Somatic piRNAs and Transposons are Differentially Expressed Coincident with Skeletal Muscle Atrophy and Programmed Cell Death.

PIWI-interacting RNAs (piRNAs) are small single-stranded RNAs that can repress transposon expression via epigenetic silencing and transcript degradation. They have been identified predominantly in the ovary and testis, where they serve essential roles in transposon silencing in order to protect the integrity of the genome in the germline. The potential expression of piRNAs in somatic cells has been controversial. In the present study we demonstrate the expression of piRNAs derived from both genic and transposon RNAs in the intersegmental muscles (ISMs) from the tobacco hawkmoth Manduca sexta. These piRNAs are abundantly expressed, ∼27 nt long, map antisense to transposons, are oxidation resistant, exhibit a 5’ uridine bias, and amplify via the canonical ping-pong pathway. An RNA-seq analysis demonstrated that 19 piRNA pathway genes are expressed in the ISMs and are developmentally regulated. The abundance of piRNAs does not change when the muscles initiate developmentally-regulated atrophy, but are repressed coincident with the commitment of the muscles undergo programmed cell death at the end of metamorphosis. This change in piRNA expression is correlated with the repression of several retrotransposons and the induction of specific DNA transposons. The developmentally-regulated changes in the expression of piRNAs, piRNA pathway genes, and transposons are all regulated by 20-hydroxyecdysone, the steroid hormone that controls the timing of ISM death. Taken together, these data provide compelling evidence for the existence of piRNA in somatic tissues and suggest that they may play roles in developmental processes such as programmed cell death.

PIWI相互作用RNA（piRNA）是小的单链RNA，能够通过表观遗传沉默和转录本降解来抑制转座子表达。它们主要在卵巢和睾丸中被发现，在这些部位它们在转座子沉默中起关键作用，以保护生殖系中基因组的完整性。piRNA在体细胞中的潜在表达一直存在争议。在本研究中，我们证明了源自基因和转座子RNA的piRNA在烟草天蛾（Manduca sexta）的节间肌（ISM）中的表达。这些piRNA大量表达，约27个核苷酸长，与转座子反向互补，具有抗氧化性，表现出5’尿苷偏好，并通过经典的乒乓途径扩增。一项RNA - seq分析表明，19个piRNA通路基因在节间肌中表达，并受到发育调控。当肌肉开始发生发育调控的萎缩时，piRNA的丰度没有变化，但在变态末期肌肉发生程序性细胞死亡时受到抑制。piRNA表达的这种变化与几种逆转录转座子的抑制以及特定DNA转座子的诱导相关。piRNA、piRNA通路基因和转座子表达的发育调控变化均受20 - 羟基蜕皮酮调控，20 - 羟基蜕皮酮是控制节间肌死亡时间的类固醇激素。综上所述，这些数据为体细胞组织中存在piRNA提供了有力证据，并表明它们可能在程序性细胞死亡等发育过程中发挥作用。