Exosomes are vesicles secreted by cells that are widely present in various bodily fluids such as blood, urine, saliva, milk, and have a particle size ranging from 30 to 150 nanometers. In recent years, with the deepening understanding of cellular structures, it has been discovered that exosomes play crucial roles in numerous physiological and pathological activities, including antigen presentation in immunity, tumor growth and migration, and tissue damage repair. Generally, the formation of exosomes undergoes a series of processes, including membrane invagination of early endosomes, the formation of multivesicular bodies, and the release and secretion of vesicles. Exosomes can encapsulate biomolecules from donor cells and contain some characteristic substances commonly found in exosomes. Common exosomal cargoes include lipids, the tetraspanin protein family (such as CD9, CD63, CD81), major histocompatibility complexes, growth factors, adhesion proteins, heat shock proteins, and nucleic acids. Among these, the tetraspanin protein family and heat shock proteins are often used as markers for the extraction, separation, and analytical identification of exosomes.

The diverse biomolecules contained within exosomes, such as proteins, lipids, DNA, mRNA, and miRNAs, have garnered significant attention in biomedical applications. Exosomes serve as promising biomarkers for disease diagnosis. As different cells secrete exosomes with distinct compositions and functions, the widespread presence and ease of detection of exosomes make them an excellent candidate for research. Utilizing exosomes for non-invasive disease detection and diagnosis is a viable and highly beneficial approach. Exosomes contain various types of proteins, including membrane proteins and cytosolic proteins, which can reflect the function and status of cells to a certain extent. Currently, numerous exosomal proteins have been identified as potential biomarker molecules for disease diagnosis, encompassing conditions such as liver diseases, kidney diseases, brain disorders, neurological diseases, and cancers.

1. Liver Diseases

In cirrhosis, early-stage hepatocellular carcinoma (HCC), and advanced HCC, the number of serous exosomes is significantly higher than in normal liver tissue. Related research indicates that exosomes play a crucial role in the early detection of HCC. Specifically, miR-155 in serum exosomes is closely associated with the progression of liver cirrhosis and clinical prognostic indicators, suggesting that exosomes enriched with miR-155 could serve as a non-invasive biomarker for the diagnosis and progression of liver fibrosis. In a mouse model of CCl4-induced liver fibrosis, macrophage-derived exosomes treated with lipopolysaccharides highly express miR-500, which promotes hepatic stellate cell activation and liver fibrosis by inhibiting MFN2. Therefore, miR-500 in serum exosomes can also be used as a biomarker for liver fibrosis. Furthermore, experiments have shown that microRNA-214 in exosomes derived from mouse or human hepatic stellate cells epigenetically regulates connective tissue growth factor, leading to hepatocellular carcinogenesis. Additionally, research has found that high expression of miR-1247-3p in serum exosomes from patients with HCC is positively correlated with lung metastasis of the tumor.

In the serum exosomes of patients with non-alcoholic fatty liver disease (NAFLD), the expressions of miR-122-5p, miR-1290, miR-27b-3p, miR-192-5p, miR-148a-3p, and miR-99a-5p are significantly upregulated. Among them, the upregulation of miR-122-5p is the most pronounced, with a sensitivity of up to 93.4% for diagnosing NAFLD. Research reports have indicated that nine exosomal miRNAs (miR-34a, miR-192, miR-27b, miR-122, miR-22, miR-21, miR-197, miR-30c, and miR-16) are associated with the severity of NAFLD. In particular, miR-34a, miR-192, and miR-122 are correlated with non-alcoholic steatohepatitis (NASH), while miR-30c is associated with liver fibrosis. Higher levels of miR-34a and miR-122 are found in serum, and miR-122, miR-192, and miR-27b can predict the occurrence and progression of NAFLD with high accuracy. Additionally, compared to patients with drug-induced liver injury, miR-192, miR-27b, miR-22, miR-197, and miR-30c exhibit better specificity for NAFLD. In terms of specificity and sensitivity for NAFLD diagnosis, the combined detection of multiple exosomal miRNAs offers superior diagnostic value.

2.Kidney Diseases

Research has found that the expression of miR-93-5p in serum and urinary exosomes is associated with tubular interstitial injury. The expression level of miR-29c in urine decreases with the progression of renal fibrosis, suggesting that miR-29c in urinary exosomes may serve as an indicator of renal interstitial fibrosis (RIF). Scholars have discovered that high levels of miR-19b-3p in urinary exosomes correlate with the severity of tubular interstitial inflammation in patients with diabetic nephropathy. Furthermore, studies have shown that compared to healthy controls, the expression of mRNA-CD2AP in urinary exosomes from patients with kidney disease is downregulated and is associated with renal function and the severity of renal fibrosis. In urinary exosomes from patients with chronic kidney disease (CKD), the expression levels of miR-29 and the miR-200 family are significantly downregulated. The expression level of miR-29c in urinary exosomes can reflect renal function and the degree of histological fibrosis, indicating that miR-29c is a novel non-invasive biomarker for renal fibrosis.

