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Logo of biolreprodBiology of ReproductionSSRSubmissionsEditorial Board
Biol Reprod. Dec 2008; 79(6): 1030–1037.
Prepublished online Aug 20, 2008. doi:  10.1095/biolreprod.108.069690
PMCID: PMC2780477

Hormonal Regulation of MicroRNA Expression in Periovulatory Mouse Mural Granulosa Cells1

Abstract

MicroRNAs (miRNAs) mediate posttranscriptional gene regulation by binding to the 3′ untranslated region of messenger RNAs to either inhibit or enhance translation. The extent and hormonal regulation of miRNA expression by ovarian granulosa cells and their role in ovulation and luteinization is unknown. In the present study, miRNA array analysis was used to identify 212 mature miRNAs as expressed and 13 as differentially expressed in periovulatory granulosa cells collected before and after an ovulatory dose of hCG. Two miRNAs, Mirn132 and Mirn212 (also known as miR-132 and miR-212), were found to be highly upregulated following LH/hCG induction and were further analyzed. In vivo and in vitro temporal expression analysis by quantitative RT-PCR confirmed that LH/hCG and cAMP, respectively, increased transcription of the precursor transcript as well as the mature miRNAs. Locked nucleic acid oligonucleotides complementary to Mirn132 and Mirn212 were shown to block cAMP-mediated mature miRNA expression and function. Computational analyses indicated that 77 putative mRNA targets of Mirn132 and Mirn212 were expressed in ovarian granulosa cells. Furthermore, upon knockdown of Mirn132 and Mirn212, a known target of Mirn132, C-terminal binding protein 1, showed decreased protein levels but no change in mRNA levels. The following studies are the first to describe the extent of miRNA expression within ovarian granulosa cells and the first to demonstrate that LH/hCG regulates the expression of select miRNAs, which affect posttranscriptional gene regulation within these cells.

Keywords: corpus luteum, granulosa cells, luteinizing hormone, microRNA, ovulation

INTRODUCTION

Ovulation of a mature and viable oocyte and the formation of a functional corpus luteum (CL) are essential for establishment of pregnancy. These events are preceded by a highly orchestrated series of growth and developmental events in the periovulatory follicle that ultimately signals the preovulatory surge of LH, resulting in the initiation of ovulation and CL formation [13]. Numerous studies have shown that the LH surge initiates the transcriptional upregulation and downregulation of genes, including cytokines, transcription factors, and matrix-remodeling proteins, within periovulatory granulosa cells [46]. Furthermore, the signaling mechanisms whereby the LH receptor transduces this endocrine signal within ovarian granulosa cells is well described [79]. However, posttranscriptional gene regulation with respect to ovarian function [10] outside of oocyte development [11] has been largely overlooked. The recent identification of microRNAs (miRNAs) within somatic tissues, particularly rapidly differentiating cells, such as embryonic tissues and cancer cells, has implicated posttranscriptional gene regulation as a central player in development [1215]. In the adult animal, the cyclic phenotypic changes that take place in the ovary and uterus are some of the more dramatic cellular and tissue changes to occur outside of embryonic development. The structural and functional transformation that takes place as the highly prolific periovulatory follicular granulosa cells terminally differentiate into luteal cells occurs exceptionally fast and requires a complete change in the cellular phenotype. The importance of transcription in this conversion is highlighted by the marked number of genes that are transcriptionally regulated during this period [1618] and by the essential role specific LH-induced genes (e.g., progesterone receptor, CCAAT/enhancer binding protein β, prostaglandin synthetase-2, and cyclin D2 [1922]) have on the processes of ovulation and luteinization. No singular method has allowed for the global evaluation of posttranscriptional gene regulation as it pertains to ovarian granulosa cell function; nonetheless, it is relatively easy to envision that during the phenotypic transformation of the granulosa cells, follicular transcript turnover and/or the blockade of follicular transcript translation would facilitate the processes of ovulation and luteinization. MicroRNAs provide such a posttranscriptional mechanism, whereby large classes of transcripts may be simultaneously and effectively regulated by individual factors [23].

