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The PIWI-interacting RNA (piRNA) pathway is essential for retrotransposon silencing. In piRNA-deficient mice, L1-overexpressing male germ cells exhibit excessive DNA damage and meiotic defects. It remains unknown whether L1 expression simply highlights piRNA deficiency or actually drives the germ-cell demise. Specifically, the sheer abundance of genomic L1 copies prevents reliable quantification of new insertions. Here, we developed a codon-optimized L1 transgene that is controlled by an endogenous mouse L1 promoter. Importantly, DNA methylation dynamics of a single-copy transgene were indistinguishable from those of endogenous L1s. Analysis of Mov10l1 -/- testes established that de novo methylation of the L1 transgene required the intact piRNA pathway. Consistent with loss of DNA methylation and programmed reduction of H3K9me2 at meiotic onset, the transgene showed 1,400-fold increase in RNA expression and consequently 70-fold increase in retrotransposition in postnatal day 14 Mov10l1 -/- germ cells compared with the wild-type. Analysis of adult Mov10l1 -/- germ-cell fractions indicated a stage-specific increase of retrotransposition in the early meiotic prophase. However, extrapolation of the transgene data to endogenous L1s suggests that it is unlikely insertional mutagenesis alone accounts for the Mov10l1 -/- phenotype. Indeed, pharmacological inhibition of reverse transcription did not rescue the meiotic defect. Cumulatively, these results establish the occurrence of productive L1 mobilization in the absence of an intact piRNA pathway but leave open the possibility of processes preceding L1 integration in triggering meiotic checkpoints and germ-cell death. Additionally, our data suggest that many heritable L1 insertions originate from individuals with partially compromised piRNA defense., Competing Interests: The authors declare no conflict of interest.

Retinoic acid (RA), an active vitamin A derivative, is essential for mammalian spermatogenesis. Genetic studies have revealed that oxidation of vitamin A to retinal by retinol dehydrogenase 10 (RDH10) is critical for embryonic RA biosynthesis. However, physiological roles of RDH10 in postnatal RA synthesis remain unclear, given that Rdh10 loss-of-function mutations lead to early embryonic lethality. We conducted in vivo genetic studies of Rdh10 in postnatal mouse testes and found that an RDH10 deficiency in Sertoli cells, but not in germ cells, results in a mild germ cell depletion phenotype. A deficiency of RDH10 in both Sertoli and germ cells in juvenile mice results in a blockage of spermatogonial differentiation, similar to that seen in vitamin A-deficient animals. This defect in spermatogenesis arises from a complete deficiency in juvenile testicular RA synthesis and can be rescued by retinoid administration. Thus, in juvenile mice, the primary, but not exclusive, source of RA in the testes is Sertoli cells. In contrast, adult Rdh10-deficient mice exhibit phenotypically normal spermatogenesis, indicating that during development a change occurs in either the cellular source of RA or the retinaldehyde dehydrogenase involved in RA synthesis.

Dmrt1 (doublesex and mab-3 related transcription factor 1) is a conserved transcriptional regulator of male differentiation required for testicular development in vertebrates. Here, we show that in mice of the 129Sv strain, loss of Dmrt1 causes a high incidence of teratomas, whereas these tumors do not form in Dmrt1 mutant C57BL/6J mice. Conditional gene targeting indicates that Dmrt1 is required in fetal germ cells but not in Sertoli cells to prevent teratoma formation. Mutant 129Sv germ cells undergo apparently normal differentiation up to embryonic day 13.5 (E13.5), but some cells fail to arrest mitosis and ectopically express pluripotency markers. Expression analysis and chromatin immunoprecipitation identified DMRT1 target genes, whose missexpression may underlie teratoma formation. DMRT1 indirectly activates the GDNF coreceptor Ret, and it directly represses the pluripotency regulator Sox2. Analysis of human germ cell tumors reveals similar gene expression changes correlated to DMRT1 levels. Dmrt1 behaves genetically as a dose-sensitive tumor suppressor gene in 129Sv mice, and natural variation in Dmrt1 activity can confer teratoma susceptibility. This work reveals a genetic link between testicular dysgenesis, pluripotency regulation, and teratoma susceptibility that is highly sensitive to genetic background and to gene dosage.

