Can Extracellular Vesicles Serve as a 'Liquid Biopsy' for Neurological Disorders?

What you need to know

• Extracellular vesicles (EVs) are small particles released by cells that carry proteins, lipids, and genetic material. They can be isolated from blood and other bodily fluids.

• Brain cell-derived EVs may serve as biomarkers for neurological disorders like Alzheimer’s disease, Parkinson’s disease, and depression.

• Analyzing the contents of brain-derived EVs in blood samples could potentially allow for earlier and less invasive diagnosis of brain diseases.

• While promising, there are still challenges in isolating brain-specific EVs and standardizing methods before they can be used clinically.

Extracellular vesicles as a window into the brain

Diagnosing diseases that affect the brain has always been challenging. Unlike other organs, we can’t easily take a sample of brain tissue to look for signs of disease. Brain imaging techniques like MRI scans can show structural changes, but often only after significant damage has already occurred. Blood tests for brain diseases have been limited because the blood-brain barrier prevents many molecules from entering the bloodstream.

However, a new approach using tiny particles called extracellular vesicles (EVs) is showing promise as a way to gain insight into the brain through a simple blood test. EVs are small sacs released by cells throughout the body, including brain cells. They contain proteins, genetic material, and other molecules from their cell of origin. Importantly, some EVs from brain cells are able to cross the blood-brain barrier and enter the bloodstream.

This means that by isolating and analyzing brain-derived EVs from blood samples, researchers may be able to detect molecular changes happening in the brain - potentially before symptoms appear. The ability to diagnose brain disorders earlier and through a simple blood draw, rather than more invasive procedures, could be transformative for conditions like Alzheimer’s disease, Parkinson’s disease, and other neurological disorders.

What are extracellular vesicles?

Extracellular vesicles are small membrane-enclosed sacs released by cells. They range in size from about 30-1000 nanometers - much smaller than cells themselves. There are a few main types of EVs:

• Exosomes: 30-150 nm vesicles formed inside cells and released when multivesicular bodies fuse with the cell membrane
• Microvesicles: 100-1000 nm vesicles that bud directly from the cell membrane
• Apoptotic bodies: Larger vesicles (500-4000 nm) released by dying cells

EVs contain proteins, lipids, and nucleic acids like DNA and RNA from their cell of origin. They play important roles in cell-to-cell communication by transferring these molecules between cells. EVs have been found in many bodily fluids including blood, urine, saliva, and cerebrospinal fluid.

Importantly, the contents of EVs often reflect the state of their parent cell. This means analyzing EVs could provide information about cellular changes happening in different tissues throughout the body. For neurological disorders, brain-derived EVs in the blood may offer a unique window into molecular changes occurring in the brain.

Isolating brain-derived extracellular vesicles

One of the key challenges in using EVs as biomarkers for brain disorders is isolating brain-specific EVs from the mixture of vesicles in blood. Researchers have developed methods to enrich for EVs from specific brain cell types using antibodies that recognize proteins found on the surface of those cells. Some examples include:

• Neuron-derived EVs (NDEs): Isolated using antibodies against L1CAM, NCAM, or SNAP-25
• Astrocyte-derived EVs (ADEs): Isolated using antibodies against GLAST
• Microglia-derived EVs (MDEs): Isolated using antibodies against TMEM119
• Oligodendrocyte-derived EVs (ODEs): Isolated using antibodies against MOG or PDGFR-α

The process typically involves first isolating total EVs from blood plasma or serum, then using antibodies to capture the brain cell-specific EVs. The captured EVs can then be analyzed for their contents using various techniques.

While these methods have shown promise, there are still challenges in ensuring the specificity and purity of isolated brain-derived EVs. Many of the surface proteins used are not exclusive to brain cells. Ongoing research is focused on identifying more specific markers.

Potential biomarkers in brain-derived EVs

Once isolated, brain-derived EVs can be analyzed for potential biomarkers of neurological disorders. Some of the main types of molecules being investigated include:

Proteins

Many studies have looked at proteins in NDEs as biomarkers for Alzheimer’s disease. Key findings include:

• Higher levels of tau and phosphorylated tau in NDEs from Alzheimer’s patients
• Increased amyloid-β peptides in NDEs from Alzheimer’s patients
• Lower levels of synaptic proteins like SNAP-25 and PSD-95 in NDEs from Alzheimer’s patients

For Parkinson’s disease, higher levels of α-synuclein have been found in NDEs from patients compared to healthy controls.

microRNAs

MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression. Altered miRNA levels in brain-derived EVs have been associated with various disorders:

• Decreased miR-132 and miR-212 in NDEs from Alzheimer’s patients
• Increased miR-223 and miR-190a-5p in NDEs from Alzheimer’s patients
• Altered levels of miR-331-5p, miR-505, and others in EVs from Parkinson’s patients

Lipids and metabolites

While less studied so far, the lipid and metabolite content of brain-derived EVs may also hold potential as biomarkers. For example, altered levels of ceramide and gangliosides have been found in ADEs exposed to amyloid-β peptides.

Potential applications

Analysis of brain-derived EVs could have several important applications for neurological disorders:

Earlier diagnosis

Many studies have found changes in EV biomarkers before clinical symptoms appear. This could allow for earlier diagnosis and intervention.

Disease monitoring

EV biomarkers may be useful for tracking disease progression and response to treatments over time through repeated blood tests.

Drug development

EV analysis could help identify new drug targets and be used to assess the effects of potential therapies.

Personalized medicine

Patterns of EV biomarkers may help predict which patients will respond best to different treatments.

Challenges and limitations

While brain-derived EVs show great promise as biomarkers, there are still several challenges to overcome before clinical use:

• Ensuring specificity of isolation methods for brain-derived EVs
• Standardizing protocols for EV isolation and analysis across labs
• Developing sensitive assays to detect low-abundance molecules in EVs
• Validating findings in large, diverse patient populations
• Determining which combinations of biomarkers are most useful clinically

Additionally, more research is needed to understand exactly how brain-derived EVs enter the bloodstream and whether they accurately reflect the state of brain cells.

Conclusions

• Brain-derived extracellular vesicles in blood samples offer a promising new approach for biomarker discovery in neurological disorders.

• Analysis of proteins, RNAs, lipids, and metabolites in these vesicles may allow for earlier and less invasive diagnosis of conditions like Alzheimer’s and Parkinson’s disease.

• While showing great potential, more research and standardization is needed before brain-derived EVs can be used as clinical biomarkers.

• If successful, this “liquid biopsy” approach could transform how we diagnose, monitor, and treat disorders of the brain.

Extracellular vesicles are opening up an exciting new window into the molecular changes happening in the brain. As isolation and analysis methods continue to improve, these tiny messengers may yield big insights into neurological health and disease.