Transposable elements in the mammalian germline: a comfortable niche or a deadly trap?

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Abstract
Retrotransposable elements comprise around 50% of the mammalian genome. Their activity represents a constant threat to the host and has prompted the development of adaptive control mechanisms to protect genome architecture and function. To ensure their propagation, retrotransposons have to mobilize in cells destined for the next generation. Accordingly, these elements are particularly well suited to transcriptional networks associated with pluripotent and germinal states in mammals. The relaxation of epigenetic control that occurs in the early developing germline constitutes a dangerous window in which retrotransposons can escape from host restraint and massively expand. What could be observed as risky behavior may turn out to be an insidious strategy developed by germ cells to sense retrotransposons and hold them back in check. Herein, we review recent insights that have provided a detailed picture of the defense mechanisms that concur toward retrotransposon silencing in mammalian genomes, and in particular in the germline. In this lineage, retrotransposons are hit at multiple stages of their life cycle, through transcriptional repression, RNA degradation and translational control. An organized cross-talk between PIWI-interacting small RNAs (piRNAs) and various nuclear and cytoplasmic accessories provides this potent and multi-layered response to retrotransposon unleashing in early germ cells.