A resource for the conditional ablation of microRNAs in the mouse

Abstract

The importance of miRNAs during development and disease processes is well established. However, most studies have been done in cells or with patient tissues, and therefore the physiological roles of miRNAs are not well understood. To unravel in vivo functions of miRNAs, we have generated conditional, reporter-tagged knockout-first mice for numerous evolutionarily conserved miRNAs. Here, we report the generation of 162 miRNA targeting vectors, 64 targeted ES cell lines, and 46 germline-transmitted miRNA knockout mice. In vivo lacZ reporter analysis in 18 lines revealed highly tissue-specific expression patterns and their miRNA expression profiling matched closely with published expression data. Most miRNA knockout mice tested were viable, supporting a mechanism by which miRNAs act redundantly with other miRNAs or other pathways. These data and collection of resources will be of value for the in vivo dissection of miRNA functions in mouse models.

Figure 1. ES cell targeting and PCR genotyping

(A) Schematic representation of a miRNA locus and targeting strategy. Two homology arms at the 5′ and 3′ ends of the targeting vector mediate gene-specific targeting by homologous recombination. Targeting leads to insertions of a promoter-less lacZ reporter with an IRES (in black color) and a polyA signal, β-actin-driven neomycin selection marker with a polyA signal, and a miRNA stem-loop flanked by loxP sites into the miRNA locus. (B) PCR genotyping strategy of targeted ES cells. Positions of the PCR primers used for genotyping wild type and targeted alleles are marked with arrows and expected size differences for PCR products are illustrated. To verify homologous recombination on the 5′ arm, PCR was done with 2 gene-specific primers and 1 universal mutant primer. For the longer 3′arm side, PCR was done to verify the insertion of the third loxP site with 2 gene-specific primers. The mutant products were verified by sequencing.

Figure 2. Generation of knockout mice

(A) Breeding strategy. Combination of FRT and loxP sites allows manipulation of targeted allele in mice. Germline-transmitted mice (lacZ-neo-flox) can be crossed with germline deleter Flp mice, which will remove the reporter and the neomycin cassette to restore the wild type allele (conditional allele/flox). Then conditional mice can be crossed with either germline- or tissue-specific Cre transgenic mice to generate knockouts (knockout allele/KO). Instead of sequential recombination steps, germline-transmitted mice can be crossed with germline deleter Cre mice to produce mice with a reporter-tagged null allele (lacZ-knockout allele/lacZ-KO). (B) Flp-mediated recombination of FRT sites in mice. Germline-transmitted mice were crossed with homozygous Rosa-Flp mice. Parents and offspring were PCR-genotyped to check deletion of the lacZ reporter. Flp excision was not fully penetrant as the lacZ was not deleted in #5 pup, although it was completely excised in #2 pup (F and M indicate father and mother respectively). (C) Cre-mediated recombination of loxP sites in mice. Germline-transmitted mice were crossed with actin-Cre transgenic mice. Parents and offspring were PCR-genotyped to evaluate deletion of the neomycin marker, but not of the lacZ reporter. Excision by actin-Cre was complete among all pups (#7 to #9). (D) qPCR analysis to confirm the loss of miRNA in lacZ-KO mice. Either lung (miR-30b/30d, miR-339, miR-130a, miR-141/200c, miR-479/195 and miR-296/298) or brain (miR-7a-2) of 2 to 6 month-old knockout mice was used to isolate total RNA. RT-PCR was done by using Taqman miRNA assays. PCR were done as triplicates and data were presented as means with standard deviation.

Figure 3. Expression analysis of miRNA lacZ reporter

(A) LacZ reporter expression in E11.5 embryos. Embryos for miR-30b/30d, miR-30e, miR-130a, miR-296/298, miR-301a, and miR-339 displayed lacZ expression ubiquitously, whereas embryos for miR-210, miR-146a, mir-688, miR-497/195, and miR-654/376b exhibited no expression. The central nervous system was positive for miR-7a-2, miR-135b, and miR-325. Embryos for miR-141/200c showed staining only in the nostrils, as shown with arrows. Posterior trunk staining was detected for miR-196a-1. Although both expressed in pharyngeal arches and limb buds, expression patterns of miR-130b/301b and miR-205 were distinct. In addition, miR-130b/301b embryos displayed forebrain staining. (B) lacZ reporter expression in E18.5 embryos and adult. Brains of E18.5 embryos for miR-7a-2, miR-135b, and miR-325 displayed broad lacZ expression, whereas miR-688 exhibited very restricted expression domain in the dorsal cortex and ventral mid-brain (for each miRNA, top and bottom panels correspond to dorsal and ventral view respectively). Expression of miR-135b was also found in the inner ears (semicircular canals, vestibule and cochlea), while miR-325 expression was found in the visual sensory system (eyes, optic tracts, and lateral geniculate nucleus). The spinal cord staining was detected for miR-325. The CNS expression pattern of miR-7a-2 was maintained in adults as shown in the vibratome-sectioned brain. Expression of miR-141/200c was found in the airways of the respiratory tract, including in the nasal cavity, trachea, bronchi, and bronchioles, as well as in olfactory bulbs in the brain. Expression of miR-497/195 was found both in the airways and the air sacs of the lungs. Kidneys of miR-196a-1 displayed a striking lacZ staining pattern. For miR-654/376b, the frontal portion of the ribcage (corresponding to cartilage at this developmental stage) stained positive in E18.5 embryos.