Electron microscopy visualization of cell-free mitochondrial DNA-containing extracellular vesicles in human plasma, serum, and saliva

Using electron microscopy to analyze human biofluids, this study demonstrates that cell-free mitochondrial DNA is often associated with double-membrane, mitochondria-like particles rather than existing as naked DNA, suggesting a potential role in intercellular mitochondrial transfer or signaling.

The Gist

Seeing the Hidden Cargo of Human Biofluids

When scientists study human blood, saliva, or serum, they often look for tiny fragments of DNA to understand health and disease. One specific type of interest is mitochondrial DNA. Mitochondria are the parts of a cell that produce energy. Because they have their own unique DNA, finding pieces of this DNA floating freely in the body can act as a signal. For a long time, many researchers assumed that this DNA traveled through the body as naked, broken fragments. Because naked DNA can trigger inflammation, this led to the idea that circulating mitochondrial DNA is primarily a marker of inflammation or cellular damage.

In this study, researchers used high-powered electron microscopy to look much closer at what is actually floating in these fluids. Instead of just measuring the amount of DNA, the researchers isolated particles from plasma, serum, and saliva and imaged them to see their physical structure.

The researchers identified 14 different types of particles in these fluids. Among these were structures that looked remarkably like intact mitochondria. These specific particles had a double membrane and internal folds, which are the physical hallmarks of a mitochondrion. The study found these double-membrane structures in all the fluids tested, though they were most common in plasma and least common in saliva.

By combining these images with DNA measurements, the researchers found a connection between the physical structures and the DNA content. In people with higher levels of mitochondrial DNA in their plasma, there were more of these double-membrane, mitochondria-like particles. This suggests that a large portion of the mitochondrial DNA found in the blood is not actually floating around as naked, broken fragments. Instead, it appears to be packaged inside whole mitochondria or small, membrane-bound bubbles called extracellular vesicles.

These findings challenge the assumption that circulating mitochondrial DNA is mostly composed of pro-inflammatory, naked fragments. If the DNA is tucked safely inside a membrane, it may not trigger the same inflammatory responses. Instead, the presence of these intact-looking structures might suggest that cells are using them to send signals or transfer energy and components between different parts of the body.

The paper provides a detailed catalog and an image bank of these particles. This inventory serves as a resource for future studies, helping researchers choose the right types of blood tubes and technical methods when they want to study how mitochondria and their DNA move through the human body.

Technical Summary

Technical Summary: Electron Microscopy Visualization of Cell-Free Mitochondrial DNA–Containing Extracellular Vesicles

Problem

The biological nature of cell-free mitochondrial DNA (cf-mtDNA) and extracellular mitochondria (ex-Mito) in human biofluids remains poorly defined. A central debate in the field is whether cf-mtDNA exists primarily as "naked," fragmented DNA—which is known to be pro-inflammatory by activating receptors like TLR9—or if it is encapsulated within intact mitochondria or extracellular vesicles (EVs). Furthermore, the composition and origin of these particles vary significantly depending on the biofluid used (plasma, serum, or saliva) and the technical methods of collection (e.g., anticoagulant type), complicating the standardization of cf-mtDNA as a biomarker.

Methodology

The researchers conducted a descriptive morphometric and quantitative analysis using samples from ten healthy adult volunteers.

• Sample Collection: The study utilized six distinct biofluid preparations: three types of plasma (citrate, heparin, and EDTA), two types of serum (red top and gold top), and saliva.
• Isolation: Extracellular vesicles (EVs) were isolated from these biofluids using Size-Exclusion Chromatography (SEC) via the Izon qEV system. This allowed for the separation of particles based on size.
• Visualization: The isolated pellets were stabilized with formaldehyde and analyzed using Transmission Electron Microscopy (TEM) to identify and categorize particle morphologies.
• Quantification: To correlate morphology with genetic content, the researchers performed qPCR via alkaline lysis to quantify the concentration of cf-mtDNA (targeting the mtND1 gene) across the original biofluids and the various SEC fractions.
• Analysis: A catalog of 14 distinct particle types was established based on size, electron density, and membrane structure. Statistical correlations were performed between particle abundance and cf-mtDNA levels.

Key Contributions

1. Morphological Catalog: The study provides a comprehensive "image inventory" of 14 distinct particle types circulating in human biofluids, ranging from large multi-vesicular structures (platelet-like) to small, electron-dense vesicles.
2. Identification of ex-Mito-like Particles: The researchers identified a specific class of "Type F" particles—double-membrane vesicles containing internal cristae-like structures—which are morphologically consistent with mitochondria.
3. Correlation of Structure and Content: The study links the presence of specific membrane-bound structures directly to the concentration of cf-mtDNA, providing visual evidence for the "encapsulated" state of mitochondrial material.

Results

• Ubiquity of Mitochondria-like Structures: Double-membrane, ex-Mito-like particles (Type F) were found in all tested biofluids, with the highest relative abundance in plasma (10.28%), followed by serum (1.71%) and saliva (~0.89%).
• cf-mtDNA Localization: qPCR results confirmed that cf-mtDNA was significantly higher in the early SEC fractions (containing the largest particles) and nearly absent in the post-centrifugation supernatant, proving that the DNA is predominantly membrane-bound rather than "naked."
• Statistical Correlation: Preliminary correlation analysis showed that variation in cf-mtDNA levels was largely explained by the abundance of specific vesicles: Type F (ex-Mito-like) in EDTA plasma and Type H (small granular vesicles) in red top serum.
• Biofluid Diversity: The study highlighted that saliva is dominated by small, electron-dense amorphous bodies (Type G), whereas plasma is dominated by small granular vesicles (Type H).

Significance

This research challenges the prevailing assumption that cf-mtDNA in healthy individuals is primarily a pro-inflammatory, naked fragment. By demonstrating that much of this DNA is encased within vesicles or whole mitochondria, the study suggests that cf-mtDNA may serve functional roles—such as intercellular signaling or mitochondria transfer—rather than merely acting as a marker of inflammation or cellular damage.

Technically, the paper provides a critical resource for future researchers, emphasizing that biofluid selection (e.g., anticoagulant type) and sample processing are vital, as they significantly alter the profile of circulating mitochondrial components. This work lays the foundation for more accurate diagnostic applications and a deeper understanding of mitochondrial biology in systemic health and disease.