MicroRNAs are highly conserved, ~21-nucleotide-long RNA molecules that posttranscriptionally regulate the expression of specific mRNAs or groups of mRNAs based on interactions with the 3′ untranslated regions (UTRs) of specific mRNAs [12]. MicroRNAs are initially transcribed as primary miRNAs (pri-miRNAs), can be several kilobases in length, and contain a characteristic hairpin loop structure [24, 25]. These pri-miRNA transcripts are cleaved by the RNase III endonuclease Drosha, which works in combination with its cofactor DGCR8 to produce a ~70-nucleotide stem-loop intermediate precursor miRNA (pre-miRNA) [26]. Following Drosha cleavage, the pre-miRNA stem-loop structure is exported from the nucleus to the cytoplasm by Exportin-5 through a Ran-GTP mechanism [27], where it is then cleaved by Dicer to produce the mature form of the miRNA [24]. The mature miRNA harbors a sequence of seven nucleotides known as the seed sequence (bases 2–8 of the miRNA), a region that binds to the 3′ UTR of target mRNAs. This mature miRNA, in association with the RNA-induced silencing complex, effectively blocks the process of translation and/or causes the degradation of the targeted mRNAs [28, 29]. Moreover, recent observations of Vasudevan et al. [30] illustrate that miRNAs can differentially regulate translation, inhibiting under conditions of active cell proliferation and promoting under conditions of cell cycle arrest. Despite having been shown to be involved with proliferation and cell differentiation in a variety of systems [12, 31, 32], the role of miRNAs within periovulatory granulosa cells is presently unknown. In the following set of experiments, we have identified several hormonally regulated miRNAs within periovulatory granulosa cells. Additionally, we have examined the expression of the two most significantly upregulated miRNAs and have begun to identify downstream targets that may establish their potential role during the processes of ovulation and luteinization.

MATERIALS AND METHODS

Animals and Granulosa Cell Isolation

Nineteen-day-old CF1 female mice were injected i.p. with 5 IU eCG to induce follicular stimulation. After 46 h, the ovaries were either collected (0-h time point), or the mice were injected i.p. with 5 IU hCG and ovaries collected at 1, 2, 4, 6, 8, and 12 h after hCG. Follicles were punctured with a 30-gauge needle, and granulosa cells were collected into ice-cold PBS [33]. Mural granulosa cells were separated from cumulus-oocyte complexes using a 50-μm Nitex-size exclusion filter (Wildlife Supply Company, Buffalo, NY). The mural granulosa cells then were pelleted by centrifugation (770 × g) and resuspended in 500 μl TRI Reagent for subsequent RNA extraction per the manufacturer's protocol (Sigma, St. Louis, MO). To obtain proliferating granulosa cells for in vitro culture, ovaries from 25- to 27-day-old unprimed female CF1 mice were collected into room temperature M-199 medium (Sigma) supplemented with 10 mM Hepes (Gibco, Carlsbad, CA) and 0.2% bovine serum albumin (Sigma). Ovaries then were transferred to M-199 media containing 1.8 mM EGTA and 0.5 M sucrose (Fluka, St. Louis, MO) and incubated for 15 min at 37°C. Following incubation, ovaries were rinsed three times in M-199 media to remove sucrose, and then they were punctured as described above. Expressed mural granulosa cells were centrifuged for 5 min and resuspended in Dulbecco modified Eagle medium/Ham F-12 (DMEM/F-12; Cellgro, Manassas, VA) supplemented with 10% heat-inactivated fetal bovine serum (Atlanta Biologicals, Lawrenceville, GA) and 1% gentamicin (Gibco). All procedures involving animals were reviewed and approved by the University Committee on the Use and Care of Animals at the University of Kansas Medical Center and were performed in accordance with the Guiding Principles for the Care and Use of Laboratory Animals. All experiments were performed using CF1 female mice from Charles River Laboratories (Wilmington, MA).

