Conditional deletion of Dicer in mouse oocytes leads to infertility with severe defects in chromosomal alignment and spindle organization [1, 2]. Because numerous endo-siRNAs and miRNAs are present in oocytes [3, 4], the Dicer knockout phenotype could be the result of either of these classes of small RNAs. To test the relative contribution of canonical miRNAs versus other Dicer-dependent small RNAs to the phenotype, we crossed the same oocyte-specific Cre transgenic line (Zp3-Cre) [6] to a Dgcr8 conditional knockout model [7, 8], hence specifically removing canonical miRNAs. The conditional knockout of Dgcr8 is based on a floxed exon 3 allele. Loss of exon 3 results in a frameshift mutation with multiple downstream premature stop codons, resulting in a truncated protein missing the WW and RNA-binding domains (Figure 1A). The floxed allele in Dgcr8^delta/flox; Zp3-Cre females was efficiently deleted in oocytes (Figure 1B). Furthermore, quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) showed complete loss of Dgcr8 transcripts carrying exon 3 (Figure 1C, left). It also showed a 68% reduction in Dgcr8 transcript levels in Dicer^−/− oocytes (Figure 1C, left; *p = 0.0058, Student’s t test). In contrast, Zp3-Cre-induced deletion of Dgcr8 had no significant effect on Dicer transcript levels (Figure 1C, right; not significant, p = 0.1432). Immunohistochemistry showed a loss of nuclear DGCR8 protein in the knockout oocytes (Figure 1D). Together, these data show a complete loss of Dgcr8 in the conditional knockouts.

To evaluate the effect of Dgcr8 loss on oocyte maturation in vivo, we collected oocytes 16 hr post-human chorionic gonadotropin (hCG) injection. Similar numbers of metaphase II (MII) oocytes were recovered from wild-type and Dgcr8^delta/flox; Zp3-Cre mutant females (Figure 1E). To follow maturation of the oocytes in vitro, we isolated fully grown germinal vesicle (GV) oocytes and allowed them to mature in culture. Wild-type and Dgcr8^−/− oocytes developed similarly in terms of both the kinetics of GV breakdown (GVBD) in vitro (Figure 1F) and maturation to the MI and MII stages both in vivo (Figure 1G) and in vitro (data not shown). This finding is in stark contrast to Dicer^−/− oocytes, which arrest at MI, rarely extruding a polar body [1, 2]. Because Dicer^−/− oocytes arrest with defects in spindle organization and chromosome condensation [1, 2], we evaluated spindle structure in Dgcr8^−/− oocytes, which were collected 16 hr post-hCG. The meiotic spindles of Dgcr8^−/− oocytes had a normal morphology (Figure 1H). These findings show that DGCR8, unlike Dicer, is not required for oocyte maturation.

Next, we asked whether maternal loss of Dgcr8, and hence maternally contributed canonical miRNAs, would affect fertility and the development of resulting embryos. Wild-type males were crossed to the Dgcr8^delta/flox; Zp3-Cre females. Initial crosses showed that unlike the Dicer^flox/flox; Zp3-Cre females, Dgcr8^delta/flox; Zp3-Cre females could be fertilized and produce healthy-appearing offspring (Figure 2A). However, analysis of brood size over many crosses showed that Dgcr8-deficient mothers produced fewer offspring compared to controls (5.5 ± 0.6 pups per litter, n = 23 versus 9.5 ± 0.5 pups per litter, n = 32, p < 0.0001) (Figure 2B, left). This defect was not secondary to zygotic haploinsufficiency, because control heterozygous crosses showed no significant difference (8.7 ± 0.5 pups per litter, n = 29 versus 9.5 ± 0.5 pups per litter, n = 32, p = 0.2432) (Figure 2B, right). Therefore, although maternal DGCR8 is not absolutely required for fertility, it does impact female fecundity.

The loss of zygotic Dgcr8 leads to embryonic arrest prior to E6.5 [9]. To determine whether more subtle defects could be seen at earlier stages, we analyzed preimplantation development, including cell number and distribution of cells between the inner cell mass (ICM) and trophectoderm lineages, which is the first definitive differentiation event during embryogenesis. Embryos were genotyped following morphological and immunofluorescence analysis. Crosses between Dgcr8 heterozygous mice produced normal-appearing zygotic Dgcr8 knockout blastocysts (zygotic Dgcr8^−/−) (Figure 3A, left). Furthermore, knockout embryos developed to the blastocyst stage with normal Mendelian ratios (Figure S2A). Immunofluorescence showed that the total number of cells and distribution between the trophectoderm (Cdx-2-positive) and the ICM were unchanged in mutant embryos (Figure 3A, right).

