Microglia polarization of hippocampus is involved in the mechanism of Apelin-13 ameliorating chronic water immersion restraint stress-induced depression-like behavior in rats

Highlights

• Apelin-13 inhibited the depression-like behavior induced by CWIRS.

• Apelin-13 reverses the activation of hippocampal microglia in rats induced by CWIRS.

• Apelin-13 inhibited the increase of the expression of iNOS which is the marker of M1 in the hippocampal microglia of rats induced by the CWIRS.

• Apelin-13 reverses the reduce of the expression of Arg1 which is the marker of M2 in the hippocampal microglia of rats induced by the CWIRS.

Abstract

Chronic stress induces the activation of hippocampal microglia, which produces many inflammatory mediators and mediates the occurrence of depression. Two phenotypes of microglia polarization, the classical M1 and alternative M2, play important regulatory roles in neuroinflammation and are involved in the occurrence and development of depression. Apelin is derived from a precursor peptide consisting of 77 amino acids and is a natural ligand for the orphan G-protein-coupled receptor APJ. Apelin-13 is one of the subtypes of Apelin and has a wide range of biological effects. Studies have shown that Apelin-13 has an antidepressant effect, but its specific mechanism has not been elucidated. In this study, the purpose of this study is to explore the possible mechanism of Apelin-13 to improve depression-like behaviors induced by chronic stress in rats from the perspective of microglial polarization in vivo. Adult male Sprague Dawley (SD) rats received 28 days of chronic water immersion restraint stress (CWIRS). Apelin group was injected with Apelin-13 (2 μg/2 μL) through the intracerebroventricular for 7 days. The results showed that CWIRS can induce depression-like behaviors in rats. Compared with the CWIRS + saline group, the CWIRS + Apelin-13 group was significantly improved the depression-like behaviors in rats. Compared with the CWIRS + saline group, the CWIRS + Apelin-13 group was significantly down-regulated the protein expression of M1-type marker iNOS and the pro-inflammatory factors IL-1β and IL-6 secret by microglia decreased. Compared with the CWIRS + saline group, the protein expression of M2-type marker Arg1 and anti-inflammatory factors IL-4 and IL-10 secreted by microglia was significantly increased in CWIRS + Apelin-13 group. Double-labelling immunofluorescence co-localization showed that, compared with the CWIRS + saline group, CWIRS + Apelin-13 group significantly inhibited the co-localization expression of Iba-1 and iNOS, and promoted the co-localization expression of Iba-1 and Arg1 in hippocampus microglia. Taken together, our study suggests that Apelin-13 improves depression-like behavior in rats induced by CWIRS and its mechanism may be related to the regulation of microglial polarization.

Introduction

Depression is a serious affective disorder mental disease. According to estimates by the WHO, >300 million people suffer from major depression disorder, and the incidence of depression is increasing year by year (Jakobsen et al., 2019). It brings enormous health and economic burdens on people, so finding an effective antidepressant is urgent.

Inflammatory processes are thought to cause a variety of diseases, including metabolic syndrome, diabetes and heart disease, in addition to which they are also associated with mental illness and neurodegenerative disease (Hines et al., 2013). There is increasing evidence that inflammatory processes play an important role in psychological disorders such as depression (Xu et al., 2018). Excessive production of pro-inflammatory cytokines is considered to be the key to the development of this psychological disorder (Harry and Kraft, 2012). Microglia serves as the body's first line of defense and the innate immune system and is activated in response to invading pathogens and danger signals (Biesmans et al., 2013). Activation of microglia was defined as the classical activation of M1 or alternative activation of M2 (Mikita et al., 2011). M1 microglia are considered as the inducer of neuroinflammation, while M2 microglia are considered as the inhibitor of neuroinflammation (Pusic et al., 2014). These findings suggest that microglia mediated neuroinflammation may be a target for treatment of depression.

Apelin is a small molecular regulatory peptide whose biological activity is mediated by its specific receptor APJ which is a G protein-coupled receptor with seven transmembrane domains (Kasai et al., 2011) (Choe et al., 2000). Apelin-13 is a completely conserved polypeptide with 13C-terminal amino groups and spans all species and exhibits the highest biological efficiency (Gu et al., 2013). Apelin and its receptors are widely distributed throughout the nervous system, and are highly expressed in neuron, microglia, oligodendrocytes, and astrocytes (Chen et al., 2015). Previous studies in our laboratory have shown that Apelin-13 regulates LPS-induced N9 microglia polarization (Zhou et al., 2019). These studies indicate that Apelin-13 is involved in regulating neuroinflammatory responses. In addition, studies have shown that Apelin-13 can improve depression-like behavior (Tian et al., 2018), but the specific mechanism is still unclear.

Therefore, based on the literature reports and the previous studies of this experiment, the aim of this study is to investigate whether Apelin-13 can ameliorate chronic water immersion restraint stress (CWIRS)-induced depression-like behavior by regulating the polarization of microglia from the perspective of neuroinflammation.