Microarray and Temporal Expression Analysis

Total RNA was quantified using a NanoDrop-1000 spectrophotometer (NanoDrop Technologies, Wilmington, DE), and quality was assessed by analysis of the 18S and 28S peaks on an Agilent 2001 Bioanalyzer (Agilent Technologies, Santa Clara, CA). Three independent replicates of mural granulosa cells were collected before hCG (0 h) and 4 h after hCG for detection of mature miRNA expression by microarray analysis. The 0-h granulosa cell samples were labeled with Cy3, and the 4-h hCG samples were labeled with Cy5. Each 0-h and 4-h replicate was then hybridized as a pair onto an individual Atactic μParaFlo microfluidics chip (Sanger versions 8.2 and 9.0 [34, 35]; LC Sciences, Houston, TX) as described in detail by the manufacturer. These chips (n = 3) included 357 experimentally validated miRNAs on the version 8.2 Sanger chip and 375 on the version 9.0 chip. An additional 100 miRNAs predicted in human miRNA but not detected/represented in the murine miRNA 8.2/9.0 Sanger databases were also printed onto the microarray chips to evaluate their expression in periovulatory granulosa cells (list provided upon request). These data were normalized using a cyclic LOWESS (Locally weighted Regression) method as described by LC Sciences Inc. to remove system-related variations, such as sample amount variations, dye labeling bias, and signal gain differences between scanners. Those signals that were greater than background plus three times the standard deviation were then derived for each color channel; the mean and the covariance (CV = SD × 100/replicate mean) were calculated for each probe having a detected signal. A log-transformed ratio then was calculated from the two sets of detected signals, and the transcript ratios with P values of less than 0.01 were considered to be differentially expressed. For clustering analysis of multiple datasets, data adjustment included data filtering, Log2 transformation, and gene centering and normalization. Data filtering was also done to remove clustering values from the data set (detected signals or detected ratios below a threshold value). An in-depth cluster analysis was then completed using the paired samples, and differentially expressed transcripts were compiled (P < 0.05). Lists of miRNAs showing their relative fluorescence values were generated after exclusion of all transcripts that failed to be present in two of three 0-h and two of three 4-h samples and those having fluorescence values of less than 50, which is approximately twice the background level. Additionally, the normalized and background subtracted data were analyzed using the National Institute of Aging Array Analysis (http://lgsun.grc.nia.nih.gov/anova/) with significant genes identified as having P values < 0.05 [36].

Quantitative RT-PCR for Mature miRNAs

Quantitative RT-PCR (qRT-PCR) was performed to validate the microarray results and to analyze the temporal expression of specific mature miRNAs. Human miRNA assay kits for specific miRNAs (mmu-miR-132, mmu-miR-212, mmu-miR-21, and mmu-miR-31, hereafter referred to Mirn132 and Mirn212, etc.) were purchased from Applied Biosystems (Foster City, CA), as were the reverse transcription kits. Total RNA was diluted to a 50 ng/μl working dilution, then 250 ng was used in the RT reactions following the manufacturer's protocol with the following modifications. Briefly, three to five miRNAs were reverse transcribed in a single RT reaction using 2 μl of each miRNA-specific 5× RT primer. This single RT reaction for multiple miRNAs then served as the starting material for independent qRT-PCR reactions using the primers and probes specific to each miRNA. In addition to the added benefit that only one fifth of the RT reagents and RNA was needed, a single reaction allowed the normalizer and target amplicons to be determined from the same RT reaction. Comparison of the qRT-PCR results for miRNAs that were reverse transcribed as a group versus those that were transcribed individually showed no differences, indicating that the RT primers and the derived cDNAs do not interfere with the quantification of the other miRNAs. To normalize for starting material, a 20-μl aliquot from each of the 50 ng/μl total RNA sample dilutions was reverse transcribed using random hexamers, and GAPDH levels were determined using the Rodent GAPDH Taqman primers and probe (Applied Biosystems). Additionally, a reverse small nuclear RNA U6, Rnu6, RT-PCR primer (see Supplementary Table 1, available online at www.biolreprod.org, for sequence) was included in some of the miRNA RT reactions, and qRT-PCR of U6 (Supplementary Table 1) following either miRNA RT or random hexamer RT demonstrated that GAPDH and U6 were equally valid for normalization (results not shown). Comparison of the qRT-PCR results using GAPDH, U6 small nuclear RNA, or miR31 (miR31 was shown to not change in granulosa cells following in vivo LH treatment) as the normalizer made no difference in the relative profiles of the target miRNAs. As most of the initial in vivo studies used GAPDH, we chose to depict all of the data normalized to GAPDH levels for consistency throughout the paper. The qRT-PCR reactions were completed on the 7900 HT Sequence Detection System (Applied Biosystems). Each sample was run in triplicate, and the average was used in subsequent calculations. Each primer set included a minus RT control. The delta-delta Ct method was used to calculate relative fold-change values between samples [37], and at least three independent replicate experiments were used to calculate means and SEM values.

Quantitative RT-PCR for pri-miRNAs and mRNAs

To confirm that a single pri-miRNA exists for Mirn132 and Mirn212 and that it is transcriptionally upregulated in response to LH, a qRT-PCR primer set was designed to amplify a region that spans from the 5′ end of the Mirn212_pre through the 3′ end of the Mirn132_pre. These primers to the primary Mirn132/212 (Mirn132/212_pri; also known as pri-miR-132/212) transcript (see Supplementary Table 1 for primer sequences) amplify a 349-bp amplicon that was confirmed by sequencing (data not shown).

