Editorial on “PNLDC1 is essential for piRNA 3' end trimming and transposon silencing during spermatogenesis in mice”

Piwi-interacting RNA (piRNA) are a class of small non-coding RNAs expressed in the germline. These piRNAs are processed to a mature piRNA by endonucleases and exonucleases that are bound to PIWI family proteins (1-3). In mouse, three Piwi proteins have been shown to be critical for spermatogenesis, namely Miwi, Mili and Miwi2 (4-6). Mili and Miwi show different temporal expression during testes development. Mili is expressed earlier until the pachytene stage of meiosis, and Miwi is expressed from pachytene until the haploid round spermatid stage. Disruption of the genes encoding Mili and Miwi results in sterile males due to spermatogenesis arresting at the pachytene stage, resulting from a failure of piRNA retrotransposon silencing (5-11). Transposon silencing within the germline requires the correct processing of piRNA. Immature piRNA requires 3'-5' trimming to generate mature piRNAs (12-14). A protein related to the poly(A)-specific ribonuclease (PARN) family, poly(A)-specific ribonuclease-like domain containing 1 (PNLDC1), has been recognized as an enzyme “trimmer” for pre-piRNA 3' end trimming and maturation processing in silkworms (14). PNLDC1 has also been found to be expressed in mammalian species including mouse and human (15). The potential role of PNLDC1 in mammals was unclear until recent reports have implicated it in piRNA processing in mice during spermatogenesis (16-18)