Piwi-interacting RNA (piRNA) is the largest class of small non-coding RNA molecules expressed in animal cells. piRNAs form RNA-protein complexes through interactions with piwi proteins. These piRNA complexes have been linked to both epigenetic and post-transcriptional gene silencing of retrotransposons and other genetic elements in germ line cells, particularly those in spermatogenesis. They are distinct from microRNA (miRNA) in size (26–31 nt rather than 21–24 nt), lack of sequence conservation, and increased complexity.
It remains unclear how piRNAs are generated, but potential methods have been suggested, and it is certain their biogenesis pathway is distinct from miRNA and siRNA, while rasiRNAs are a piRNA subspecies.
piRNAs have been identified in both vertebrates and invertebrates, and although biogenesis and modes of action do vary somewhat between species, a number of features are conserved. piRNAs have no clear secondary structure motifs, the length of a piRNA is, by definition, between 26 and 31 nucleotides, and the bias for a 5’ uridine is common to piRNAs in both vertebrates and invertebrates. piRNAs in Caenorhabditis elegans have a 5’ monophosphate and a 3’ modification that acts to block either the 2’ or 3’ oxygen, and this has also been confirmed to exist in Drosophila melanogaster, zebrafish, mice and rats. This 3’ modification is a 2’-O-methylation; the reason for this modification is not clear, but it has been suggested to increase piRNA stability. It is thought that there are many hundreds of thousands of different piRNA species found in mammals. Thus far, over 50,000 unique piRNA sequences have been discovered in mice and more than 13,000 in D. melanogaster.
piRNAs are found in clusters throughout the genome; these clusters may contain as few as ten or up to many thousands of piRNAs and can vary in size from one to one hundred kb. The detection and annotation of piRNA clusters in genomes based on bioinformatic methods became more and more sophisticated during the past years. While the clustering of piRNAs is highly conserved across species, the sequences are not. While D. melanogaster and vertebrate piRNAs have been located in areas lacking any protein coding genes, piRNAs in C. elegans have been identified amidst protein coding genes.
In mammals, piRNAs are found both in testes and ovaries, although they only seem to be required in males. In invertebrates, piRNAs have been detected in both the male and female germlines.
The biogenesis of piRNAs is not yet fully understood, although possible mechanisms have been proposed. piRNAs show a significant strand bias, that is, they are derived from one strand of DNA only, and this may indicate that they are the product of long single stranded precursor molecules. A primary processing pathway is suggested to be the only pathway used to produce pachytene piRNAs; in this mechanism, piRNA precursors are transcribed resulting in piRNAs with a tendency to target 5’ uridines. Also proposed is a ‘Ping Pong’ mechanism wherein primary piRNAs recognise their complementary targets and cause the recruitment of piwi proteins. This results in the cleavage of the transcript at a point ten nucleotides from the 5’ end of the primary piRNA, producing the secondary piRNA. These secondary piRNAs are targeted toward sequences that possess an adenine at the tenth position. Since the piRNA involved in the ping pong cycle directs its attacks on transposon transcripts, the ping pong cycle acts only at the level of transcription. One or both of these mechanisms may be acting in different species; C. elegans, for instance, does have piRNAs, but does not appear to use the ping pong mechanism at all.
A significant number of piRNAs identified in zebrafish and D.melanogaster contain adenine at their tenth position, and this has been interpreted as possible evidence of a conserved biosynthetic mechanism across species. Ping-pong signatures have been identified in very primitive animals such as sponges and cnidarians, pointing to the existence of the ping-pong cycle already in the early branches of metazoans.
The wide variation in piRNA sequences and piwi function over species contributes to the difficulty in establishing the functionality of piRNAs. However, like other small RNAs, piRNAs are thought to be involved in gene silencing, specifically the silencing of transposons. The majority of piRNAs are antisense to transposon sequences, suggesting that transposons are the piRNA target. In mammals it appears that the activity of piRNAs in transposon silencing is most important during the development of the embryo, and in both C. elegans and humans, piRNAs are necessary for spermatogenesis.
piRNA has a role in RNA silencing via the formation of an RNA-induced silencing complex (RISC). piRNAs interact with piwi proteins that are part of a family of proteins called the Argonautes. These are active in the testes of mammals and are required for germ-cell and stem-cell development in invertebrates. Three piwi subfamily proteins - MIWI, MIWI2 and MILI - have been found to be essential for spermatogenesis in mice. piRNAs direct the piwi proteins to their transposon targets. A decrease or absence of PIWI protein expression is correlated with an increased expression of transposons. Transposons have a high potential to cause deleterious effect on their host, and, in fact, mutations in piRNA pathways are found to reduce fertility in D.melanogaster. However, piRNA pathway mutations in mice do not demonstrate reduced fertility; this may indicate redundancies to the piRNA system. Further, it is thought that piRNA and endogenous small interfering RNA (endo-siRNA) may have comparable and even redundant functionality in transposon control in mammalian oocytes.
piRNAs appear to have an impact on particular methyltransferases that perform the methylations which are required to recognise and silence transposons, but this relationship is not well understood.
piRNAs can be transmitted maternally, and based on research in D. melanogaster, piRNAs may be involved in maternally derived epigenetic effects. The activity of specific piRNAs in the epigenetic process also requires interactions between piwi proteins and HP1a, as well as other factors.
Recent discovery also show, the existence of snoRNA, microRNA, piRNA characteristics in a novel non-coding RNA: x-ncRNA and its biological implication in Homo sapiens.
Major advances in the piRNA field have been achieved thanks to the use of next-generation sequencing techniques, such as Solexa and 454. These techniques allow analysis of highly complex and heterogeneous RNA populations like piRNAs. Due to their small size, expression and amplification of small RNAs can be challenging, so specialised PCR-based methods have been developed in response to this difficulty.
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- piRNA Bank A web resource on classified and clustered piRNAs
- proTRAC - a software for probabilistic piRNA cluster detection, visualization and analysis
- piRNA cluster - database