Intron spanning primers for Rnasen (also called Drosha), Dgcr8, Xpo5 (Exportin-5), Dicer1, Methyl CpG-binding protein (Mecp2), C-terminal-binding protein 1 (Ctbp1), and p250 GTPase-activating protein (GAP; also known as Grit within the murine genome) were designed using the Primer Express 3.0 software (Applied Biosystems), and qRT-PCR was conducted as previously described (primer sequences provided in Supplementary Table 1).

Granulosa Cell Culture

Ovarian granulosa cells expressed from 25- to 27-day-old mouse ovaries as previously described were seeded at 2.5 × 104 cells per well into six-well tissue culture plates (Corning) for temporal miRNA expression analysis and for anti-miRNA/locked nucleic acid (LNA) analysis or at 1x106 cells per 10-cm tissue culture dishes (Corning) for Western blot analysis. All tissue culture plastics were precoated with fibronectin (2.5 μl/ml) for 30 min at 37°C prior to plating cells. Cells were cultured at 37°C in an incubator with 5% CO2, and media were replaced 24 h after plating to remove any unattached cells. At 48 h after plating, the cells were placed in serum-free DMEM/F-12 media for 24 h, at which point they were treated with 1 mM 8-bromo-cAMP (Sigma) for 1, 2, 4, 6, 8, or 12 h or remained untreated (serum-free media) for the same time periods. Total RNA was isolated, and expression of Mirn132 and Mirn212 was analyzed using qRT-PCR.

Transfection

Transfection of the mural granulosa cells with anti-miRNAs and LNA oligoribonucleotides to Mirn132 and Mirn212 was completed with Lipofectamine 2000 (Gibco) as described in the manufacturer's instructions. Briefly, isolated mural granulosa cells were grown for 48 h prior to transfection, rinsed twice with serum-free DMEM/F-12, and transfected with a nonspecific anti-miRNA negative control (42 nM) or with hsa-miR-132 anti-miRNA (21 nM) and hsa-miR-212 anti-miRNA (21 nM; Ambion) individually or in combination. The anti-miRNA to Mirn212 failed to consistently block the Mirn212 action and/or detection of the mature miRNA transcript. Therefore, we designed a set of LNA knockdown probes complementary to Mirn132 (tggctgtagactgtta, underlined bases denote the LNA nucleotides) and Mirn212 (tgactggagactgtta). Cells were transfected with nonspecific LNA scramble (40 nM; IDT) or with a combination of Mirn132-LNA (20 nM) and Mirn212-LNA (20 nM). Transfection efficiency within this cell type was analyzed using a fluorescein-labeled BLOCK-iT Fluorescent Oligo (Invitrogen) transfected at 100 nM into cultured granulosa cells using the same protocol as stated above, and the transfection efficiency was determined to be 70% using fluorescent microscopy. Following a 24-h transfection, the cells were rinsed twice with serum-free DMEM/F-12 and then treated with 1 mM 8-bromo-cAMP for 24 h. Total RNA was isolated, and miRNA expression was analyzed by qRT-PCR as previously described.

Western Blots

Protein for Western blot analysis was collected from cAMP-treated granulosa cells transfected with either the nonspecific scramble LNA or with the combination of Mirn132-LNA and Mirn212-LNA then treated with 8-Br-cAMP for 24 h. Following treatment, granulosa cells were collected in cell lysis buffer (200 μl/10-cm dish; Cell Signaling Technology, Danvers, MA), and protein levels were quantified using the Becton Dickinson Protein Assay (Hercules, CA). Standard Western blot protocols were used with anti-CtBP1 (1:1000; BD Biosciences) and anti-nucleoporin (1:500; BD Biosciences) antibodies diluted in TBST + 5% BSA. Secondary horseradish peroxidase-conjugated antibodies were diluted in TBST + 5% milk at 1:5000 for CTBP1 and nucleoporin detection. CTBP1/nucleoporin blots were exposed using pico ECL detection (Thermo Scientific).

Steroid Assays

Concentrations of estradiol and progesterone in granulosa cell culture media were analyzed by radioimmunoassay as previously described [38]. Estradiol levels were quantified in 1 ml media following ether extraction, whereas progesterone levels were measured following a 1:10 dilution of the medium in 0.1 M PBS + 0.1% gelatin.

