mature follicle stages. Primordial follicles are composed of a single flat layer of GCs surrounded by immature oocytes. Further differentiation of primary follicles into preantral follicles involves acquisition of a theca layer around the GC layer and GC division through mitosis to form multiple layers of GCs (the theca layer remains as a single layer at this stage). The growth and development of oocytes affect the functional activities of surrounding somatic cells [36].The interaction between oocytes and surrounding somatic cells progresses as the oocyte is released from the state of quiescence toward ovulation, fertilization and zygote formation. As follicles grow and cavities form, somatic cells are divided into two distinct subtypes: the CCs, which surround and are in close metabolic contact with the oocyte, and the parietal GCs, which are follicle-forming cells [37].Numerous factors affect oocyte developmental capacity and granulosa cell and follicular microenvironment changes, including heat stress [38].GCs are tightly connected to oocytes in mammalian ovaries through gap junctions and provide oocytes with the essential signals and metabolites required for oocyte growth and maturation [39].CCs support meiotic arrest and cytoplasmic maturation of the oocyte by exporting cyclic adenosine nucleotides [40], calcium [41], other metabolites [42], and unknown signals that control transcription in the occluded oocyte [43].
Proper folliculogenesis and ovulation can only be achieved through reciprocal signaling events generated by intimate granulosa cell-oocyte communication [44]. Before ovulation, differentiated GCs can secrete a large amount of sex hormones and growth factors in follicles to ensure successful ovulation [45].The functions of the two different cells in the whole follicle development process are not consistent: CCs play a key role in the normal growth and development of oocytes, while GCs mainly play an endocrine function to support the growth of the follicle [8].After ovulation, GCs undergo terminal differentiation into luteal cells and continue to play an endocrine role, while CCs and co-ovulate with oocytes to assist in egg retrieval and the sperm acrosome reaction. Although some functional differences between these two types of follicular somatic cells have been well described, it is not clear which genes regulate their developmental pathways. Previous studies have found that the ability of oocytes to promote follicular cell proliferation was first directly demonstrated in mice [46].
GC proliferation, steroid production, and morphological changes are primarily linked to molecular pathways. Sheep GCs overexpress the sonic hedgehog (Shh) signaling pathway, which has the same potential role as the Hedgehog pathway in increasing GCs proliferation at the preluminal stage [47]. An important factor in maintaining follicle and oocyte growth for reproductive success is proper regulation of granulosa cell activity [48]. In the ovary, the β-catenin-dependent canonical pathway of WNT4 leads to an ovarian-dependent pathway. WNT proteins can also signal through Rho-GTPases, utilizing non-canonical pathways associated with changes in polarized cell shape and migration [49].This ameliorate small changes in cell shape that affect GCs during early folliculogenesis. The study uncovered two classes of cellular interactions, demonstrating a complex dialogue between compartments: molecular dialogue (signaling pathways) and physical communication (gap junctions and trans-regional projections) [50].
LncRNAs are more than 200 nucleotides in length and constitute a family of transcripts that are unable to encode protein [51]. This theory suggests that lncRNAs act as natural sponges or decoys to competitively bind certain miRNAs and reduce the binding of miRNAs to corresponding target genes, resulting in altered miRNA target gene expression [52, 53]. However, it remains unclear whether abnormal lncRNAs exert ceRNA effects on some miRNAs and exert certain effects on signal transduction during cell proliferation and differentiation by indirectly regulating the expression of target mRNAs.
In this study, a total of 1,411 lncRNAs and 643 significantly DE lncRNAs were obtained from the six libraries, of which 204 DE lncRNAs were upregulated and 439 DE lncRNAs were downregulated. A total of 20,519 mRNAs and 6,223 significantly differentially expressed mRNAs were identified, among which 2,197 DE mRNAs were upregulated and 4,026 DE mRNA were downregulated. Additionally, a total of 2,248 miRNAs and 559 significantly differentially expressed miRNAs were identified, among which 311 DE miRNAs were upregulated, and 248 DE miRNAs were downregulated significantly between the two groups. Some target genes are involved in cell adhesion, cell differentiation, regulation of developmental processes, cell proliferation, embryo development, signal transduction, programmed cell death, and aromatic compound biosynthetic processes. These RNAs were involved in many processes and pathways, including leukocyte trans-endothelial migration, ECM-receptor interaction, the MAPK signaling pathway, the Hippo signaling pathway, the cell cycle, cell adhesion, the PI3K-Akt signaling pathway, and regulation of the actin cytoskeleton. Few lncRNAs showed clear spatiotemporal expression and specificity in the process of tissue growth and differentiation.
Cyclin D1 is encoded by the CCND1 gene located on chromosome band 11q13 and promotes cell cycle progression during the G1-S phase [54]. Activation of the PI3K/Akt pathway promotes an up-regulation mechanism of CCND1, in which mTor can activate CCND1 translation and promote the synthesis of cyclin D1/CDK4 and CDK6 complexes involved in cell cycle progression [55]. Amplification of CCND1 is prevalent in human cancers. Dai et al propose a role for CCND1 in promoting ovarian cancer cell proliferation, which could be alleviated by treatment [56]. CDKN1A encodes P21 and is a cyclin-dependent kinase (Cdk) inhibitor [57].In cancer cell lines, p21 acts as an activator to synthesize and activate cyclin D/Cdk4 or Cdk6 complexes to enhance proliferation efficiency [58], and hyperphosphorylated p21 activates Cdk1 upon G2/M transition [59]. p21 is both an inhibitor and activator of the cell cycle, depending on the cellular environment and its expression levels. In contrast, p21 is involved in checkpoint control and initiates temporary cell cycle arrest. [60]. Interestingly, we also enriched the network graph for ITGA6. When ITGA6 forms α6β1 and α6β4 integrin complexes with other integrin subunits, embryogenesis, organogenesis, and cancer cell invasion can be mediated[61]. From these differential genes, we will provide more predictive target genes for the differentiation of GCs and CCs.
Little research has been conducted to explore the mechanism of lncRNAs in GCs and CCs. The present study had several strengths. We constructed a ceRNA network in which a network diagram of mRNA-miRNA-lncRNA was drawn.There were 540 relationship pairs, which provided a reference for exploring biological processes such as proliferation, differentiation, and signal transduction between CCs and GCs. The evidence we provide here suggests that differential mRNA-miRNA-lncRNA may play an important role in follicle development and ovulation. However, the specific mechanism requires further exploration in cell and animal experiments.