Regulatory effect of icariin on phenotype transformation of microglia in depression model

Regulatory effect of icariin on phenotype transformation of microglia in depression model
Depression is a kind of emotional mental disorder, with a high incidence of incidence rate, which is easy to recur, and even life-threatening in serious cases. It has become a global public health problem. The clinical manifestations of depression patients mainly include depressive emotions, decreased pleasure, feelings of worthlessness or guilt, and suicidal ideation. Due to the complex biological mechanisms and involvement of numerous systems, the pathogenesis of depression is not fully understood, and research on antidepressants is a global focus, but progress is slow. Finding new approaches and drug targets for treating depression has become an urgent issue.
The neuroinflammatory response caused by stress is increasingly being recognized in the pathogenesis of depression. Microglia are a type of innate immune cell in the central nervous system, closely related to neuroinflammatory responses in the brain, and have the potential to become a new therapeutic target. The classic M1 polarization releases inflammatory factors such as interleukin-6 (IL6), interleukin-1 (IL-1), tumor necrosis factor alpha (TNF – α), etc., causing neuroinflammatory reactions; Replace activated M2 polarization, secrete interleukin-10 (interle, ukin-10, IL-10), transforming growth factor beta (TGF – β), etc., induce tissue repair, and exert neuroprotective effects. Regulating the phenotype transformation of microglia is one of the important ways to intervene in the occurrence and development of depression. Traditional Chinese medicine has always had significant advantages in the prevention and treatment of depression. In traditional Chinese medicine, kidney yang deficiency is one of the important mechanisms of depression, and the treatment of depression from the perspective of kidney theory provides ideas. In clinical practice, Epimedium, a traditional Chinese medicine for tonifying kidney yang, is often combined with other Chinese medicines such as Xianmao, Morinda officinalis, and Yejiaoteng to treat depression of kidney yang deficiency type. In previous studies, it was observed that icariin, the active ingredient of Epimedium brevicomu Maxim, a traditional Chinese medicine for tonifying kidney yang, has good antidepressant effects, but its mechanism is still unclear. This study intends to use a chronic unpredictable mild stress-induced depression model, combined with a lipopolysaccharide (LPS) – induced BV-2 cell polarization model, to explore the mechanism of icariin’s antidepressant effect.

Depression is a serious mental illness that lacks ideal treatment methods due to its complex pathogenesis. Traditional Chinese medicine has its own characteristics and advantages in treating depression. At present, the pathogenesis of depression is not fully understood. The hypotheses currently formed mainly include: inflammatory immunity hypothesis, hypothalamic pituitary adrenal (HPA) axis dysfunction hypothesis, monoamine neurotransmitter hypothesis, neurotrophic factor hypothesis, etc. With the deepening of research, the inflammatory immunity hypothesis has received widespread attention both domestically and internationally. Research has shown that patients with depression not only experience peripheral inflammatory reactions, but also immune activation of the central nervous system. Microglia are an important type of immune cell in the central nervous system, closely related to the inflammatory response of the central nervous system. Under long-term stress, their structure and function are abnormal, leading to changes in emotions and mental states, which are important factors in the occurrence and development of depression.
Classic activation type (M1 type) and alternative activation type (M2 type) are the two main polarization phenotypes of microglia. M1 type accounts for the vast majority of activated cells and can synthesize and secrete pro-inflammatory mediators such as TNF – α, IL-1, IL-6, and nitric oxide synthase iNOS, triggering an immune inflammatory cascade reaction; M2 type (three subtypes M2a, M2b immunophenotype, M2c inactive phenotype) can be stimulated by interleukin-4 (IL-4), interleukin-13 (IL-13), and other factors, releasing anti-inflammatory factors such as IL-10 and CD206, exerting anti-inflammatory and neuroprotective functions, and playing an important role in brain nerve repair and plasticity. Animal experiments have shown that microglia in the prefrontal cortex of mice and rats exposed to CUMS are highly activated and accompanied by high expression of related inflammatory mediators, which can lead to inflammation related neuronal degeneration associated with depression. A post-mortem examination showed that in patients with severe depression, the density of microglia in the anterior cingulate cortex and the white matter area of the ventral prefrontal lobe significantly increased and showed an activated state, but the typical marker of M2 microglia, CD206, was significantly downregulated. The above research suggests that excessive activation of microglia and polarization of M1 and M2 microglia play an important role in the occurrence and development of depression.