MicroRNA Bioinformatics

To determine whether mRNAs expressed within periovulatory granulosa cells might be regulated by LH-regulated miRNAs, a comprehensive list of Mirn132 and Mirn212 target transcripts was created using the PicTar algorithm [39]. We then compared this list of transcripts to two mRNA gene lists that included all genes that were marginal or present within murine granulosa cells at 0 and 1 h after hCG or at 8 h after hCG based on Affymetrix gene chip analysis. The resulting list of mRNA transcripts found to be predicted targets of Mirn132 and Mirn212 and to be expressed within granulosa cells was then compared to two additional miRNA target prediction algorithms (miRBase and miRanda) [40, 41]. A detailed description of the computational procedures used to compare these lists is available upon request.

Statistical Analysis

Three independent microarray replicates were analyzed using an unpaired Student t-test (LC Science). Statistical analysis of qRT-PCR data was performed with GraphPad Prism (version 4). All data were examined for heterogeneity of variance using the Bartlett test. If heterogeneity of variance existed, the data were log-transformed and analyzed via one-way ANOVA. Upon determination of a significant (P < 0.05) F test, Newman-Keuls multiple comparison tests were used to determine differences among the means. P values of less than 0.05 were considered significant.

RESULTS

Identification of miRNAs Expressed Within Mural Granulosa Cells

Microarray analysis of miRNA expression in mural granulosa cells revealed that 196 and 206 detectable transcripts were present at 0 and 4 h, respectively, in two of three samples (n = 3). Each miRNA transcript was arbitrarily categorized into a group based on its fluorescence, a value indicative of its relative level within the cells. Supplementary Tables 2, 3, and 4 (available online at www.biolreprod.org) list the highly abundant (n = 31, fluorescence ranging from 10 000 to 50 000), intermediately abundant (n = 64, fluorescence ranging from 1000 to less than 10 000), and low-abundant (n = 117, fluorescence ranging from 50 to less than 1000) miRNAs. A pairwise t-test cluster analysis of the data from the microarrays found 13 miRNA transcripts to be differentially expressed between 0 and 4 h after hCG (P < 0.05; Fig. 1A). Analysis using the National Institutes of Health Array Analysis tool also found Mirn132 and Mirn212 to be significantly upregulated between the 0-h and 4-h time points, with fold changes of 16.9 and 21.7, respectively. Quantitative RT-PCR was used to validate the microarray results for these two specific LH-induced miRNAs, as well as one other highly upregulated miRNA transcript (Mirn21, data not shown). The fold increases seen for the mature forms of both Mirn132 (11.4 ± 4.2) and Mirn212 (20.5 ± 4.0) at 4 h after hCG (Fig. 1, B and C) were similar to the expression changes predicted by miRNA microarray analysis.

FIG. 1.
Microarray cluster analysis for mural granulosa cells collected 46 h after in vivo eCG (0-h control) or after eCG + 4 h of hCG treatment of mice. Paired t-test analyses of the microarray data (n = 3) revealed: (A) 13 miRNA transcripts were statistically ...

Temporal Expression of miRNAs in Granulosa Cells In Vivo

In vivo expression patterns for mature Mirn132 and Mirn212 were determined by qRT-PCR within mural granulosa cells. Examination of miRNA levels over the 12-h periovulatory period demonstrated that Mirn132 and Mirn212 expressions remained unchanged 1 h following hCG administration, then were significantly upregulated between 2 and 12 h after hCG to 17- and 34-fold over the basal levels (0-h control) for Mirn132 and Mirn212, respectively (Fig. 2, A and B).

FIG. 2.
Quantitative RT-PCR analysis of mature Mirn132 (A), mature Mirn212 (B), and the Mirn132/212_pri (C) transcript levels in mural granulosa cells collected from mice following in vivo treatment of eCG for 46 h (i.e., 0 h) and eCG + hCG for 1–12 h ...

Quantitative RT-PCR analysis of the pri-miRNA transcript in periovulatory granulosa cells collected after in vivo hCG treatment indicated that both the Mirn132 and Mirn212 originate from a single transcript and that increased expression occurred approximately 1 h prior to the detection of the mature forms (Fig. 2C). Changes in pri-miRNA expression preceded those changes that were observed in the mature forms (when comparing Fig. 2, C to A and B).

