Aging-Associated Mitochondrial Circular RNAs Regulate Cellular Senescence

The study was led by first author Hyejin Mun at the University of Oklahoma, with Je-Hyun Yoon (University of Oklahoma) and Young-Kook Kim (Chonnam National University Medical School) serving as corresponding authors. The investigators systematically characterized mitochondrial-derived circular RNAs in Peripheral Blood Mononuclear Cells (PBMCs) obtained from young and elderly human cohorts. Their objective was to determine how mitochondrial circular RNAs (circRNAs) and the mitochondrial RNA-binding protein GRSF1 influence mitochondrial metabolism and cellular senescence.

The authors state: “Here, we report profiles of circular RNAs annotated to mitochondrial chromosome, chrM, in young and old cohorts.”

Through total RNA sequencing of PBMCs from both age groups, complemented by mechanistic cell-based assays, the researchers observed that a substantial proportion of circular RNA back-splice junctions originate from the mitochondrial genome. Among these, MT-RNR2 generated the highest abundance of circular junctions. Notably, circMT-RNR2 expression was significantly reduced in older individuals and in replicatively senescent human fibroblasts.

Mechanistic analyses demonstrated that GRSF1, a mitochondria-localized RNA-binding protein, associates with both linear and circular forms of MT-RNR2. Functional depletion of GRSF1 led to reduced circMT-RNR2 levels, diminished tricarboxylic acid (TCA) cycle intermediates specifically fumarate and succinate and promoted mitochondrial dysfunction alongside accelerated cellular senescence. Collectively, these findings establish a functional link between mitochondrial circRNAs, mitochondrial bioenergetics, and the proliferative capacity characteristic of younger cells.

Mitochondria-localized RNA-binding protein GRSF1 binds transcripts
originated from mitochondrial genome

class= — (A) GRSF1-GFP, expressed using the split GFP system (MTS-GFP1–10/GRSF1-GFP11), localizes to mitochondria, as confirmed by co-staining with anti-GRSF1 antibody and MitoTracker. DAPI was used to stain the nucleus. White arrows in the magnified view indicate regions of GRSF1 co-localization within mitochondria. The scale bar represents 20 μm. (B) Autoradiograph of 32P-labeled RNA fragments identified from GRSF1 or IgG immunoprecipitation. (C) Example of GRSF1-bound mitochondrial transcripts with T-to-C conversion ratio and existence of circular RNA. (D) Common RNA-sequences from GRSF1 PAR-CLIP located in mitochondrial transcripts. The sequence motif and p-values are presented from. (E) Location of RNA fragments from GRSF1 PAR-CLIP located in MT-RNR2. T-to-C conversion ratio is denoted next to the coordinates.

“Taken together, our findings demonstrate the existence and possible function of circular MT-RNR2 during human aging and senescence, implicating its role in promoting the TCA cycle.”

The authors point out some important limitations and explain what needs to be studied next. First, scientists still do not fully understand how mitochondrial circular RNAs are formed, including whether a process called trans-splicing helps create them. Second, it is not yet clear how these mitochondrial RNAs directly interact with metabolic enzymes inside the cell. Third, more detailed experiments—both in living organisms and in additional groups of people are needed to confirm how circMT-RNR2 and GRSF1 affect mitochondrial energy production and the aging process.

Future research will help determine whether mitochondrial circular RNAs could be used as targets to improve mitochondrial function or slow down certain aspects of cellular aging.