Elsevier

Journal of Psychiatric Research

Volume 133, January 2021, Pages 166-173
Journal of Psychiatric Research

Analysis of blood mature BDNF and proBDNF in mood disorders with specific ELISA assays

https://doi.org/10.1016/j.jpsychires.2020.12.021Get rights and content

Abstract

Previous studies showed that blood BDNF levels in mood disorders were reduced. However, little is known about the changes of BDNF and its precursor proBDNF in lymphocytes. In addition, earlier studies using commercial ELISA kits cannot distinguish mature BDNF from proBDNF. We aimed to investigate the change of mBDNF and proBDNF levels in the peripheral blood and their diagnostic value in the mood disorders using a specific Enzyme-Linked Immunosorbent Assay (ELISA). Serum mBDNF levels were significantly decreased in major depressive disorder (MDD) (n = 90) and bipolar disorder (BD) (n = 15) groups (P < 0.0001), whereas there was no significant change in suicidal group (n = 14) compared to the control group (n = 96). In the subgroups of MDD, the serum mBDNF level in MDD patients with severe symptoms was significantly lower than that with moderate symptoms (P < 0.05). The serum mBDNF levels in antidepressant-free patients were significantly lower than in antidepressant-treated patients (P < 0.01). Serum mBDNF yielded good diagnostic effectiveness for MDD and BD with sensitivity and specificity around 80–83%. The levels of mBDNF, proBDNF and its receptor sortilin were upregulated in lymphocytes of MDD patients relative to control subjects. Specific ELISA assays for mature BDNF confirmed the reduction of serum mBDNF level in MDD and BD. The measurement of mBDNF level could be a potential diagnostic marker with a cut-off point at 12.4 ng/ml. Upregulations of proBDNF and mBDNF in lymphocytes of MDD patients might be considered as novel pathological biomarkers for MDD.

Introduction

Mood disorders are a group of human mental diseases that affect millions of individuals worldwide, and the leading causes of morbidity and mortality in young people (James et al., 2018). The most prevalent mood disorder is major depressive disorder (MDD). Clinical management and therapeutic intervention work well only in a subpopulation of patients, and about one-third of MDD patients are refractory to antidepressants or alternate therapies (Caraci et al., 2018; Johnston et al., 2019). The reason underlying the effectiveness in response to interventions is not fully understood, and it may be related to the complicated pathogenic mechanisms in these patients.

The neurotrophic hypothesis was proposed 20 years ago based on the evidence from both animal and human studies (Groves, 2007; Cai et al., 2015). Accumulating evidence shows that patients with MDD have lower levels of brain-derived neurotrophic factor (BDNF) in their blood (Molendijk et al., 2014; Castrén and Kojima, 2017; Schroter et al., 2020). Reduced blood BDNF levels were also found in other mood disorders such as bipolar disorder (BD) (Polyakova et al., 2015; Chiou and Huang, 2019). BDNF gene products consist of three different forms, such as mature BDNF (mBDNF), the precursor of BDNF (proBDNF) and the prodomain of BDNF and play different biological functions (Hempstead, 2015). Mature BDNF is well known for its neurotrophic and neuroprotective functions, whereas proBDNF and the prodomain may have an opposing function to mBDNF (Hempstead, 2014). As mBDNF and proBDNF have such different biological activities, it is essential to distinguish between these two proteins and detect their level changes accordingly (Hashimoto, 2010).

A variety of measurement methods were used for the detection of BDNF in the peripheral blood (Teche et al., 2013). Most previous human studies used commercially available ELISA assay kits to quantify blood BDNF levels from patients with mood disorders. Studies have shown that some of these kits are not able to distinguish proBDNF from mBDNF (Hashimoto, 2010; Lim et al., 2015). To solve this problem, we have developed BDNF and proBDNF ELISA kits which can specifically detect these proteins respectively. The new sandwich ELISA uses a combination of antibodies which capture both full-length and mature BDNF and detect them with biotin-labelled antibodies respectively. The cross-reactivity between mBDNF and proBDNF is less than 5% compared to the other commercial ELISA kits, which were around 33–91%. Therefore, our new ELISA assay showed high specificity in the detection of precise levels of mBDNF and proBDNF in human and rodent samples which was validated by western blots using commercially available antibodies (Lim et al., 2015).

