piRNA 3' uridylation facilitates the assembly of MIWI/piRNA complex for efficient target regulation in mouse male germ cells

Poly(U) polymerase activity in Caenorhabditis elegans regulates abundance and tailing of sRNA and mRNA

Terminal nucleotidyltransferases add nucleotides to the 3' end of RNA to modify their stability and function. In Caenorhabditis elegans, the terminal uridyltransferases/poly(U) polymerases PUP-1 (aka CID-1, CDE-1), PUP-2, and PUP-3 affect germline identity, survival, and development. Here, we identify small RNA (sRNA) and mRNA targets of these PUPs and of a fourth predicted poly(U) polymerase, F43E2.1/PUP-4. Using genetic and RNA sequencing approaches, we identify RNA targets of each PUP and the U-tail frequency and length of those targets. At the whole organism level, PUP-1 is responsible for most sRNA U-tailing, and other PUPs contribute to modifying discrete subsets of sRNAs. Moreover, the expression of PUP-2, PUP-3, and especially PUP-4 limits uridylation on some sRNAs. The relationship between uridylation status and sRNA abundance suggests that U-tailing can have a negative or positive effect on abundance depending on context. sRNAs modified by PUP activity primarily target mRNAs that are ubiquitously expressed or most highly expressed in the germline. mRNA data obtained with a Nanopore-based method reveal that the addition of U-tails to nonadenylated mRNA is substantially reduced in the absence of PUP-3. Overall, this work identifies PUP RNA targets, defines the effect of uridylation loss on RNA abundance, and reveals the complexity of PUP regulation in C. elegans development.

The functions and mechanisms of piRNAs in mediating mammalian spermatogenesis and their applications in reproductive medicine

As the most abundant small RNAs, piwi-interacting RNAs (piRNAs) have been identified as a new class of non-coding RNAs with 24-32 nucleotides in length, and they are expressed at high levels in male germ cells. PiRNAs have been implicated in the regulation of several biological processes, including cell differentiation, development, and male reproduction. In this review, we focused on the functions and molecular mechanisms of piRNAs in controlling spermatogenesis, including genome stability, regulation of gene expression, and male germ cell development. The piRNA pathways include two major pathways, namely the pre-pachytene piRNA pathway and the pachytene piRNA pathway. In the pre-pachytene stage, piRNAs are involved in chromosome remodeling and gene expression regulation to maintain genome stability by inhibiting transposon activity. In the pachytene stage, piRNAs mediate the development of male germ cells via regulating gene expression by binding to mRNA and RNA cleavage. We further discussed the correlations between the abnormalities of piRNAs and male infertility and the prospective of piRNAs' applications in reproductive medicine and future studies. This review provides novel insights into mechanisms underlying mammalian spermatogenesis and offers new targets for diagnosing and treating male infertility.

piRNA loading triggers MIWI translocation from the intermitochondrial cement to chromatoid body during mouse spermatogenesis

The intermitochondrial cement (IMC) and chromatoid body (CB) are posited as central sites for piRNA activity in mice, with MIWI initially assembling in the IMC for piRNA processing before translocating to the CB for functional deployment. The regulatory mechanism underpinning MIWI translocation, however, has remained elusive. We unveil that piRNA loading is the trigger for MIWI translocation from the IMC to CB. Mechanistically, piRNA loading facilitates MIWI release from the IMC by weakening its ties with the mitochondria-anchored TDRKH. This, in turn, enables arginine methylation of MIWI, augmenting its binding affinity for TDRD6 and ensuring its integration within the CB. Notably, loss of piRNA-loading ability causes MIWI entrapment in the IMC and its destabilization in male germ cells, leading to defective spermatogenesis and male infertility in mice. Collectively, our findings establish the critical role of piRNA loading in MIWI translocation during spermatogenesis, offering new insights into piRNA biology in mammals.

Emerging roles and functional mechanisms of PIWI-interacting RNAs

PIWI-interacting RNAs (piRNAs) are a class of small non-coding RNAs that associate with proteins of the PIWI clade of the Argonaute family. First identified in animal germ line cells, piRNAs have essential roles in germ line development. The first function of PIWI-piRNA complexes to be described was the silencing of transposable elements, which is crucial for maintaining the integrity of the germ line genome. Later studies provided new insights into the functions of PIWI-piRNA complexes by demonstrating that they regulate protein-coding genes. Recent studies of piRNA biology, including in new model organisms such as golden hamsters, have deepened our understanding of both piRNA biogenesis and piRNA function. In this Review, we discuss the most recent advances in our understanding of piRNA biogenesis, the molecular mechanisms of piRNA function and the emerging roles of piRNAs in germ line development mainly in flies and mice, and in infertility, cancer and neurological diseases in humans.