Polyadenylation, the process of eukaryotic mRNA 3' end formation, is essential for gene expression and cell viability. Polyadenylation of male germ cell mRNAs is unusual, exhibiting increased alternative polyadenylation, decreased AAUAAA polyadenylation signal use, and reduced downstream sequence element dependence. CstF-64, the RNA-binding component of the cleavage stimulation factor (CstF), interacts with pre-mRNAs at sequences downstream of the cleavage site. In mammalian testes, meiotic XY-body formation causes suppression of X-linked CstF-64 expression during pachynema. Consequently, an autosomal paralog, tauCstF-64 (gene name Cstf2t), is expressed during meiosis and subsequent haploid differentiation. Here we show that targeted disruption of Cstf2t in mice causes aberrant spermatogenesis, specifically disrupting meiotic and postmeiotic development, resulting in male infertility resembling oligoasthenoteratozoospermia. Furthermore, the Cstf2t mutant phenotype displays variable expressivity such that spermatozoa show a broad range of defects. The overall phenotype is consistent with a requirement for tauCstF-64 in spermatogenesis as indicated by the significant changes in expression of thousands of genes in testes of Cstf2t(-/-) mice as measured by microarray. Our results indicate that, although the infertility in Cstf2t(-/-) males is due to low sperm count, multiple genes controlling many aspects of germ-cell development depend on tauCstF-64 for their normal expression. Finally, these transgenic mice provide a model for the study of polyadenylation in an isolated in vivo system and highlight the role of a growing family of testis-expressed autosomal retroposed variants of X-linked genes.

In mammals, meiosis is initiated at different time points in males and females, but the mechanism underlying this difference is unknown. Female germ cells begin meiosis during embryogenesis. In males, embryonic germ cells undergo G0/G1 mitotic cell cycle arrest, and meiosis begins after birth. In mice, the Stimulated by Retinoic Acid Gene 8 (Stra8) has been found to be required for the transition into meiosis in both female and male germ cells. Stra8 is expressed in embryonic ovaries just before meiotic initiation, whereas its expression in testes is first detected after birth. Here we examine the mechanism underlying the sex-specific timing of Stra8 expression and meiotic initiation in mice. Our work shows that signaling by retinoic acid (RA), an active derivative of vitamin A, is required for Stra8 expression and thereby meiotic initiation in embryonic ovaries. We also discovered that RA is sufficient to induce Stra8 expression in embryonic testes and in vitamin A-deficient adult testes in vivo. Finally, our results show that cytochrome p450 (CYP)-mediated RA metabolism prevents premature Stra8 expression in embryonic testes. Treatment with an inhibitor specific to RA-metabolizing enzymes indicates that a cytochrome p450 from the 26 family (CYP26) is responsible for delaying Stra8 expression in embryonic testes. Sex-specific regulation of RA signaling thus plays an essential role in meiotic initiation in embryonic ovaries and precludes its occurrence in embryonic testes. Because RA signaling regulates Stra8 expression in both embryonic ovaries and adult testes, this portion of the meiotic initiation pathway may be identical in both sexes.

Within the mammalian testis, specialized tight junctions between somatic Sertoli cells create basal and apical polarity within the cells, restrict movement of molecules between cells, and separate the seminiferous epithelium into basal and adluminal compartments. These tight junctions form the basis of the blood-testis barrier, a structure whose function and dynamic regulation is poorly understood. In this study, we used microarray gene expression profiling to identify genes with altered transcript levels in a mouse model for conditional androgen insensitivity. We show that testosterone, acting through its receptor expressed in Sertoli cells, regulates the expression of claudin 3, which encodes a transient component of newly formed tight junctions. Sertoli cell-specific ablation of androgen receptor results in increased permeability of the blood-testis barrier to biotin, suggesting claudin 3 regulates the movement of small molecules across the Sertoli cell tight junctions. These results suggest that androgen action in Sertoli cells regulates germ cell differentiation, in part by controlling the microenvironment of the seminiferous epithelium. Our studies also indicate that hormonal strategies for male contraception may interfere with the blood-testis barrier.

Rana catesbeiana tadpoles were injected with [(14)C]thyroxine, and the subcellular distribution of the labeled hormone was determined. At 25 degrees the amount of isotope found in the liver was maximal after 1-2 hr and then rapidly decreased to a relatively constant value. A large percentage of the hormone was found associated with the purified nuclei isolated 24 hr after injection of [(14)C]thyroxine. Injection of [(14)C]thyroxine into tadpoles maintained at 5 degrees resulted in a much slower but constant accumulation of isotope in the liver, with virtually no movement of thyroxine into the cell nucleus. Thyroxine was bound very tightly to the chromatin fraction of the nucleus, but extraction and chromatography revealed no chemical modification of the thyroxine itself. These results suggest the presence of two temperature-dependent processes: one concerned with the transport of thyroxine into the liver cell and a second concerned with the transport of the intracellular thyroxine into the cell nucleus. It is proposed that the latter process is involved in the low-temperature inhibition of thyroxine-induced metamorphosis.