piRNA-Guided Genome Defense: From Biogenesis to Silencing

Abstract

PIWI-interacting RNAs (piRNAs) and their associated PIWI clade Argonaute proteins constitute the core of the piRNA pathway. In gonadal cells, this conserved pathway is crucial for genome defense, and its main function is to silence transposable elements. This is achieved through posttranscriptional and transcriptional gene silencing. Precursors that give rise to piRNAs require specialized transcription and transport machineries because piRNA biogenesis is a cytoplasmic process. The ping-pong cycle, a posttranscriptional silencing mechanism, combines the cleavage-dependent silencing of transposon RNAs with piRNA production. PIWI proteins also function in the nucleus, where they scan for nascent target transcripts with sequence complementarity, instructing transcriptional silencing and deposition of repressive chromatin marks at transposon loci. Although studies have revealed numerous factors that participate in each branch of the piRNA pathway, the precise molecular roles of these factors often remain unclear. In this review, we summarize our current understanding of the mechanisms involved in piRNA biogenesis and function.

Movement of small RNAs in and between plants and fungi

“…siRNA, miRNA as well as piRNA all act to control gene expression and play important roles in many fundamental biological processes in eukaryotic organisms. They have been tied to vital processes such as cell growth, tissue differentiation, heterochromatin formation, cell proliferation, and disease resistance (Blair and Olson, ; Yuan et al , ; Tassetto et al , ; Czech et al , ; Mondal et al , ; Almeida et al , ). Research over the past few decades has led to powerful insight into the structure and function of small RNAs, which has been summarized in several reviews (Eamens et al , ; Ghildiyal and Zamore, ; Peters and Meister, ; Pratt and MacRae, ; Holoch and Moazed, ; Quinn and Chang, ; Zhang, Cozen et al , ; Zhang et al , ).…”

Essential roles of Windei and nuclear monoubiquitination of Eggless/ SETDB 1 in transposon silencing

“…piRNAs are small non-coding RNAs enriched in the germline. piRNAs assemble the piRNA-induced silencing complexes (piRISCs) with members of the PIWI family of proteins [13][14][15][16]. piRNAs show high complementarity to transposon RNA transcripts in both sense and antisense orientations and repress transposons transcriptionally, by inducing local heterochromatinization, or post-transcriptionally, by cleaving target RNAs [13][14][15][16].…”

“…piRNAs assemble the piRNA-induced silencing complexes (piRISCs) with members of the PIWI family of proteins [13][14][15][16]. piRNAs show high complementarity to transposon RNA transcripts in both sense and antisense orientations and repress transposons transcriptionally, by inducing local heterochromatinization, or post-transcriptionally, by cleaving target RNAs [13][14][15][16]. In Drosophila ovarian somatic cells (OSCs), piRNAs assemble piRISCs solely with Piwi, the only nuclear member of the PIWI protein family, and, with multiple co-factors, transcriptionally repress transposons [12,[17][18][19][20][21].…”

Specialization of theDrosophilanuclear export family protein, Nxf3, for piRNA precursor export

“…In the animal germline, transposon mobilization is prevented by a conserved small RNA-based immune system, the PIWI-interacting RNA (piRNA) pathway. piRNAs are 23-to 30nucleotide (nt) small RNAs that associate with Argonaute proteins of the PIWI clade (Czech et al 2018;Ozata et al 2019). The majority of piRNAs are produced from discrete genomic loci, called piRNA clusters, which are composed of transposon remnants, and thus constitute a genetic memory of past transposon exposure (Brennecke et al 2007).…”