It takes a special kind of small RNA to try out for the PIWI team. PIWI-interacting RNAs (piRNA) are small, have and adenine at their 10th position, an uracil at their 5’ end, and 2’-O-methylation at their 3’ end. This unique physique helps piRNA work with PIWI proteins to defend the germline against an aggressive transposon offense, but two recent studies suggest that the team might have an important role in the brain as well.
piRNA in the Brain are Small and Specific
In the testes, PIWI proteins and their piRNA teammates keep long interspersed nuclear elements (LINES) from disrupting dividing cells by pulling in DNA methyltransferases to stop them from jumping through the genome. Even though the testes have the most piRNAs, a study from the lab of Dana Dolinoy (University of Michigan, USA), developed a technique to explore the characteristics of more lowly expressed piRNAs from other parts of the body.
By first selecting for small RNAs (<200bp), and then selecting for 2’-O-methylated RNA by treatment with sodium periodate, the talented team enriched for piRNAs in their sequencing libraries. Using their technique on mouse testes, kidney, heart, liver, hippocampus, and whole brain, they found that:
- Somatic piRNAs, especially those from the hippocampus, are shorter than those from the testes ( < 20bp vs. 24-32 bp), but otherwise contain the characteristic adenine and uracil signatures
- While germline piRNAs tend to align with masked repetitive sequences, somatic piRNAs are more likely to align to unique regions of the genome
- Even though most of the piRNAs identified in this study can be found in one of the four existing piRNA databases, many of them are found in just one or two tissues
- 89.8% of the piRNAs in the hippocampus are not found in any other tissue
Although they’re smaller and less expressed than in the germline, their diversity and specificity suggests that piRNA are important players in the brain, too. Their structure suggests that they interact with PIWI proteins, but their alignment to non-repetitive sequences hints that there’s more than one play in the PIWI playbook.
PIWI Proteins Play a Part in Fear Memory
The lab of Timothy Bredy (University of Queensland, Australlia) has been exploring how PIWI proteins contribute to the functioning of brain cells, and they’ve already shown that two of the team captains, Piwil1 and Piwil2 are expressed in cultured mouse neurons and can be induced by neural activity. Their new report takes it a step further and shows how knocking down (KD) the PIWI proteins in the hippocampus of adult mice affects behavior. They injected AAV vectors containing shRNA against one or both proteins to the dentate gyri of the hippocampus and found that:
- Double KD mice that are trained to associate a sound to a foot shock learn to freeze when the sound is played alone, just as well as wildtype (WT) mice, but they freeze more often than WT mice in recall tests up to a week after they stop receiving shocks
- Single KD of either of the PIWI proteins has no effect on freezing or recall
- Although the double KD has no effect on generalized anxiety behaviors or locomotion, Piwil2 KD mice were generally more active than their WT counterparts
This research suggests that, although Piwil1 and Piwil2 will have each other’s back on game day, they each support the brain in slightly different ways.
It’s too early to tell exactly how the proteins work with piRNA to tackle out of bounds transcription, but these studies suggest that the PIWI league is in for an exciting season in brain research!