MicroRNA and microRNA Clusters in the Aging Heart Figure 12. The miR-21* passenger strand expression in comparison with miR-21guide strand and pri-mir-21 transcript in mouse hearts, from 1 month to 24 months of age. doi:10.1371/journal.pone.0034688.g012 mechanism could enhance our understanding of the role of miRNAs in the process of adult aging. The effects of these altered microRNAs and miRNA clusters on target gene expression during adult aging and senescence warrant further investigation. Exogenous miRNAs can be introduced into live cells or animals; likewise, endogenous miRNAs can also be knocked-down or deleted in cells or animals with various approaches. However, the effects of the various approaches might not be the same [38]. It is believed that the length of the 39 untranslated regions (39 UTRs) of the target messenger RNAs, the cellular proliferative status, and the abundance of target messenger RNA may affect miRNA regulation [39–42]. For instance, functional miRNA target sites tend to reside in 39 UTRs, therefore the length of the 39 UTRs often correlates with density of the miRNA target sites [41,43]. Highly proliferating cells usually express more messenger RNAs with shorter 39 UTRs, which might escape miRNA targeting, whereas low proliferating cells, including senescent cells, may express miRNAs with longer 39 UTRs [39–41]. In addition, transfected (exogenous) miRNAs may compete with endogenous miRNAs for the same protein complex, including RISC protein complex that is needed for miRNA regulation. Therefore, genes that are only targeted by endogenous miRNAs but not by the exogenous miRNA may end up relatively increased [44]. In addition, the target messenger RNA abundance could potentially dilute microRNA and siRNA activity, thereby reducing the effectiveness of miRNA regulation [45]. Nevertheless, miRNA transfection assays have been shown to target various messenger RNAs and intracellular pathways. For example, miR- 494 down-regulates IGF2BP1 and induces cellular senescence in A549 cell line [46]. In neural stem cells, miR-106b,25 target the insulin/insulin-like growth factor-1 (IGF) signaling pathways and promote neuron growth [47]. These factors (various approaches of knock-down vs. deletion, cellular proliferative status, length of 39 UTR of target, and abundance of messenger RNA target) all Figure 13. The expression of Argonaute genes. The cardiac expression of Ago1 and Ago2 in the mouse, from 1 month to 24 months of age. doi:10.1371/journal.pone.0034688.g013 PLoS ONE | www.plosone.org 9 April 2012 | Volume 7 | Issue 4 | e34688