Proteomic analysis of olfactory bulb suggests CACNA1E as a promoter of CREB signaling in microbiota-induced depression.

Major depressive disorders impact approximately 17% of the population worldwide, whose high morbidity and considerable adversity have resulted in enormous social and economic burden. In addition, clinically depressed patients often show reduced volume of olfactory bulb (OB) and decreased olfactory sensitivity. Although mounting evidence conveyed that the gut microbiota may implicate the pathophysiology of major depressive disorder (MDD) via the microbe-gut-brain axis, knowledge about its distinctive molecular mechanism is rudimentary. Herein, iTRAQ coupled with LC-MS/MS was applied to compare the OB proteome between "pathological microbiota" and "healthy microbiota" germ-free mice. A set of 367 proteins were differentially identified in the OB, including 119 up-regulated and 248 down-regulated proteins compared with the levels in controls. A combined analysis with significantly changed OB proteins from CUMS depression model supported the role of CREB signaling, whose dysregulation is likely to disrupt the axonogenesis of OB under microbiota condition. With that, the down-regulated CACNA1E and its downstream proteins (CALM/ CaMKII/ CREB/ BDNF) in CREB pathway were validated by Western blot. Meanwhile, the canonical pathways involved Nuclear Receptor Signaling highlighted the fecal microbiota transplantation (FMT) model, which would be a new breakthrough for depressive research. These findings enrich the previous research achievements about the gut microbiota in psychiatric disorders, providing a creative insight into the intricate mechanisms of OB dysfunction in depression. SIGNIFICANCE: Emerging evidence has shown that gut microbiota can greatly influence brain functions and even behaviors. As one of the post-developmental neurogenesis areas for the adult brain, the OB is becoming increasingly important in the study of the pathogenesis of depression. Using an iTRAQ-based proteomics, we identified 367 altered proteins in the OB of fecal microbiota transplanted mouse, which provide a novel insight for further research of the "microbiota-gut-brain axis". In addition, combined analyses with the CUMS depression model and the validation of key proteins by Western blot may assist in the investigation of OB dysfunction in mental sickness.