Quantitative RT-PCR analysis for the factors involved in the synthesis of miRNAs indicated the presence of the essential miRNA-processing components (i.e., Rnasen, Xpo5, Dgcr8, and Dicer1) within the purified population of granulosa cells and showed that an ovulatory surge of LH/hCG had no effect on the expression of these factors in vivo (see Supplementary Fig. 1 available online at www.biolreprod.org).

Temporal Expression of miRNAs in Granulosa Cells in Vitro

To determine whether LH/hCG-induced miRNAs were downstream of the cAMP signal transduction pathway, isolated granulosa cells from preovulatory follicles (27-day-old mice) were placed in culture. Plates were typically ~20% confluent 1 day following plating. Cells continued proliferating in culture so that by the second day the cells were near 50% confluent. On the third day of culture, cells were exposed to 8-Br-cAMP treatment for 0–12 h, mimicking the predominant second messenger system downstream of LH/hCG used in our in vivo experiments. The in vitro granulosa cell expression patterns for Mirn132 and Mirn212 were similar to those observed in vivo. However, the increases seen in vitro were more rapid as Mirn132 and Mirn212 were increased within 1 h of 8-Br-cAMP treatment. Mature Mirn132 and Mirn212 levels remained elevated for the entire 12-h cAMP treatment, reaching their peak levels (10- and 16-fold increases, respectively) at 8 h after treatment (Fig. 3).

FIG. 3.
Quantitative RT-PCR analysis of mature Mirn132 (A) and Mirn212 (B) expression in cultured granulosa cells following treatment with 8-Br-cAMP. Mature Mirn132 and Mirn212 levels were normalized to GAPDH. a,b,cMeans ± SEM with different superscripts ...

Analysis of miRNA Expression Following In Vitro miRNA Knockdown with LNA Oligonucleotides

To gain insight as to what Mirn132 and Mirn212 are regulating in granulosa cells and to begin to dissect the functional role these miRNAs have on granulosa cell function, we chose to knock down expression of these factors using LNA oligonucleotides complementary to the miRNAs. Transfection combining LNA oligonucleotides for both Mirn132 and Mirn212 were effective in knocking down their respective target miRNAs (Fig. 4). The cAMP-stimulated levels of mature Mirn132 (Fig. 4A) and Mirn212 (Fig. 4B) were completely blocked by the LNA treatment. Conversely, treatment with the LNA oligos specific to Mirn132 and Mirn212 had no effect on the expression of Mirn21 (Fig. 4C), an miRNA that is unresponsive to cAMP stimulation in vitro but is upregulated within granulosa cells following the LH surge (data not shown).

FIG. 4.
Quantitative RT-PCR analysis of mature Mirn132, Mirn212, and Mirn21 expression in cultured granulosa cells following transfection with LNAs to Mirn132 and Mirn212 and subsequent treatment with 8-Br-cAMP (n = 3). Granulosa cells were transfected with ...

Following knockdown with a combination of Mirn132-LNA and Mirn212-LNA oligonucleotides, we evaluated whether there was any effect on steroidogenesis. Medium concentrations of estrogen and progesterone were measured at the completion of the each experiment, and knockdown of the miRNAs failed to affect medium estradiol or progesterone at 12 and 24 h after cAMP treatment. Treatment of granulosa cells with cAMP increased progesterone synthesis ~6-fold, as expected (steroid data not shown).

Identification of Mirn132 and Mirn212 Targets

To gain insight into the possible target mRNAs that these miRNAs may regulate, a comprehensive list of Mirn132 and Mirn212 target genes was created through the use of the PicTar algorithm [39]. This analysis yielded 156 or 157 mRNA transcripts as predicted targets of murine Mirn132 and Mirn212, respectively. A comparison of these transcripts to genes expressed in granulosa cells at either 0 and 1 h after hCG or 8 h after hCG revealed 77 common transcripts (see Supplementary Table 5 available online at www.biolreprod.org). These transcripts were then evaluated using several other miRNA target prediction algorithms, and those results are also shown in Supplementary Table 5. Ingenuity pathway analysis indicated that a large number of these transcripts were in the cellular development and gene expression ontology groups.

Additionally, three recently identified Mirn132 target transcripts were examined to determine whether they were differentially expressed in murine granulosa cells. Quantitative RT-PCR indicated that LH/hCG increased expression of Grit (the mouse homolog to p250 GAP) at 6 and 12 h after hCG (Fig. 5A). Conversely, Mecp2 and Ctbp1 were not transcriptionally regulated (Fig. 5, B and C). Western blot analysis for CTBP1 protein in granulosa cells following in vitro LNA treatment indicated that loss of miRNA function caused a marked decline in the translation of this target protein (Fig. 5D).