3.Brain Diseases

Research has revealed that serum exosomal miR-24 can reflect the severity of carotid plaque sclerosis in patients with lacunar infarction, making it highly valuable for predicting the occurrence of progressive lacunar infarction in elderly patients. Study data indicate that miR-454-3p can serve as an exosomal biomarker for glioma and may potentially be developed into a novel therapy for this condition. Additionally, scholars have discovered that compared to the control group, the expression levels of serum exosomal miR-9 and miR-12 are significantly elevated in patients with acute ischemic stroke. These findings suggest that serum exosomal miR-9 and miR-12 can be used for diagnosing acute ischemic stroke and assessing its severity.

4.Neurological Diseases

Studies have shown that the expression of exosomal miR-1224-5p is significantly elevated in patients with myasthenia gravis (both ocular and generalized types), with the highest levels observed in the generalized type. This indicates that the relative expression level of miR-1224-5p positively correlates with the Quantitative Myasthenia Gravis Score (QMGS). Additionally, research suggests that plasma exosomal miRNA miR-342-3p may emerge as a biomarker for the diagnosis of Alzheimer's disease (AD). It has been found that the levels of exosomal miR-135a and miR-384 are upregulated, while miR-193b is downregulated in the peripheral blood of AD patients. Exosomal miRNAs in both peripheral blood and cerebrospinal fluid (CSF) hold promise as potential biomarkers for diagnosing AD.

Furthermore, reports indicate that compared to healthy controls, patients with Parkinson's disease (PD) exhibit downregulated expression of miR-1 and miR-19b-3p, and upregulated expression of miR-153, miR-409-3p, and miR-10a-5p in their CSF exosomes. These findings suggest that miRNAs such as miR-19b, miR-195, and miR-24 could serve as potential biomarkers for the diagnosis of PD, offering both specificity and sensitivity.

5.Cancer Diseases

Exosomes carry DNA, RNA, proteins, and metabolites that can influence recipient cells through autocrine and paracrine mechanisms, playing crucial roles in tumor initiation, progression, immunity, and drug resistance. Research has demonstrated that exosome analysis can aid in tumor diagnosis. For instance, the levels of Glypican-1 (GPC1) in circulating exosomes from patients with pancreatic ductal adenocarcinoma (PDAC) and colorectal cancer (CRC) are significantly higher compared to healthy controls. Additionally, Collapsin Response Mediator Protein-2 (CRMP-2) has been detected in exosomes secreted by colon adenocarcinoma cell lines Colo205 and SW480, providing a basis for further distinguishing CRC patients from healthy individuals.

Furthermore, the detection of prostate-specific membrane antigen (PSMA)-positive exosomes in urine has been reported to enhance the diagnosis of prostate cancer. Scholars have developed an extracellular array technology capable of analyzing 37 lung cancer-related protein levels in exosomes. This method, based on a 10 μL plasma sample, can distinguish patients with non-small cell lung cancer from healthy individuals with an accuracy rate of 75%. Membrane proteins and miRNAs in exosomes serve as biomarkers for early detection and prognosis monitoring of pancreatic cancer, potentially contributing to reduced mortality rates. An aptamer-based nanoplatform has also been constructed to detect PSMA-positive exosomes in urine for the diagnosis of prostate cancer.

With the continuous deepening of exosome research, there has been increasing insight into their structural characteristics, basic composition, biogenesis, morphological features, and intercellular functions. Through the analysis of exosomes, numerous tumor-related biomarkers have been discovered, such as HER2, EpCAM, MUC1, CA125, PTK7, and PSA. Based on these newly identified exosomal marker proteins, the recognition and detection of exosomes from different cellular origins have been achieved, enriching the efficient isolation and analysis methods of exosomes. This has laid a solid foundation for the application of exosomes in cancer diagnosis and treatment.

However, the complete picture of exosome biogenesis, secretion, target cell uptake, and functions remains elusive. Questions such as whether exosomes can grow, divide in appropriate environments, participate in signal transduction mechanisms, or possess autonomous biochemical reactions await further clarification. The existing exosome isolation techniques also require refinement, with the primary challenge being the achievement of high yield, low damage, high recovery rates, and high throughput simultaneously. In terms of exosome analysis, the challenges lie in the selection of surface targets and the identification of exosome origins.

References

[1] Peng Wenjin, Wu Gege, Li Li. Research Progress in Exosomes and Disease Detection and Diagnosis [J]. Scientific Consulting (Science and Technology · Management), 2023, (06): 142-147.

[2] Zhou Yubin. Exosomes and Their Applications in Diagnosis and Treatment [J]. Journal of Guangdong Medical University, 2022, 40(05): 481-485+491.

[3] Wang Yutong, Hou Shiyuan, Lv Yunhua, Ma Ruixue, Liu Ziyu, Wu Xing'an, Liu Rongrong. Research Progress on Exosome Isolation and Detection Methods [J]. Journal of Air Force Medical University, 2022, 43(06): 641-645.

About the Author

Xiaonisha, a food technology professional holding a Master's degree in Food Science, is currently employed at a prominent domestic pharmaceutical research and development company. Her primary focus lies in the development and research of nutritional foods, where she contributes her expertise and passion to create innovative products.