A Mitochondrial Circular RNA Linked to the TCA Cycle Depletes With Age in Human Blood Cells

Most circular RNA research focuses on transcripts from the cell's nucleus. The mitochondrial genome - a small, circular DNA molecule present in hundreds of copies per cell and responsible for encoding thirteen essential proteins of the respiratory chain - has received far less attention as a source of circular RNA. A study published in Aging-US suggests that this omission may have consequences for understanding how cells age.

Led by Hyejin Mun from the University of Oklahoma, with corresponding authors Je-Hyun Yoon at Oklahoma and Young-Kook Kim at Chonnam National University Medical School, the study profiles circular RNAs derived from the mitochondrial chromosome in peripheral blood mononuclear cells (PBMCs) from young and old human donors. The central finding: a specific mitochondrial circular RNA called circMT-RNR2 decreases significantly with age and may play a role in sustaining the energy-generating TCA cycle in younger cells.

A largely unmapped territory of the mitochondrial transcriptome

Using total RNA sequencing from both young and old PBMC cohorts, the authors found that a substantial fraction of circular RNA junctions originates from the mitochondrial genome - a finding that itself is notable, since the mechanisms that produce circular RNAs from mitochondrial transcripts are not well understood. Among all mitochondrial circular RNA junctions detected, MT-RNR2 - a gene encoding a ribosomal RNA - produced the most abundant.

The circular form, circMT-RNR2, showed lower levels in older PBMC cohorts compared with younger donors. The same depletion pattern appeared in replicative senescence experiments using human fibroblasts, suggesting the association is not specific to immune cells but may reflect a broader feature of cellular aging across tissue types.

GRSF1 connects the circular RNA to mitochondrial metabolism

A mitochondria-localized RNA-binding protein called GRSF1 interacts with both the linear and circular forms of MT-RNR2. This interaction is functionally significant: when GRSF1 expression was reduced experimentally, circMT-RNR2 levels fell, levels of two TCA cycle intermediates - fumarate and succinate - decreased, and markers of cellular senescence and mitochondrial dysfunction increased.

The chain of associations suggests a pathway: GRSF1 helps maintain circMT-RNR2, which in turn supports normal TCA cycle activity, which sustains mitochondrial energy production, which delays or prevents senescence. Loss of any link in this chain - whether through declining GRSF1 expression, reduced circMT-RNR2 stability, or direct mitochondrial damage - could accelerate the metabolic deterioration associated with cellular aging.

Limitations and questions that remain open

The study profiles associations rather than establishing direct causal mechanisms. The authors are explicit about what remains unknown: how mitochondrial circular RNAs form biologically (whether trans-splicing, back-splicing from mitochondrial transcripts, or another mechanism), how circMT-RNR2 specifically influences TCA enzyme activity at the molecular level, and whether the observed associations in human PBMCs and fibroblasts hold in vivo in animal models and in additional human cohorts representing diverse ages and health states.

The results from cell lines and human blood samples may not generalize to all tissues. Mitochondrial function varies substantially across cell types, and the contribution of circMT-RNR2 to aging in metabolically active tissues like cardiac muscle or neurons has not been tested.

Potential significance for aging research

Despite those caveats, the study opens a line of investigation that has been largely untouched. If mitochondrial circular RNAs contribute to age-related metabolic decline, they could represent targets for interventions aimed at preserving TCA cycle function in aging cells - either by maintaining circMT-RNR2 levels, stabilizing GRSF1 activity, or augmenting the downstream metabolic pathways the system appears to support.

The study's authors frame their next steps clearly: clarify biogenesis mechanisms, map direct interactions between the circular RNA and metabolic enzymes, and test the system in animal models. Those experiments will determine whether circMT-RNR2 and GRSF1 are actionable targets or primarily markers of a more complex aging process.