The high heterogeneity of MDD in symptoms and etiology makes the pathogenesis elusive[49]. At present, reducing the severity and duration of current depression and the possibility of recurrence mainly depends on the treatment of antidepressants. While antidepressants provide efficacy, they come with side effects such as low receptor specificity, a wide range of adverse reactions, and high toxicity[50]. It is well known that many components of Chinese herbal medicine have been neglected in experimental and clinical studies because of their physicochemical properties, such as poor water solubility or oral availability. Based on data mining, network pharmacology helps us find valuable information from big data, which is especially suitable for the multi-target and multi-pathway characteristics of traditional chinese medicine and then guides us to conduct meaningful research[51]. Currently, MeV is extracted mainly from the dried root of Radix Saposhnikoviae, which is commonly used as a Chinese herbal remedy for immune, nervous, and respiratory diseases[11]. Due to the low oral availability, when screening the effective ingredients of Saposhnikovia divaricata (Turcz.) Schischk. by DL (drug-likeness) and OB (oral bioavailability) values in the Traditional Chinese Medicine systems pharmacology database and analysis platform (TCMSP) database, they were overlooked. Here, we take MeV as an independent research object to further explore the mechanism of action on MDD. Previous studies have confirmed that MeV is a novel histone H3 phosphorylation epigenetic suppressor, which plays a neuroprotective role by attenuating inflammation and cleaved caspase-3- and − 9a-related apoptosis on focal cerebral ischemia in rats[14]. In addition, Sun et al. found that MeV inhibited the over-activation of BV-2 microglia stimulated by LPS, reduced the content of inflammatory factors by the NF-κB/IKBα pathway, and reversed the depression-like behavior induced by LPS in mice[15]. Therefore, the above findings suggest that the MeV may be vital for the therapeutic effect of MDD, which is worthy of further exploration. The main research idea of this study was to evaluate the antidepressant effect of MeV and the potential mechanism based on the perspective of disease-target-drug. Three LPS exposures were employed to establish significant depressive symptoms accompanied by microglial activation in mice. MeV administration significantly alleviated depression-like behaviors and reduced microgliosis in mice by inhibiting SRC phosphorylation. MeV has an antidepressant effect that may be related to the SRC-mediated NF-κB signaling pathway (Fig. 7). The results provide references for further study of the pathogenesis and treatment of depression.
After searching public databases, 85 potential MeV and MDD intersection targets were identified, which were screened by the value of "degree" to acquire pivotal targets. Then, to annotate the functions of 85 validated targets and related pathways by the GO and KEGG enrichment, the results showed that the target genes had been implicated in various biological processes (such as the positive regulation of hormone response, hypoxia response, lipopolysaccharide response, and phosphorylation) and signaling pathways. Comorbid depression is common actually, such as endocrine diseases (thyroid hormone[52]), metabolic diseases (diabetes[53]), visceral diseases (alcoholic liver disease[54]), vascular diseases[55] and the pessimism of cancer patients[56]. Here, we mainly discuss the results of enrichment analysis excluding other diseases. From the pathogenesis perspective, MDD has also been considered a disease of microglia or a disease caused by changes in neuroplasticity[57, 58]. Microglia are the primary resident innate immune cells in the central nervous system and actively regulate microenvironmental changes in healthy and disordered brains[59]. Such cellular activation regulates inflammation, synaptic thinning or pruning, and neuronal connections[60]. Polarized microglia-mediated the inflammatory response of the central nervous system mainly through the NF-κB signaling pathway[61]. Specifically, NF-κB enters the nucleus when activated, promotes the expression of inflammatory genes, destroys neuronal synapses, leads to structural disruption and dysfunction of neural networks, and increases the risk of depression. Exposure to psychological stress can induce the activation of damage (or danger)-associated molecular patterns, and affect the production of cytokines, neurotransmitters, neuropeptides, and centrally-acting peripheral hormones[58]. TLR4, as a pattern recognition receptor, plays a central part in innate immunity. TLR4 recognizes LPS, which triggers activation of c-SRC-kinase responses mediated by [Ca2+]i increase and NF-κB signaling pathway activation, mediating cell proliferation and motility[62, 63]. TLR4, when located on the surface of microglia, activates various signaling pathways mediating the production of inflammatory cytokines under the stimulation of LPS in the central nervous system. As an "integration factor" of endogenous and exogenous stimulators, SRC participates in numerous cellular processes, such as proliferation, survival, adhesion, migration[64]. Therefore, to maintain normal cellular processes, SRC activity is tightly regulated by phosphorylation sites. Phosphorylated SRC-Tyr416 is considered a marker for c-SRC activation[65]. In parallel, a study has shown that the Src function is necessary and sufficient for triggering microglial cell activation[66]. Our results showed that multiple LPS stimulation up-regulated the expression levels of TLR4, PSRC, NF-κB, IKBα, and IL-1β proteins in the hippocampus, also activated microglia. MeV eliminated these effects, while the SRC inhibitor PP2 plays a similar role except TLR4. Meanwhile, when PP2 and MeV coexist, the regulatory effect of MeV on NF-κB pathway proteins and microglia is inhibited. Previous study has shown that PP2 treatment significantly reduced PSRC expression and LPS-induced microglial neuroinflammation triggered by LPS in mice[67], which is consistent with our results. Therefore, these results, and other previously reported results, strongly support that SRC may act as a major direct target involved in the MeV-mediated regulation of the NF-κB signaling pathway in depression.
However, there still are some limitations in our current research. On the one hand, although network pharmacology provides a predictable approach based on biochemistry, pharmacology, and network biology, the target data of drugs and diseases are collectively obtained from multiple public databases, especially the open database has little target data of MeV, which is insufficient to provide an objective and comprehensive analysis of the drug's intervention outcomes for MDD. On the other hand, numerous studies have shown that the hippocampus[68, 69], anterior cingulate cortex[70], and lateral habenula[71] participate in regulating neuroinflammatory depression. We focus our research on the hippocampus, as impairment of this area may be a core factor in the pathophysiology of depression. Although we had explored the effects of MeV in the hippocampus, we could not completely rule out the role of other brain regions, such as the anterior cingulate cortex and lateral habenula, during the period of MeV affecting LPS-induced depressive symptoms. These mechanisms should be validated in subsequent research.