Maternally provided Dgcr8 and/or miRNAs may have enabled preimplantation development. Therefore, Dgcr8 conditional knockout mothers (Dgcr8^delta/flox; Zp3-Cre) were crossed to heterozygous knockout males, and the resulting maternal-zygotic knockout (maternal-zygotic Dgcr8^−/−) embryos were evaluated. Once again, there was no difference between the knockout and control embryos in terms of morphology (Figure 3B, left) or the number of embryos that developed to the blastocyst stage (Figure S2B). Furthermore, the number of cells per embryo and the distribution of cells between the ICM and the trophectoderm were unchanged (Figure 3B, right). Together, these findings show that the earliest differentiation event leading to epiblast and trophectoderm lineages proceed normally in the absence of canonical miRNAs.

The finding that the maternal and zygotic DGCR8 were dispensable for oocyte maturation and early embryonic development was surprising considering the central role of miRNAs in other developmental decisions [10]. The dramatic difference in phenotypes between Dicer and Dgcr8 knockout oocytes could be partly explained by the loading of pre-miRNAs in the growing oocyte prior to Zp3-Cre-induced deletion, which would then be processed by Dicer at later stages without any further requirement of DGCR8. To test this possibility, we evaluated 40 highly expressed miRNAs in oocytes by performing a quantitative RT-PCR assay [11]. The levels of 39 of these 40 miRNAs were similarly depleted in the Dgcr8^−/− and Dicer^−/− oocytes (Figure 4A). The only miRNA not depleted, miR-484, is a noncanonical miRNA and is hence DGCR8 independent [12]. Therefore, ongoing Dgcr8 expression is indeed required for the production of canonical miRNAs in developing oocytes.

We next asked whether miRNAs repress mRNAs in oocytes. We reasoned that although there were no obvious phenotypic consequences, the large number of highly expressed miRNAs [1–4] in wild-type oocytes would still impact the molecular constitution of the cells. Because miRNAs function to both destabilize mRNAs as well as inhibit translation, we compared the mRNA profiles of the Dgcr8^−/− and wild-type oocytes. Surprisingly, the profiles were almost identical (Figure 4B). Only three mRNAs were identified as significantly changed with a false discovery rate (FDR) cutoff of 5%, including Dgcr8 itself representing remnant exon 3 deleted transcript (0.1× relative to wild-type), MT1 (10.7×), and a predicted transcript ENSMUST00000101675 (1.7×). Furthermore, cluster analysis via Pearson’s correlation for the distance measurement could not distinguish the wild-type from Dgcr8^−/− transcriptomes, whereas the Dicer^−/− transcriptome was clearly diverged (Figure 4C). Similar cluster analysis on wild-type, Dgcr8^−/−, and Dicer^−/− embryonic stem cells showed that the transcriptomes of Dgcr8^−/− and Dicer^−/− were more similar to each other than to that of wild-type, as would be expected if miRNAs play a central role in the cells (Figure S3). These data show that canonical miRNAs have little effect on the transcriptome of oocytes, in stark contrast to other cell types.

Dicer^−/− oocytes showed dramatic changes in their expression profile (Figure 4D). One thousand and forty nine transcripts were upregulated and 657 genes were downregulated compared to wild-type controls (FDR < 5%) (Figures 4D and 4E). These changes were likely due to DGCR8-independent, Dicer-dependent small RNAs. Indeed, qRT-PCR analysis for a number of predicted endo-siRNA mRNA targets [3, 4] showed no change in Dgcr8^−/− but significant upregulation in the Dicer^−/− oocytes (Figure S4A). Furthermore, qRT-PCR of the mouse transposon (MT) family of retrotransposons (also predicted targets of endo-siRNAs [3, 4]) showed upregulation in Dicer^−/−, but not Dgcr8^−/−, oocytes (Figure S4B). In contrast, other transposons including intracisternal A-particle transposons and short interspersed repetitive elements (SINEs) appeared unchanged in both mutants. Surprisingly, long interspersed repetitive elements (LINEs) were downregulated in Dicer^−/−, but not Dgcr8^−/−, oocytes. This latter finding is likely an indirect consequence of Dicer loss.

The lack of mRNA changes in the Dgcr8^−/− oocytes was not simply due to the absence of mRNA targets for miRNAs, because bioinformatic analysis identified many oocyte-expressed mRNAs with multiple seed matches in their 3′ untranslated regions (3′UTRs) to highly expressed miRNAs (Figure 5A). Furthermore, a characteristic property of miRNA function in cells is the phenomenon of antitargeting, the depletion of miRNA target sites in the 3′UTRs of the most highly expressed genes [13]. In embryonic stem (ES) cells, we found evidence of antitargeting because there was a depletion of predicted ES cell miRNA target sites among the most highly expressed mRNAs and an enrichment of sites among the intermediate expressed mRNAs (Figures 5A and 5B, dark gray). In contrast, there was little depletion of predicted miRNA target sites among the most highly expressed mRNAs in oocytes (Figures 5A and 5B, light gray). This lack of antitargeting in oocytes is consistent with the finding that miRNA function—which drives the phenomenon of antitargeting—is lost in mature oocytes.

