Transposons and the PIWI Pathway – Genome Defence in Gametes and Embryos

Abstract

Hiding in plain sight within the genome of virtually every eukaryotic organism are large numbers of sequences known as transposable elements (TEs). These sequences often comprise 50% or more of the DNA in many mammals and are transcriptionally constrained by DNA methylation and repressive chromatin marks. Individual TEs, when relieved of these epigenetic constraints, can readily move from one genomic location to another, either directly or through RNA intermediates. Demethylation and removal of repressive histone marks during epigenetic reprogramming stages of gametogenesis and embryogenesis render the genome particularly susceptible to increased TE mobilization, which has significant implications for the fidelity of genome replication and subsequent viability of the progeny. Importantly however, TEs have functionally integrated themselves into developmental events to the extent that complete suppression precludes normal gamete and embryo development. Consequently, multiple mechanisms have evolved to limit the extent of TE expression and mobilization during reprogramming without completely suppressing it. One of the most important TE repression mechanisms is the PIWI/piRNA pathway, in which 25 - 32 nucleotide RNA molecules known as piRNAs associate with Argonaute proteins from the PIWI clade to form piRISC complexes. These complexes target and silence TEs post-transcriptionally and through the induction of epigenetic changes at the loci from which they are expressed. This review will briefly discuss the intricate molecular détente between TE expression and its suppression by the PIWI pathway, with particular emphasis on mammalian species including human, bovine and murine.

Non-coding RNA regulation in reproduction: Their potential use as biomarkers

“…In mammals, it is known that miRNAs control about 50% of all protein-coding genes and participate in most cellular processes [2]. Regarding piRNAs, this regulatory pathway is responsible for germline specification and gametogenesis and plays an important role in defending against transposable elements during the epigenetic reprogramming stage in the fetal testis [7] (Fig. 1).…”

Silencing of Transposable Elements Mediated by 5-mC and Compensation of the Heterochromatin Content by Presence of B Chromosomes in Astyanax scabripinnis

“…On the other hand, RNA sequences denominated as piRNA that have 25 to 32 nucleotides participate in silencing TEs through post-transcriptional mechanisms and through epigenetic changes. In flies and mice, where the chromatin conformational change mechanism is better understood, the piRISC complex helps to induce methylation patterns in specific loci of DNA from which TEs are expressed [ 43 ].…”

Genome size evolution in the beetle genusDiabrotica

“…TE de-repression is triggered by environmental stimuli, in particular stress (Bundo et al, 2014;Fouché et al, 2020;Voronova et al, 2014) impacting transcription levels and increasing transpositional activity (Dubin et al, 2018). Additionally, there are other factors influencing TE mobilization for example demethylation and the removal of repressive histone marks during epigenetic reprogramming stages (Russell & LaMarre, 2018) Transposable elements are a major component of many eukaryotic genomes and have been associated with several mechanisms shaping eukaryotic genome function, structure, and evolution (Charlesworth & Charlesworth, 1983;Chuong et al, 2017). These transposons are capable of changing position within the genome, leading to mutations by inserting themselves into functional regions and causing change by either modifying or eliminating gene expression (Feschotte, 2008;Oliver & Greene, 2009).…”