Tight control of transposon activity is essential for the integrity of the germline. Recently, a germ-cell-specific organelle, nuage, was proposed to play a role in transposon repression. To test this hypothesis, we disrupted a murine homolog of a Drosophila nuage protein Maelstrom. Effects on male meiotic chromosome synapsis and derepression of transposable elements (TEs) were observed. In the adult Mael(-/-) testes, LINE-1 (L1) derepression occurred at the onset of meiosis. As a result, Mael(-/-) spermatocytes were flooded with L1 ribonucleoproteins (RNPs) that accumulated in large cytoplasmic enclaves and nuclei. Mael(-/-) spermatocytes with nuclear L1 RNPs exhibited massive DNA damage and severe chromosome asynapsis even in the absence of SP011-generated meiotic double-strand breaks. This study demonstrates that MAEL, a nuage component, is indispensable for the silencing of TEs and identifies the initiation of meiosis as an important step in TE control in the male germline.

Postmigratory mouse primordial germ cells (PGCs) undergo extensive epigenetic remodeling that includes DNA methylation (DM) reprogramming of imprinted genes and, surprisingly, of transposable elements (TEs). Given the danger posed by TEs to the integrity of the germline, even a brief derepression of TEs is counterintuitive and puzzling. In the male fetal gonocytes, a sophisticated repressive mechanism that uses DM and TE-targeting piRNAs has evolved to stably silence TEs. A recent study has further increased the complexity of this problem by revealing that TE silencing is alleviated specifically at the onset of meiosis in testes lacking MAEL, a piRNA pathway protein. These observations and prior work of others are consistent with existence of an additional reprogramming event, transient relaxation of transposon silencing (TRTS), at the onset of both male and female meiosis in mice. In this Point of View we propose that TE expression is inherent to mammalian meiosis and discuss potential functional significance of this phenomenon.

Repression of transposable elements is crucial for the survival of germ cells. In this issue of Developmental Cell, Shoji et al. provide evidence that a Tudor domain protein TDRD9 partners with MIWI2 to specifically silence LINE-1 transposons in the fetal germline of male mice.

Meiosis yields haploid gametes following two successive divisions of a germ cell in the absence of intervening DNA replication. Balanced segregation of homologous chromosomes in Meiosis I is aided by a proteinaceous structure, the synaptonemal complex (SC). The objective of this study was to determine total average autosomal SC lengths in spermatocytes in three commonly used mouse strains (129S4/SvJae, C57BL/6J, and BALB/c). Our experiments revealed that the total autosomal SC length in BALB/c spermatocytes is 9% shorter than in the two other strains. Shorter SCs are also observed in spermatocytes of (BALB/c x 129S4/SvJae) and (C57BL/6J x BALB/c) F1 hybrids suggesting a genetic basis of SC length regulation. Along these lines, we studied expression of a selected group of genes implicated in meiotic chromosome architecture. We found that BALB/c testes express up to 6-fold less of Rec8 mRNA and 4-fold less of REC8 protein. These results suggest that the mechanism that defines the SC length operates via a REC8-dependent process. Finally, our results demonstrate that genetic background can have an effect on meiotic studies in mice.

Epigenetic reprogramming of embryonic mouse germ cells involves DNA demethylation of the genome that is accompanied by derepression of transposable elements (TEs). Threatening the genome's integrity, TE activation is efficiently countered by the concerted action of de novo DNA methylation and PIWI-interacting small RNAs (piRNAs). Recent studies have closely examined the subcellular localization of various piRNA pathway proteins in fetal prospermatogonia of wild-type and piRNA pathway mutant mice. Our survey highlights hierarchies and partnerships between the members of this ancient defensive mechanism. Overall, the elaborate cytoplasmic compartmentalization of the piRNA pathway in fetal prospermatogonia provides a highly informative context to aid our understanding of the mouse piRNA pathway.

Integrity of the germline genome is essential for the production of viable gametes and successful reproduction. In mammals, the generation of gametes involves extensive epigenetic changes (DNA methylation and histone modification) in conjunction with changes in chromosome structure to ensure flawless progression through meiotic recombination and packaging of the genome into mature gametes. Although epigenetic reprogramming is essential for mammalian reproduction, reprogramming also provides a permissive window for exploitation by transposable elements (TEs), autonomously replicating endogenous elements. Expression and propagation of TEs during the reprogramming period can result in insertional mutagenesis that compromises genome integrity leading to reproductive problems and sporadic inherited diseases in offspring. Recent work has identified the germ cell associated PIWI Interacting RNA (piRNA) pathway in conjunction with the DNA methylation and histone modification machinery in silencing TEs. In this review we will highlight these recent advances in piRNA mediated regulation of TEs in the mouse germline, as well as mention the repercussions of failure to properly regulate TEs. (C) 2011 Elsevier B.V. All rights reserved.

5-Ethynyl-2'-deoxycytidine triphosphate (EdCTP) was synthesized as a probe to be used in conjunction with fluorescent labeling to facilitate the analysis of the in vivo dynamics of DNA-centered processes (DNA replication, repair and cytosine demethylation). Kinetic analysis showed that EdCTP is accepted as a substrate by Klenow exo(-) and DNA polymerase beta. Incorporation of 5-ethynyl-2'-deoxycytidine (EdC) into DNA by these enzymes is, at most, modestly less efficient than native dC. EdC-containing DNA was visualized by using a click reaction with a fluorescent azide, following polymerase incorporation and T4 DNA ligase mediated ligation. Subsequent experiments in mouse male germ cells and zygotes demonstrated that EdC is a specific and reliable reporter of DNA replication, in vivo.