FIG. 5.
Quantitative RT-PCR and Western blot analysis of several Mirn132/212 target transcripts in granulosa cells collected from mice throughout the periovulatory period. A, B, C) Grit, Mecp2, and Ctbp1 mRNA levels were normalized to GAPDH. a,bMeans ± ...

DISCUSSION

This study is the first to describe the hormonal regulation of miRNA expression in the ovary, specifically the ability of the LH/hCG surge to mediate changes in the expression of miRNAs within periovulatory granulosa cells. The ovarian granulosa cell is likely a prime target for miRNA-mediated changes in gene expression due to the rapid conversion this cell must undergo following the LH surge. This phenotypic change requires the global loss of its follicular phenotype and its attendant gene expression/protein profile as well as the rapid induction of a luteal phenotype. We chose to examine the expression and function of the two miRNAs (Mirn132 and Mirn212) that exhibited the greatest difference in expression following the hCG/LH surge. The recent functional description of Mirn132 in neuronal cells, combined with the fact that both Mirn132 and Mirn212 are regulated by the cAMP regulatory element-binding protein (CREB), also made these miRNAs attractive to study [42].

From the microarray data, we identified 212 independent, mature miRNA transcripts in periovulatory mural granulosa cells that were above background levels in at least two of three samples for the 0-h and 4-h time points. These results are in striking contrast to the list of 122 miRNAs cloned from whole ovarian tissues recently by Ro et al. [43]. Of interest, approximately 20% of these transcripts are variants at the 5′ and 3′ ends, and as noted by the authors, a consequence that may be due to the cloning procedure. Alternatively, editing of the miRNA ends, which was recently described, may also be involved [44]. A more conservative interpretation indicates that Ro et al. [43] identified 97 miRNAs combined from 2-wk-old and adult mouse ovaries. Comparison of our list to their list of 97 bona fide miRNAs cloned from whole ovaries indicates that ~80% of the transcripts overlap. Moreover, the relative abundance of each miRNA transcript (i.e., fluorescent value) observed in our experiments was predictive of the cloning method's ability to detect the transcript in the whole ovary. For example, 24 of 31 miRNA transcripts in our high-fluorescence group, 28 of 64 miRNA transcripts in our intermediate-fluorescence group, and 25 of 117 miRNA transcripts in the low-fluorescence group were detected by Ro et al. [43]. Those transcripts found in both lists are bolded in Supplementary Tables 2–4. Interestingly, several miRNAs, including Mirn132 and Mirn212, the two differentially expressed miRNAs with intermediate to low fluorescence levels we detected, were not detected in the analysis by Ro et al. [43]. Because our preparations of granulosa cells are not contaminated with oocytes as they were eliminated by size exclusion filtering, we can be confident that our results reflect the changes seen in the somatic cell compartments and are not the result of an effect of the germ cells/oocytes. Therefore, we believe that the miRNA microarray approach we used has provided us with a robust analysis of miRNA expression within the periovulatory granulosa cell. We recognize that novel granulosa cell miRNAs may have been missed in our approach; however, presently this approach yields a much higher number of miRNAs compared with a cloning method.

The miRNAs that fail to experience a differential expression pattern within granulosa cells after the LH surge have the potential to be very interesting, as they may also play a role in ovarian function. Indeed, it has been demonstrated recently that miRNAs can both block and/or enhance translation dependent upon the cell cycle state [30]. MicroRNAs regulate translation by recruiting the RNA-induced silencing complex to the 3′ UTR of mRNAs, where they can prohibit further translation [29]. Recently, Vasudevan et al. [30] demonstrated that miRNAs could enhance translation under conditions that promote cellular differentiation by recruiting Argonaute 2 to this complex. Under this paradigm, highly expressed miRNAs in particular, although not exhibiting hormonally dependent regulation in expression, may still exhibit a dynamic regulatory role in response to a hormone-dependent shift of a key regulatory factor (i.e., an argonaute protein). This type of cellular event occurs in periovulatory granulosa cells as they transition from highly proliferative follicular cells to terminally differentiated luteal cells and could indicate that these highly abundant ovarian granulosa cell miRNAs, which are present prior to the LH surge, may be preventing the translation of genes critical for luteinization and ovulation. It is possible that these miRNAs only release their block on mRNA translation following the LH surge; alternatively these miRNAs may be necessary for the efficient translation of specific mRNAs in the ovary after the LH surge. Subsequent studies will be necessary to determine the importance specific miRNAs have on ovarian granulosa cell function.