DGCR8-independent, Dicer-dependent small RNAs include the noncanonical miRNAs, mirtrons, and small hairpin RNAs (shRNAs), along with the endo-siRNAs [5]. To determine whether the noncanonical miRNAs may have a role in the Dicer^−/− phenotype, we evaluated published deep-sequencing data from oocytes [3]. To identify mirtrons, we mapped sequence reads to all short introns (<500 bp). Two previously identified mirtrons [12] were expressed at low levels in oocytes: Abcf1 and Mosc2. We searched for target sites of these two mirtrons in transcripts that are present and expressed in oocytes. However, there was no enrichment for seed sequence matches in the open reading frames or 3′UTRs of transcripts that were increased in abundance in the absence of Dicer (Figure S5A), suggesting that these mirtrons were not underlying the mRNA changes. We next asked whether two previously identified shRNAs in ES cells [12], mir-320 and mir-484, could regulate mRNAs in oocytes. Once again, there was no enrichment in the complementary seed matches in the Dicer upregulated transcripts (Figure S5B). In contrast to the mirtrons and shRNA, the endo-siRNAs could be mapped to many of the upregulated transcripts [3] (Table S1). Therefore, we conclude that the dramatic gene expression changes in Dicer^−/− oocytes are the result of the direct and indirect effects of endo-siRNA loss, not canonical or noncanonical miRNAs.

In summary, our findings provide two major insights into the roles of small RNAs in early embryonic development. First, miRNA function is globally suppressed in mature oocytes and early embryos. Second, endo-siRNAs, not miRNAs, are the central small RNA players at this early stage of mammalian development.

Remarkably, both mature miRNAs and siRNAs are present in the oocyte [3, 4], but only the loss of the latter impacts the transcriptome, suggesting differential regulation of these two classes downstream of Dicer maturation. This differential regulation may occur at the level of the silencing complex. For example, it may involve modifications to the Argonaute (Ago) proteins, which are key components of the silencing complex [14]. Deletion of Ago2 in the oocytes results in a Dicer knockout-like phenotype, presumably because of loss of Slicer activity and hence siRNA function [15]. The role for the remaining Argonautes in oocytes remains to be elucidated, but Argonautes appear to be highly redundant in terms of miRNA activity [16]. An alternative mechanism may involve RNA-binding proteins that suppress the capacity of miRNAs to bind mRNAs or recruit the destabilization machinery. Such a mechanism has been described for a subset of miRNAs and mRNA targets [17, 18].

Although endo-siRNAs play a central role in mammalian oocytes, it is unclear whether they play roles in other mammalian tissues. Endo-siRNAs have also been uncovered in ES cells [12]. There is no overlap between the specific endo-siRNAs expressed in oocytes and ES cells showing that they are developmentally regulated [12]. Consistent with a potential role for endo-siRNAs in ES cells, loss of Dicer has more severe phenotypes than Dgcr8 knockout cells [9, 16, 19, 20]. However, these differences may also be explained by a role for noncanonical miRNAs [12]. DGCR8 and its enzymatic partner, Drosha, have been deleted in other tissues and compared to corresponding Dicer knockout phenotypes. In particular, Dgcr8 has been knocked out in the skin and cardiomyocytes [7, 21], whereas Drosha has been knocked out in T cells [22]. In all three cases, the Dgcr8 or Drosha null phenotypes appeared to be very similar to that of the corresponding Dicer null phenotypes. These findings suggest that endo-siRNAs and/or other classes of DGCR8/Drosha-independent, Dicer-dependent small RNAs play less important roles in somatic tissues. However, because the phenotypes are all quite severe, subtle differences may have been missed. Indeed, siRNAs have been uncovered in somatic tissues of Drosophila [23]. It remains to be seen whether siRNAs are present in mammalian somatic tissues.

Consistent with our findings, Ma et al. [24] (this issue of Current Biology) found that the activities of two highly expressed miRNAs (let-7 and miR-30), as measured by reporter assays, become suppressed during oocyte development. However, it remains unclear when miRNA function is reactivated. In the absence of both maternal and zygotic Dgcr8, embryos develop normally to the blastocyst stage, although this phenotypic finding does not exclude more subtle molecular roles for miRNAs prior to this stage. Interestingly, cytoplasmic densities called P bodies containing the miRNA/mRNA inhibitory complexes disappear in the developing oocyte and only reappear around the blastocyst stage (P. Svoboda and R.M. Schultz, personal communication), consistent with suppression of miRNA activity throughout preimplantation development. Surprisingly, although maternal miRNAs do not play a phenotypic role in preimplantation development, they do appear to be important in postimplantation development as brood sizes of maternal Dgcr8 knockouts are diminished. The role of maternal miRNAs at these later stages of development remains to be elucidated.

Supplementary Data