Transposable element regulation in mammalian germ cells and other contexts: multilayered repression, fertility, and regulated activation.

内容摘要

Transposable elements (TEs), particularly retrotransposons that dominate mammalian genomes, are pervasive components of mammalian genomes whose activation is constrained by multilayered repression systems. In germ cells, this repression architecture is particularly elaborate, integrating chromatin-based silencing, DNA methylation, and small RNA-guided pathways to safeguard genome integrity during epigenetic reprogramming. These mechanisms are coordinated yet mechanistically specialized, targeting distinct phases of the transposon life cycle and different TE families across developmental stages. Yet the distinctive chromatin landscape of germ cells also creates windows of developmental permissiveness during which TE transcription can occur. Beyond the germline, TE expression can emerge in defined stages of early embryogenesis, extraembryonic development, neural differentiation, and aging, with consequences ranging from chromatin remodeling and regulatory co-option to inflammatory signaling and genome instability. Together, these observations raise a central question: how do different mammalian lineages balance epigenetic plasticity with genome defense? Here, we synthesize current understanding of the molecular logic of TE repression, emphasizing the germline, and integrate evidence across development and aging. We highlight shared principles-such as epigenetic permissiveness and RNA-guided targeting-while underscoring a key difference in regulatory outcome: somatic contexts may tolerate, co-opt, or pathologically amplify TE activity, whereas the germline converts transient activation into heritable, sequence-specific silencing. Transposable elements are repeated DNA sequences that can move or copy themselves within the genome. Because of this ability, they can damage DNA and disrupt normal cell function. For many years, they were mainly viewed as harmful “genetic parasites.” However, recent studies have shown that they can also contribute to normal biological processes, including early development and gene regulation. This review discusses how mammals control transposable elements, with a particular focus on reproductive cells that give rise to sperm and eggs. These cells undergo major changes in their DNA packaging and gene regulation during development. Such changes can temporarily weaken the systems that normally keep transposable elements silent, creating periods when these elements become more active. To prevent harmful effects, reproductive cells use several layers of protection, including chemical modifications of DNA and RNA-based defense systems. We also compare transposable element activity in reproductive cells with other biological settings, including early embryos, the placenta, the nervous system, and aging tissues. In some situations, transposable element activity may contribute to normal development or gene control. In others, excessive activation is linked to infertility, inflammation, aging, and disease. Together, these findings show that mammals must carefully balance two competing needs: allowing enough flexibility in the genome for development and adaptation, while still protecting the genome from harmful transposable element activity. Understanding this balance may improve our knowledge of fertility, aging, and human disease.