Plasma and serum exosome markers analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry coupled with electron multiplier

Although matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is a simple, rapid, and high-throughput assay, its microchannel plate (MCP) detector is limited by the low sensitivity and ion saturation effect when analyzing macromolecules. Herein, we introduced a strategy that combined MALDI-TOF MS with electron multiplier (EM) for the direct analysis of exosomal proteins isolated from human plasma and serum. The results demonstrated that EM yielded a higher sensitivity than MCP detector in high-mass range (m/z 5000–100000). Through the analysis of MALDI-TOF MS coupled with EM, chemokine (C-X-C motif) ligand 12 (CXCL12) ion at m/z 7960 and its degradation products at m/z 7927, 7587, and 7553 were identified as characteristic exosomal proteins in plasma. CXCL4 ion at m/z 7765 was identified as a characteristic protein in serum exosomes. Additionally, the peak intensity of CXCL12 and CXCL4 standards exhibited great linear relationship (CXCL12, R2 = 0.989; CXCL4, R2 = 0.986) with the concentrations (ranging from 0.1 to 20 μg/mL) when using EM as detector. In conjunction with ultrasonic assisted matrix coating technology (UAMCT), this assay repeatability in our lab has been excellent with coefficient of variation (CV%) of 4.6% for CXCL12 and 9.3% for CXCL4. Finally, the spectra demonstrated that the intensity of exosome related peaks was significantly enhanced in plasma and serum of patients with Parkinson's disease (PD) (m/z 7553, P < 0.01; m/z 7587, P < 0.01; m/z 7927, P < 0.001; m/z 7980, P < 0.001; m/z 7765, P < 0.01), Alzheimer's disease (AD) (m/z 7553, P < 0.001; m/z 7587, P < 0.001; m/z 7927, P < 0.001; m/z 7980, P < 0.001), and ischemic cerebrovascular disease (ICD) (m/z 7553, P < 0.05; m/z 7587, P < 0.05; m/z 7927, P < 0.01; m/z 7980, P < 0.05; m/z 7765, P < 0.05) compared to that in healthy persons. The fingerprint information of CXCL12 in plasma exosomes has better clinical relevance than serum exosome CXCL4 in MALDI-TOF MS analysis.