Search of brain-enriched proteins in salivary extracellular vesicles for their use as mental disease biomarkers: A pilot study of the neuronal glycoprotein M6a

Abstract

Background: Mental disorders affect millions of people worldwide. Their etiology is complex and the fact that the main effects occur in the brain hampers biochemical diagnosis. Therefore, biomarker finding in peripheral fluids such as serum or saliva is desirable. Here, we searched for biomarkers in salivary extracellular vesicles (EVs). Then, we focused on the protein M6a, related to neuronal connectivity and associated with several mood disorders to study its usefulness in saliva for the diagnosis of depression and anxiety. Methods: Biomarker candidates were searched by proteomic analysis of human salivary EVs. M6a presence in salivary EVs was validated by transmission electron microscopy and Western blot. M6a levels were measured by ELISA in saliva samples of 88 individuals classified as control, depressed or anxious. Results: We identified ten protein candidates in salivary EVs: OLIG2, PMP2, CNP, CAMK2A, SLC25A22, MLLT11, HTR2A, MAPT, ATP2B2 and M6a, all associated with emotional disorders. Salivary M6a levels positively correlated with the scores for the perceived stress scale in individuals diagnosed with depression. Furthermore, saliva samples of depressed patients treated with serotonin reuptake inhibitors (SSRI) or benzodiazepines differed in their M6a levels with respect to untreated patients. Limitations: The main limitation of this study lies in the low number of patients involved, which warrants replication. Conclusions: Salivary EVs contain promising biomarker candidates for mental disorders. Further studies will help validate them for their potential use in diagnosis. Our results lead us to propose M6a as a workable molecule to take into account as a possible stress biomarker.