Human blood contains circulating cell-free mitochondria, but are they really functional?

Abstract

Dache et al. (2020, FASEB J. 15, e2002338–15) recently reported the presence of respiratory-competent cell-free mitochondria in human blood (up to 3.7 x 106 per mL of blood), providing exciting perspectives on the potential role of these extra-cellular mitochondria. While their evidence for the presence of cell-free mitochondria in human blood is compelling, their conclusion that these cell-free mitochondria are respiratory-competent or functional has to be re-evaluated. To this end, we evaluated the functionality of cell-free mitochondria in human blood using high-resolution respirometry and mitochondria extracted from platelets of the same blood samples as positive controls. While cell-free mitochondria were present in human plasma (i.e. significant complex IV activity), there was no evidence suggesting that their mitochondrial electron transport system (ETS) was functional (i.e. respiration rate not significantly different from 0; no significant responses to ADP, uncoupler or mitochondrial inhibitors oligomycin and antimycin A). Yet, in vitro complex IV activity was detectable and even slightly higher than levels found in mitochondria extracted from platelets, suggesting that cell-free mitochondria in human blood only retain a non-functional part of the electron transport system. Despite being unlikely to be fully functional in the narrow-sense (i.e. capable of oxidative phosphorylation), circulating cell-free mitochondria may have significant physiological roles that remain to be elucidated.

Functional platelet‐derived mitochondria induce the release of human neutrophil microvesicles

“…Similarly, the expression of cytochrome c oxidase subunit 4 (COX-IV), the final enzyme in the respiratory electron transport chain, was not affected in PMN treated with free-Mitos, whereas DAMPs treatments induced a 4.8-fold increase in COX IV levels when compared to vehicle control (Fig 1G and H). These results indicate that despite their uptake by PMN, freeMitos do not modulate the oxygen consumption rate (OCR) of PMN, further supporting evidence that freeMitos may not convey respiratory competent mitochondria to recipient cells (Al Amir Dache et al, 2020;Stier, 2021).…”

Mitochondria Can Cross Cell Boundaries: An Overview of the Biological Relevance, Pathophysiological Implications and Therapeutic Perspectives of Intercellular Mitochondrial Transfer

“…The authors provided an evaluation of 200,000 up to 3,700,000 respiratory-competent mitochondria per ml of extracted plasma, and suggested that circulating cell-free respiratory-competent mitochondria could represent a novel class of signaling organelles involved in regulatory activities and intercellular communication. Although the evidence for the presence of cell-free mitochondria in human blood is convincing, the conclusion that these mitochondria are energy competent or functional for respiration has been more recently refuted [ 140 ]. In its report, Stein evaluated the functionality of cell-free mitochondria in human blood using high-resolution respirometry and mitochondria extracted from platelets from the same blood samples as positive controls.…”

Environmental Chemical Exposures and Mitochondrial Dysfunction: a Review of Recent Literature

“…Additionally, the presence of circulating cell-free mitochondria in blood may serve as an alternative matrix for the biomarkers discussed above. Cell-free mitochondria are the presence of whole and functioning mitochondria out of the cell, which has been detected within human blood [ 56 •, 57 ]. In addition to whole mitochondria, cell-free mtDNA fragments are also detected in human blood, either encapsulated within extracellular vesicles or free-circulating.…”