This study found that in the hippocampus of CUMS induced depression mice, the levels of pro-inflammatory cytokine IL-6 were elevated, and IL-6 mRNA, iNOS mRNA, and iNOS protein were overexpressed, accompanied by an increase in depressive like behavior (increased forced swimming immobility time, reduced sugar water consumption, and exercise distance), suggesting a significant increase in classic activated M1 microglia in the hippocampus of depression mice. Epimedium glycoside can inhibit M1 type activation of microglia, reduce levels of pro-inflammatory cytokine IL-6, decrease overexpression of IL-6 mRNA, iNOS mRNA, and iNOS protein, and suppress brain inflammation in model mice; On the other hand, icariin can promote M2 activation of microglia, increase IL-10 levels in the hippocampus of CUMS exposed mice, promote IL-10 mRNA, CD206 mRNA, and CD206 protein expression, thereby inhibiting depression like behavior in model mice. In vitro experiments also showed that under LPS stimulation, M1 polarization of microglia increased, and icariin could inhibit M1 polarization of microglia and promote M2 polarization. This suggests that icariin alleviates excessive neuroinflammation by regulating the phenotypic transformation of microglia.

The metabolic disorders of monoamine neurotransmitters 5-HT, DA, and NE are closely related to the occurrence and development of neurological and psychiatric disorders such as depression. The levels of monoamine neurotransmitters 5-HT, DA, and NE in the brain of CUMS induced depression model rats were significantly reduced. Rehmannia glutinosa can lower the levels of 5-HT, DA, and NE in the hippocampus of the model rats and improve depressive like behavior. Danshen polyphenolic acid can effectively improve depression like behavior in rats, possibly by inhibiting the non receptor tyrosine kinase 2 (Janus kinase 2)/signal transducer and activator of transcription 3 (STAT3) signaling pathway, alleviating neuroinflammation, and regulating levels of monoamine neurotransmitters. More and more immune cells – microglia. Activated microglia induce the release of many inflammatory factors such as interleukin-1 β (IL-1 β), TNF – α, TGF – β, etc. These inflammatory factors can affect the function of monoamine neurons and induce a decrease in monoamine neurotransmission through oxidative stress, which is considered an important factor in depression. It has been confirmed that monoamine drugs can block the activation of microglia and reverse depression in some way. Research has found that tetrahydrobiopterin (BH4), as a recognized biomarker of depression, is a key enzymatic cofactor required for the synthesis of 5-HT, DA, and NE. BH4 deficiency may lead to depression. M1 type microglia may lead to BH4 catabolism. Inhibiting M1 polarization in microglia can limit BH4 catabolism and promote the synthesis and secretion of monoamine neurotransmitters. The results of this study showed that a decrease in monoamine neurotransmitters in the hippocampus of CUMS induced mouse depression model was accompanied by an increase in depressive like behavior. Epimedium glycoside can increase the level of monoamine neurotransmitters in the hippocampus of model mice, which may be one of the mechanisms of its antidepressant effect, consistent with previous reports. However, whether icariin promotes the synthesis and secretion of monoamine neurotransmitters in the hippocampus of depression model mice by regulating the phenotype transformation of microglia remains to be further studied.

In summary, this study used CUMS to establish a mouse model of depression and an LPS induced polarization model of microglia. It was found that icariin, an active ingredient in the traditional Chinese medicine Epimedium for tonifying kidney yang, inhibits M1 polarization and promotes M2 polarization of microglia, reduces inflammation in the hippocampus of depressed mice, and promotes the production of monoamine neurotransmitters, which is one of its mechanisms of antidepressant action.