A novel class of somatic small RNAs similar to germ cell pachytene PIWI-interacting small RNAs.

PIWI-interacting RNAs (piRNAs) are small noncoding RNAs that bind PIWI family proteins exclusively expressed in the germ cells of mammalian gonads. MIWI2-associated piRNAs are essential for silencing transposons during primordial germ cell development, and MIWI-bound piRNAs are required for normal spermatogenesis during adulthood in mice. Although piRNAs have long been regarded as germ cell-specific, increasing lines of evidence suggest that somatic cells also express piRNA-like RNAs (pilRNAs). Here, we report the detection of abundant pilRNAs in somatic cells, which are similar to MIWI-associated piRNAs mainly expressed in pachytene spermatocytes and round spermatids in the testis. Based on small RNA deep sequencing and quantitative PCR analyses, pilRNA expression is dynamic and displays tissue specificity. Although pilRNAs are similar to pachytene piRNAs in both size and genomic origins, they have a distinct ping-pong signature. Furthermore, pilRNA biogenesis appears to utilize a yet to be identified pathway, which is different from all currently known small RNA biogenetic pathways. In addition, pilRNAs appear to preferentially target the 3'-UTRs of mRNAs in a partially complementary manner. Our data suggest that pilRNAs, as an integral component of the small RNA transcriptome in somatic cell lineages, represent a distinct population of small RNAs that may have functions similar to germ cell piRNAs.

In silico identification of novel endo-siRNAs.

Many classes of small noncoding RNAs (sncRNAs), such as microRNAs (miRNAs) and endogenous small interfering RNAs (endo-siRNAs), have been identified as important regulators of gene expression. Endo-siRNAs represent an integral part of the endogenous RNAi pathway and have been identified in multiple organisms and cell types. Wide adoption of the next-generation deep sequencing (NGS)-based sncRNA profiling has made the identification of novel sncRNA species more accessible. However, it remains a challenge to identify novel endo-siRNAs that are not collected in the current endo-siRNA databases. We have developed an in silico method for identification of novel endo-siRNAs using small RNA NGS data. Here, we describe our protocol in detail.

Computer-assisted annotation of small RNA transcriptomes.

Small noncoding RNAs (sncRNAs) are widely expressed in the cell of almost all known species. Most sncRNAs appear to have regulatory roles, ranging from facilitating RNA production and modifications (e.g., snoRNAs) to control of mRNA stability and translational efficiency (e.g., miRNAs and endo-siRNA) and to transposon silencing (e.g., piRNAs). The affordability and efficiency of next-generation RNA deep sequencing (RNA-Seq) technologies have made sncRNA deep sequencing (sncRNA-Seq) analyses a routine in biomedical research. SncRNA-Seq analyses generate millions of reads and gigabytes of data; annotation of sncRNA-Seq data remains challenging due to a lack of comprehensive sncRNA annotation pipelines. To solve this problem, we have developed a computer-assisted sncRNA annotation pipeline, which uses open-source software and allows for not only proper classification of known sncRNAs, but also discovery of novel sncRNA species. In this chapter, we describe our sncRNA annotation protocol in detail.