Somatic activity of the piRNA pathway in the mouse

The piRNA pathway is necessary for the suppression of retrotransposon activity in germ cells and the maintenance of germ cell populations, but the existence of the pathway in adult somatic cells has not been demonstrated. This study investigates the activity of the piRNA pathway in somatic cells of the adult mouse. We find that the PIWI protein MIWI2 is expressed in a variety of somatic tissues. Immunoprecipitation of MIWI2 from extracts of spleen cells and Friend virus-transformed mouse erythroblasts (MEL cells) identifies abundant piRNAs encoded by multiple intergenic clusters. In comparison to MIWI2-bound testis piRNAs, the somatic piRNAs are slightly smaller, and markedly deficient in repeat-derived sequences, but they have the same preference for a uracil at the 5' end; "ping-pong" amplification is absent. Determination of MIWI2-bound piRNAs in mouse erythropoietic cells