Active turnover modulates mature microRNA activity in C.elegans

A paper published in a september issue of nature implicates 5-3' exoribonuclease XRN-2 as a regulator of functional mature microRNAs in c. elegans. The biogenesis pathway of microRNAs has been characterized up until the point where the mature strand is loaded into RNA binding protein Argonaute forming the RNA Induced Silencing Complex. This complex acts to either signal target mRNA for degradation or translational repression. Understanding the microRNA biogenesis pathway is incredibley important for many reasons, and elucidating the fate of mature microRNAs after they have been processed is probably most important as the mature form of some microRNAs like let-7 are shown to be downregulated in various cancer cells.

In this publication, the authors show that XRN-2 depletion leads to an increase in mature let-7 in vivo and in vitro. For there in vivo assay, larval worms were synchronized and fed bacteria that expressed double stranded RNA in order to knockdown different nucleases for there screen. Worms were let-7 n2853 mutants that rupture through their vulva due to less let-7 being made relative to wild-type. This particular let-7 allele was used for the RNAi screen because any nuclease that suppressed the bursting vulva phenotype would reveal a negative regulator of let-7. From the screen the authors found that 98% of the worms showed suppression on XRN-2. Northern blot analysis confirmed that there was an increase in mature levels of let-7 on XRN-2 RNAi without any affects on the pri and pre forms of microRNAs. Following there in vivo assay, the authors developed a biochemical approve to recapitulate what they showed from there screen. Using worm lysate that was both positive and negative for XRN-2, the researchers revealed that microRNA degradation was dependent on XRN-2 being present in the lysate. Their expirements evinced a role for XRN-2 specific for degradation of single stranded mature microRNAs that are not bound to any targets. Overall this finding is very significant if in fact XRN-2 does cause downregulation of active microRNAs. However, other publications done in other models show XRN-2 to have completely different roles. In arabidopsis, XRN-2 degrades the loop sequence of pre-miRNA without affecting levels of the mature form. In yeast, XRN-2 has been showed to be in involved in clearing away inactive tRNAs and finally, in HeLa cells, XRN-2 was shown to be a nuclear protein that degrades miRNA transcripts. It will be interesting to see other researchers can confirm that XRN-2 is affecting mature microRNAs as this could possible lead to new theraputic measures for diseases caused by aberrant microRNA expression.