NCBI Summary:
MicroRNAs (miRNAs) are small noncoding regulatory RNAs that downregulate transcription by targeting specific mRNAs. Let7, one of the founding members of the miRNA family, was first identified in C. elegans. There are several human homologs of C. elegans let7, including LET7C, and all of these LET7 miRNAs share an identical seed sequence critical for target recognition. In human, mouse, and C. elegans, expression of LET7 is barely detectable in embryonic stages, but it increases after differentiation and in mature tissues (Lagos-Quintana et al., 2001 [PubMed 11679670]; Lee and Dutta, 2007 [PubMed 17437991]). For further information on LET7, see LET7A1 (MIM 605386).[supplied by OMIM]
General function
Cell proliferation, RNA processing, RNA binding
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Cellular localization
Cytoplasmic, Nuclear
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Ovarian function
Oocyte maturation
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MicroRNAs transfected into granulosa cells may regulate oocyte meiotic competence during in vitro maturation of mouse follicles. Kim YJ 2013 et al.
STUDY QUESTION
Do microRNAs (miRNAs) in granulosa cells (GCs) affect oocyte maturation during ovarian follicle development?
SUMMARY ANSWER
Sophisticated regulation by miRNAs in ovarian GCs may improve oocyte maturation efficiency during ovarian follicle development.
WHAT IS KNOWN ALREADY
The meiotic competence of oocytes depends on the follicle's potential to undergo appropriate maturation and is an important factor in infertility therapies such as IVF. The exact function of the GCs during follicular development remains unknown.
STUDY DESIGN, SIZE, DURATION
After in vitro maturation (IVM) and ovulation induction of isolated ovarian pre-antral follicles from 12-day-old female C57BL6 mice (n = 40), miRNA expression in the GCs was compared according to the maturity of the oocyte (metaphase I (MI) versus metaphase II (MII)). The miRNAs, which showed notable different expression, were modulated by transfection during IVM of follicles.
MATERIALS, SETTING, METHODS
miRNA expression and candidate target gene expression in GCs of isolated murine ovarian pre-antral follicles were evaluated by real-time PCR after IVM. miR mimics and -inhibitors for selected miRNAs were transfected into the in vitro-maturated follicles, and ovulation, oocyte maturation and fertilization rates were compared. Candidate target gene expressions in GC were evaluated by quantitative PCR and immunohistochemistry using confocal microscopy.
MAIN RESULTS AND THE ROLE OF CHANCE
The relative expression of mmu-let-7b (0.78 0.10, P = 0.016), mmu-let-7c (0.78 0.12, P = 0.029), mmu-miR-27a (0.57 0.18, P = 0.016) and mmu-miR-322 (0.59 0.14, P = 0.008) was significantly lower in the GCs of follicles containing MII oocytes compared with those of MI oocytes. Transfection with a mmu-miR-27a-mimic sequence decreased the oocyte maturation rate compared with that for the control (9.4 versus 18.9%, P = 0.042), and transfection with mmu-let-7c-, mmu-miR-27a- and mmu-miR-322-inhibitor sequences increased the oocyte maturation rate by 1.5- to 2.0-folds compared with that for the control (40.6, 31.6, and 30.5%versus 18.9%, P < 0.001, P = 0.013, P = 0.021, respectively). The expression of IGFBP-2 was higher in GCs of MII than in the GCs of MI, and higher in miR-inhibitor transfection groups than in miR-mimic transfection groups and controls.
LIMITATIONS, REASONS FOR CAUTION
An in vitro model was used in lieu of an in vivo model because of the ease of performing miRNA transfection in cell culture. However, studies have shown similarities and differences in in vivo versus in vitro cultured follicles. The findings of the present study need to be confirmed using in vivo maturation models and extended to evaluate developmental competence.
WIDER IMPLICATIONS OF THE FINDINGS
Our findings suggest that sophisticated miRNA regulation in GCs may improve oocyte maturation efficiency during ovarian follicle development.
STUDY FUNDING/COMPETING INTEREST(S)
This work was supported by a grant from the Korean Health Technology R&D Project, Ministry of Health & Welfare, Republic of Korea (A111539). None of the authors has any conflicts of interest to declare.
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Expression regulated by
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Ovarian localization
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Identification and characterization of miRNAs expressed in the bovine ovary. Hossain MM et al. ABSTRACT: BACKGROUND: MicroRNAs are the major class of gene-regulating molecules playing diverse roles through sequence complementarity to target mRNAs at post-transcriptional level. Tightly regulated expression and interaction of a multitude of genes for ovarian folliculogenesis could be regulated by these miRNAs. Identification of them is the first step towards understanding miRNA-guided gene regulation in different biological functions. Despite increasing efforts in miRNAs identification across various species and diverse tissue types, little is known about bovine ovarian miRNAs. Here, we report the identification and characterization of miRNAs expressed in the bovine ovary through cloning, expression analysis and target prediction. RESULTS: The miRNA library (5'-independent ligation cloning method), which was constructed from bovine ovary in this study, revealed cloning of 50 known and 24 novel miRNAs. Among all identified miRNAs, 38 were found to be new for bovine and were derived from 43 distinct loci showing characteristic secondary structure. While 22 miRNAs precursor loci were found to be well conserved in more than one species, 16 were found to be bovine specific. Most of the miRNAs were cloned multiple times, in which let-7a, let-7b, let-7c, miR-21, miR-23b, miR-24, miR-27a, miR-126 and miR-143 were cloned 10, 28, 13, 4, 11, 7, 6, 4 and 11 times, respectively. Expression analysis of all new and some annotated miRNAs in different intra-ovarian structures and in other multiple tissues showed that some were present ubiquitously while others were differentially expressed among different tissue types. Bta-miR-29a was localized in the follicular cells at different developmental stages in the cyclic ovary. Bio-informatics prediction, screening and Gene Ontology analysis of miRNAs targets identified several biological processes and pathways underlying the ovarian function. CONCLUSIONS: Results of this study suggest the presence of miRNAs in the bovine ovary, thereby elucidate their potential role in regulating diverse molecular and physiological pathways underlying the ovarian functionality. This information will give insights into bovine ovarian miRNAs, which can be further characterized for their role in follicular development and female fertility as well.