The qRT-PCR results for Mirn132 and Mirn212 indicated that in vivo these transcripts are rapidly upregulated by the LH surge and that their mature forms remain elevated even after transcriptional decline of the precursor form. This suggests a fairly long half-life for the mature forms. Our in vitro experiments demonstrate that the robust induction of Mirn132 and Mirn212 seen in response to in vivo LH/hCG treatment is likely due to the cAMP cell signaling cascade. This coincides with the recent observation that cAMP was able to mediate an increase in Mirn132 levels within rat pheochromocytoma cells (PC12 cells) [42, 45]. Vo et al. [42] also demonstrated several CREB response elements located immediately upstream of its pre-miRNA sequence to be critical for Mirn132 expression. Our analysis of the pri-miRNA transcript and that of the exonic regions of AK006051, 11700016P03Rik, the gene that contains the pri-miRNA transcript for Mirn132 and Mirn212 within its first intron, suggests the gene promoter that guides the expression of the mRNA transcript for AK006051 likely plays a significant role in the biogenesis of these miRNAs (data not shown). Vo et al. [42] demonstrated by 5′ cap trapping RACE and 3′ RACE that the pri-miRNA transcript did not contain a significant open reading frame. Transcriptional regulation of miRNAs is just now beginning to be unraveled, and the transcription of pri-miRNAs appears to be just as complex as the transcription of protein-encoding genes [42, 45]. This work is a beginning point for future studies concerning the regulation of expression of these two particular miRNAs within ovarian granulosa cells.

To facilitate the identification of proteins and to determine the physiologic role that these LH-regulated miRNAs might be having on granulosa cell function, we used both a computational approach and a knockdown approach. The results of our computational analyses indicate that a large number of 3′ UTRs contain Mirn132/212 seed sequences. Comparison of these lists to the total number of genes present in ovarian granulosa cells after follicular stimulation (i.e., 46 h of eCG and 1 h after hCG and 8 h after hCG treatment) yielded a list of 77 transcripts with putative Mirn132/212 recognition sites. Most of these genes are not specific to ovarian function, and their role in ovarian cell function is unknown. However, several transcripts have been confirmed as Mirn132/212 targets in other tissues. The MECP2, CTBP1, and p250 GTPase-activating proteins (GAP; also known as GRIT within the murine genome) have been shown to regulate neuronal dendritic branching [42, 45]. Interestingly, CTBP1 was recently shown to interact with steroidogenic factor-1 [46] in adrenal cells to modulate its ability to stimulate promoter activity. Our studies indicate that CTBP1 is posttranscriptionally regulated, as evidenced by the lack of change in its mRNA levels in response to in vivo hCG treatment combined with the pronounced loss in the protein when miRNA function-blocking LNA oligonucleotides were administered. CTBP1 acts as a corepressor of nuclear receptor target genes [45]. Therefore, our data suggest that Ctbp1 translation is upregulated in response to LH/hCG/cAMP induction of Mirn132 and Mirn212. In turn, this corepressor could play key regulatory roles in mediating changes in global gene transcription. We were unable to confirm whether p250GAP (GRIT) and MECP2 were posttranscriptionally regulated due to the lack of suitable antibodies.

In summary, these studies are the first to elucidate the extent of miRNA expression within ovarian granulosa cells and to identify hormonally regulated miRNAs within periovulatory granulosa cells. The two highly LH-induced miRNAs, Mirn132 and Mirn212, share the same seed sequence and were shown to be transcribed as a single pri-miRNA. The loss of induction of CTBP1 protein synthesis following ablation of functional Mirn132 and Mirn212 suggests that these miRNAs may play a key role through indirect regulation of gene transcription in ovarian granulosa cells. Further studies are necessary to identify how these miRNAs mediate changes in expression of CTBP1 as well as to identify the genes downstream of this corepressor.

Supplementary Material

Supplemental Data:

Acknowledgments

We would like to express our gratitude to Drs. Warren Nothnick and T. Rajendra Kumar for providing feedback during the writing process. We would also like to thank Todd Fiedler for assistance with data analysis.

Footnotes

1Supported by National Institute of Health grant HD051870-01.

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