Inflammation has been proposed as one of the pathological mechanisms of MDD (Arteaga-Henriquez et al., 2019; Pitsillou et al., 2020). Systemic administration of Toll receptor 4 activator lipopolysaccharide (LPS) to rodents causes similar cytokines alterations in the serum and brain tissues (He et al., 2020), as well as elicits depressive behaviours (Girard et al., 2019) and increases the level of proBDNF and its receptor in lymph nodes (Wang et al., 2019). Our previous studies also observed increased levels of proBDNF and its receptors in lymphocytes of MDD patients by Western blot analysis (Zhou et al., 2013). These findings suggest that mBDNF and proBDNF in lymphocytes may play a role in MDD.

In the present study, our overall hypothesis is that proBDNF and mBDNF levels are altered in patients with mood disorders and can be a biomarker for diagnosis and therapeutic prognosis. Specifically, we have proposed and asked several questions: 1) Is the serum mBDNF level reduced in patients with mood disorders? 2) Does mBDNF level change in the subpopulation of MDD subjects related to the severity of disease and antidepressant treatments? 3) Is there any difference in the serum mBDNF among MDD, BD and suicidal subjects? 4) Do mBDNF and proBDNF change in lymphocytes of patients with MDD? Our main aim was, therefore, to evaluate the levels of mBDNF and proBDNF in serum and lymphocytes in a cohort of the expanded number of mood disorder subjects and healthy control individuals, using specific mBDNF and proBDNF ELISA assays.

Section snippets

Participants

A total of 215 participants were recruited into the study from May 2018 to April 2019. The study was approved by the Ethics Committee of Yunnan Mental Hospital (China) and carried out at the clinical laboratory based at the Mental Hospital of Yunnan Province, China. All procedures were carried out following the latest version of the Declaration of Helsinki. Written informed consents were obtained from all subjects before participating in the study. The eligible subjects who matched the

Study population

The demographic and clinical characteristics of all participants are shown in Table 1. Overall, 215 subjects met the inclusion criteria and were categorized into different comparison groups such as Control, MDD, BD and Attempted suicide. The patients from the MDD group were further divided into subgroups based on the clinical characteristics to compare the study results. These included first episode and recurrent, moderate depressive episode and severe depressive episode, patients with

Discussion

BDNF is a neurotrophic factor that plays an essential role in the function of the peripheral and central nervous system (Huang and Reichardt, 2001). Previous studies showed that blood BDNF is a likely biomarker for mood disorders, including MDD and bipolar disorders (Hashimoto, 2010; Polyakova et al., 2015). However, the BDNF precursor proBDNF is less studied, and its role in mood disorders is not known. In the present study, we expanded the study on the serum mBDNF levels to include 90 MDD

Conclusion

With specific ELISA assays for mBDNF and proBDNF, we have confirmed that serum mature BDNF could be reliably used as a biomarker for mood disorders, including MDD and BD. The measurement of serum mBDNF could be considered as a predicting factor for the severity of MDD. The serum level of mBDNF less than 12.4 ng/ml might be a potential diagnostic biomarker for both MDD and BD. This study has revealed the increased levels of both proBDNF and mBDNF in lymphocytes of MDD, suggesting proBDNF and

Declaration of interest

All authors declare no conflict of interest.

Acknowledgement

LYL was supported by UniSA Postgraduate Scholarship and UniSA International Travel grant; and the project was supported by the National Nature Science Foundation of China, grant No. 81960806 and NHMRC project grants.

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