Normalization strategies for protein quantification in plasma astrocyte-derived extracellular vesicles: a clinical applicability study

Astrocyte-derived extracellular vesicles (ADEVs) have emerged as a novel research tool in the field of central nervous system disorders. However, significant differences in yield and purity exist among extracellular vesicles (EVs) isolated by different methods, leading to considerable heterogeneity in clinical study outcomes. Therefore, establishing appropriate normalization strategies to enhance comparability across results is a key prerequisite for their clinical translation. This study included 15 patients with major depressive disorder (MDD) and 15 healthy controls (HCs). ADEVs were isolated from plasma using ultracentrifugation combined with immunoaffinity capture. Subsequently, the levels of brain-derived neurotrophic factor (BDNF), five EV biomarkers (CD9, CD63, CD81, Alix, and TSG101), and particle counts in ADEVs were quantified. In addition, plasma lipoprotein levels were measured. Our results demonstrated a lack of significant correlation between particle counts and the levels of five EV biomarkers in plasma ADEVs, whereas strong correlations were observed among the five biomarkers themselves. Normalization of BDNF levels to CD81 or CD9 revealed a significant decrease in the MDD group, whereas normalization to EV particle counts or other EV biomarkers did not show such differences. Notably, plasma levels of apolipoprotein B (APOB), low-density lipoprotein (LDL), and total cholesterol (TC) significantly interfered with the measurement of particle counts. In summary, under conventional EV isolation and detection conditions, our findings support the use of EV biomarker levels rather than particle counts as a normalization method for quantifying target proteins of ADEVs in plasma.