The physiological ageing process of mammalian ovaries are under strict regulation. Once it is disturbed, ovarian dysfunction might arise, including early menopause and premature ovarian insufficiency (POI). POI refers to the loss of ovarian activity before 40 years of age, and is the main reason for female infertility(Coulam et al. 1986; van der Steeg et al. 2007; Shaw et al. 2010). It is reported that about 1–3% of women experience POI before 40(Coulam et al. 1986; Jacobsen et al. 2003). Although pathogenic variants in more than 100 POI causative genes have been reported, the etiology of many POI patients remains unknown due to high genetic heterogeneity.
In our previous work, the transcriptomic landscape of postnatal mouse ovarian development and ageing was constructed by utilizing a mouse model and bulk RNA sequencing. It is revealed that many POI causative/candidate genes exhibited specific expression patterns during the reproductive ageing process, providing a new strategy for identification of novel regulators. Neither the expression profile nor the biological function of lncRNA in mammalian ovaries has been fully studied, but evidence is accumulating to suggest that lncRNAs participate in ovary function maintenance. Loss of lncRNA HCP5 induced POI through regulating the expression of MSH5 and thus regulation of DNA damage repair pathways (Wang et al. 2020). Additionally, lncRNA GCAT1 is another novel POI causative gene(Li et al. 2021). LncRNA GCAT1 could regulate the proliferation of granulosa cells by regulating p27 translation via competitive binding to PTBP1. All these studies strongly suggest prospective in clinics to uncover the functions of lncRNAs in ovary development and ageing. In this study, we investigated the lncRNA expression patterns during mouse ovarian development and ageing, and 1836 differentially expressed lncRNAs in mouse ovaries were identified for the first time.
Stage-specific expressed lncRNA in ovaries throughout the reproductive lifespan was further identified in our study, and some lncRNAs were shown to be involved in the regulation of ovary development or ageing process. For example, H19 is abundantly expressed in the newborn mouse ovaries (Fig. 3c, 6c). There has been evidence to suggest that H19 is associated with the risk of polycystic ovary syndrome and male infertility(Li et al. 2016; Ghasemi et al. 2020). Furthermore, female H19 knockout mice have been shown to exhibit accelerated follicular recruitment and atresia and a more rapid decline in fertility as compared to wild type mice (Xia et al. 2020). Similarly, Mir17hg, which was shown to be highly expressed at week 3, and with expression level gradually decreasing after that, had a human homologue, MIR17HG. MIR17HG was revealed to be in a positive feedback loop with SIRT1, promoting the repair of DNA double-stranded breaks in oocytes (Xie et al. 2019).
A lncRNA-mRNA coexpression network was also established in our studies to identify candidate lncRNA-mRNA pairs involved in the gene regulation throughout the reproductive lifespan. For example, Neat1 (Fig. 4), which was predicted to regulate expression of more than one gene associated with reproduction in the co-expression network, has been reported to be associated with the formation of paraspeckles, which are specifically formed in large quantities during follicular development (Ernst et al. 2018). Knocking NEAT1 down has been shown to impair the formation of paraspeckles (Chen and Carmichael 2009; Clemson et al. 2009; Sasaki et al. 2009). It will be fascinating in the near future to find out the target genes of Neat1 and how it regulates ovarian function. Another example is Ptgs2os, which was identified as a co-expression pair together with protein-coding gene Ptgs2 (Fig. S1). Interestingly, Ptgs2os was located at the opposite strand of promoter region of Ptgs2, indicating a cis-regulatory role of Ptgs2os on Ptgs2 expression (Fig. 5a). It has been found that Ptgs2 deficient female mice had severe developmental disorders, including a series of reproductive dysfunction such as inferitily (Lim et al. 1997). Moreover, PTGS2, the human homolog of Ptgs2, was demonstrated to be a POI candidate gene, and was highly concerned in the clinical diagnosis of POI (Tucker et al. 2016). From this study, our next steps would be to verify the regulatory relationship between lncRNA and mRNA through experiments at the cellular level, clarify the causal relationship, and explain its regulatory mechanism, which would help in further elucidating the etiology of female reproductive diseases.
We have previously revealed 12 protein-coding gene clusters in mouse ovaries throughout the reproductive lifespan (Zhou et al. 2021). Co-expressed gene clusters could be used to further annotate the lncRNA modules. We found that ME2 was highly correlated with Cluster5 and Cluster6 (Fig. S5), which showed overrepresentation in embryonic organ morphogenesis. ME4 is consistent with Cluster3, experiencing transcriptional activation from NN to PB, and henceforth fixing their expression at a stable level. GO terms enriched for cluster 3 are mainly about steroid metabolic process and hormone metabolic process, which suggest that these lncRNAs may also help ovary to get prepared for the puberty stage. Importantly, ME5 is similar to Cluster8 and Cluster9, which were enriched with POI causative genes. It is also possible that lncRNAs in ME5 may regulate female ovarian function through regulating these protein-coding genes, and thus further experiments are required to verify these findings.
To our knowledge, this is the first report of a comprehensive chronological lncRNA expression analysis of mouse ovaries throughout the female reproductive lifespan. However, we have realized that some lncRNA-mediated ovarian development and ageing found in mice might not be analogous to human ovarian development and ageing because the conservation of lncRNA is not as strong as that of protein-coding genes. Moreover, neither can we simply judge whether lncRNAs are functionally conserved by sequence conservation, since lncRNAs with conserved sequences and positions may not have the same function in different species(Guo et al. 2020). It is better to combine the sequence, structure, function and expression of the lncRNA loci to to get an overall understanding. Our next step is to establish a multi-faceted system to evaluate the functional conservation of lncRNAs between mice and humans. It is hoped that the data in model organisms can contribute to understanding human ovarian development and ageing, and further improve the clinical diagnosis of human ovarian ageing-related diseases.