Objectives. Alterations of BDNF signalling in major depression (MD) are supported by studies demonstrating decreased levels of the neurotrophin serum and plasma content in MD patients. We conducted a replication study and we performed two meta-analyses on studies analysing serum and plasma BDNF levels in MD patients. Methods. The samples were composed by 489 patients/483 controls for the meta-analysis on serum and by 161 patients/211 controls for that on plasma levels. We performed also subgroup analyses to examine whether the decrease in BDNF levels in MD was influenced by gender. Results. In the replication study we found decreased serum BDNF levels in MD patients (P<0.01) and we demonstrated that is down-regulated the mature form of the neurotrophin (mBDNF). No significant difference was evidenced for plasma BDNF levels. The meta-analyses showed a reduction of both BDNF serum (P<0.0001) and plasma levels (P=0.02) in MD. No difference in the effect size on serum BDNF was observed between males and females (P=0.18). Conclusions. In conclusion, our results provide evidence of peripheral BDNF alteration in MD and support the rationale for further investigation aiming to the identification of biomarkers for differential diagnosis and personalization of therapies in this disorder.

Consistent evidence indicates the involvement of the brain-derived neurotrophic factor (BDNF) in neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson’s disease (PD). In the present study, we compared serum BDNF in 624 subjects: 266 patients affected by AD, 28 by frontotemporal dementia (FTD), 40 by Lewy body dementia (LBD), 91 by vascular dementia (VAD), 30 by PD, and 169 controls. Our results evidenced lower BDNF serum levels in AD, FTD, LBD, and VAD patients (P<0.001) and a higher BDNF concentration in patients affected by PD (P=0.045). Analyses of effects of pharmacological treatments suggested significantly higher BDNF serum levels in patients taking mood stabilizers/antiepileptics (P=0.009) and L-DOPA (P<0.001) and significant reductions in patients taking benzodiazepines (P=0.020). In conclusion, our results support the role of BDNF alterations in neurodegenerative mechanisms common to different forms of neurological disorders and underline the importance of including drug treatment in the analyses to avoid confounding effects.

Small extracellular vesicles (EVs) are able to pass from the central nervous system (CNS) into peripheral blood and contain molecule markers of their parental origin. The aim of our study was to isolate and characterize total and neural-derived small EVs (NDEVs) and their micro RNA (miRNA) cargo in Alzheimer’s disease (AD) patients. Small NDEVs were isolated from plasma in a population consisting of 40 AD patients and 40 healthy subjects (CTRLs) using high throughput Advanced TaqMan miRNA OpenArrays^®, which enables the simultaneous determination of 754 miRNAs. MiR-23a-3p, miR-223-3p, miR-100-3p and miR-190-5p showed a significant dysregulation in small NDEVs from AD patients as compared with controls (1.16 ± 0.49 versus 7.54 ± 2.5, p = 0.026; 9.32 ± 2.27 versus 0.66 ± 0.18, p <0.0001; 0.069 ± 0.01 versus 0.5 ± 0.1, p < 0.0001 and 2.9 ± 1.2 versus 1.93 ± 0.9, p < 0.05, respectively). A further validation analysis confirmed that miR-23a-3p, miR-223-3p and miR-190a-5p levels in small NDEVs from AD patients were significantly upregulated as compared with controls (p = 0.008; p = 0.016; p = 0.003, respectively) whereas miR-100-3p levels were significantly downregulated (p = 0.008). This is the first study that carries out the comparison between total plasma small EV population and NDEVs, demonstrating the presence of a specific AD NDEV miRNA signature.