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piRNA - Piwi-interagerende RNA's
PIWI-interagerende RNA's, of piRNA's, zijn de meest voorkomende korte niet-coderende RNA's. Er zijn bij mensen ruim 20.000 genen gevonden die coderen…
Transposable DNA elements, or transposons, show random movement throughout the genome. These insertions that disrupt a gene can result in genomic instability, which is dangerous to a cell.
In somatic cells, transposon induced genomic instability remains limited to a single generation; however, in germ cells, these changes can be passed to future generations leading to harmful effects.
Germ cell-specific transposable elements are silenced by small non-coding regulatory RNAs known as piwi-interacting RNA or piRNA.
piRNAs are essential for proper germ cell development, and their absence can cause infertility in animals.
piRNA are a class of silencing RNAs that differ from miRNA and siRNA by three defining characteristics: length, processing mechanism, and binding with Argonaute subfamily proteins.
piRNA are 24 to 32 nucleotides in length, longer than both miRNA and siRNA which are usually 20 to 25 nucleotides long.
piRNA is processed from single-stranded mRNA without Dicer while both miRNA and siRNA are processed from double-stranded RNA by Dicer.
Each of these three types of non-coding RNA bind to Argonaute family proteins, but piRNA bind to the piwi subfamily while miRNA and siRNA bind to the AGO subfamily of proteins.
piRNA originate from piRNA clusters, specific regions of the genome that are rich in transposons.
Two phases have been proposed for the biogenesis of piRNA: the primary processing pathway and the amplification loop.
In the primary processing pathway, the transcripts from piRNA clusters are used to produce piRNA. These are loaded onto selected piwi proteins to form piRISC, an alternative form of the RNA induced silencing complex.
Primary piRNA then takes part in an amplification loop to rapidly increase the concentration of the piRNA.
piRISC binds and cleaves the complementary target RNA creating the 5’ end of a premature secondary pi-RNA. The 3’ end of the pi-RNA is processed further by other piwi proteins to resulting in a mature secondary pi-RNA.
This process repeats itself resulting in the amplification of both sense and antisense piRNA.
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Q1: How do piRNAs differ from miRNAs and siRNAs?
piRNAs are 24 to 32 nucleotides long, compared to miRNAs and siRNAs which are 20 to 25 nucleotides. piRNAs are processed from single-stranded RNA without Dicer, while small interfering RNAs post transcription require Dicer to process double-stranded RNA. Additionally, piRNAs bind to the piwi subfamily of Argonaute proteins, whereas miRNAs and siRNAs bind to the AGO subfamily.
Q2: What role do piRNAs play in germ cells?
piRNAs silence transposable elements in germ cells, preventing genomic instability that could be passed to future generations. Without piRNAs, transposon-induced mutations can accumulate and cause infertility in animals. piRNAs are essential for proper germ cell development and protect the germline from harmful genetic changes.
Q3: What are piRNA clusters and where do they originate?
piRNA clusters are specific genomic regions rich in transposons that serve as the source for piRNA biogenesis. Transcripts from these clusters are processed to produce piRNAs, which are then loaded onto piwi proteins to form piRISC complexes. These complexes are transported to the nucleus or cytoplasm depending on the specific piwi protein involved.
Q4: How does the ping-pong amplification pathway generate secondary piRNAs?
In the ping-pong amplification pathway, Aubergine-piRNA complexes bind and cleave complementary target RNA, creating the 5' end of a premature secondary piRNA. The resulting cleaved fragments are loaded onto AGO3 proteins, which further process the 3' end to generate mature secondary piRNAs. This cycle repeats, amplifying both sense and antisense piRNA populations.
Q5: What is the piRISC complex and how is it formed?
piRISC is the piRNA-induced silencing complex formed when primary piRNAs are loaded onto selected piwi proteins. This complex binds and cleaves complementary target RNAs to silence transposable elements. The formation of piRISC is a critical step in both the primary processing pathway and the ping-pong amplification loop.
Q6: Why are transposable elements dangerous to cells?
Transposable elements show random movement throughout the genome, and their insertions can disrupt genes, resulting in genomic instability. In somatic cells, this instability remains limited to a single generation. However, in germ cells, transposon-induced changes can be passed to future generations, leading to harmful effects that compromise cell viability and organism health.
Q7: What proteins are involved in the piRNA biogenesis pathway?
Multiple proteins participate in piRNA biogenesis, including piwi subfamily members like Piwi, Aubergine, and AGO3 in Drosophila, or Miwi, Mili, and Miwi2 in mammals. Tudor family proteins act as scaffolds in the ping-pong amplification pathway. These proteins work together to process piRNA transcripts and generate mature piRNAs that silence transposons.