MicroRNAs identified in highly purified liver-derived mitochondria may play a role in apoptosis

MicroRNAs (miRNAs) are a class of small approximately 22 nt noncoding (nc) RNAs that regulate gene expression post-transcriptionally by direct binding to target sites on mRNAs. They comprise more than 1,000 novel species in mammalian cells and exert their function by modulating gene expression through several different mechanisms, including translational inhibition, and/or degradation of target mRNAs. Mitochondria maintain and express their own genome, which is distinct from the nuclear transcriptional and translational apparatus. Thus, they provide a potential site for miRNA mediated post-transcriptional regulation. To determine whether they maintain a unique miRNA population, we examined the miRNA profile from highly purified and RNase treated mitochondria from adult rat liver. Fifteen miRNAs were identified by microarray analysis of which, five were confirmed by TaqMan 5'nuclease assays using rat specific probes. Functional analysis of the miRNAs indicated that they were not targeted to the mitochondrial genome nor were they complementary to nuclear RNAs encoding mitochondrial proteins. Rather, the mitochondria-associated miRNAs appear to be involved in the expression of genes associated with apoptosis, cell proliferation, and differentiation. Given the central role that mitochondria play in apoptosis, the results suggest that they might serve as reservoirs of select miRNAs that may modulate these processes in a coordinate fashion.