Identification of Koumine as a Translocator Protein 18 kDa Positive Allosteric Modulator for the Treatment of Inflammatory and Neuropathic Pain

Structure-based drug design for TSPO: Challenges and opportunities

The translocator protein 18 kDa (TSPO) is an evolutionarily conserved mitochondrial transmembrane protein implicated in various neuropathologies and inflammatory conditions, making it a longstanding diagnostic and therapeutic target of interest. Despite the development of various classes of TSPO ligand chemotypes, and the elucidation of bacterial and non-human mammalian experimental structures, many unknowns exist surrounding its differential structural and functional features in health and disease. There are several limitations associated with currently used computational methodologies for modelling the native structure and ligand-binding behaviour of this enigmatic protein. In this perspective, we provide a critical analysis of the developments in the uses of these methods, outlining their uses, inherent limitations, and continuing challenges. We offer suggestions of unexplored opportunities that exist in the use of computational methodologies which offer promise for enhancing our understanding of the TSPO.

The alleviatory effects of koumine on MSU-induced gouty arthritis via the TLR4/NF-κB/NLRP3 pathway

The aim of this study was to validate the preventive effects of koumine (KM), a monoterpene indole alkaloid, on gouty arthritis (GA) and to explore its possible mechanisms. C57BL/6 mice were intraperitoneally administered KM (0.8, 2.4 or 7.2 mg/kg), colchicine (3.0 mg/kg) or sterile saline. One hour later, a monosodium urate (MSU) suspension was injected into the right hind paws of the mice to establish an acute gout model. Inflammation symptoms were evaluated at 0, 3, 6, 12 and 24 h, and the mechanical withdrawal threshold was evaluated at 0, 6 and 24 h. After 24 h, the mice were euthanized, and the joint tissue, kidney and blood were collected for subsequent experiments. Histological examination and antioxidant enzyme, kidney index and serum uric acid (UA) measurements were taken. The expression levels of the signalling pathway components were determined. KM effectively alleviated the symptoms of redness, swelling and pain; counteracted inflammatory cell infiltration; and increased antioxidant enzyme levels, reduced kidney index and serum UA levels through regulating UA excretion in MSU-induced mice. The expression of toll-like receptor 4 (TLR4)/nuclear factor kappa-B (NF-κB)/nucleotide-binding oligomerization domain, leucine-rich repeat and pyrin domain-containing 3 (NLRP3) signalling pathway proteins and mRNA were reduced in the KM group. These results suggest that KM may be effective in alleviating GA through the TLR4/NF-κB/NLRP3 pathway.

Recent progress in chemistry and bioactivity of monoterpenoid indole alkaloids from the genus gelsemium: a comprehensive review

Monoterpenoid indole alkaloids (MIAs) represent a major class of active ingredients from the plants of the genus Gelsemium. Gelsemium MIAs with diverse chemical structures can be divided into six categories: gelsedine-, gelsemine-, humantenine-, koumine-, sarpagine- and yohimbane-type. Additionally, gelsemium MIAs exert a wide range of bioactivities, including anti-tumour, immunosuppression, anti-anxiety, analgesia, and so on. Owing to their fascinating structures and potent pharmaceutical properties, these gelsemium MIAs arouse significant organic chemists' interest to design state-of-the-art synthetic strategies for their total synthesis. In this review, we comprehensively summarised recently reported novel gelsemium MIAs, potential pharmacological activities of some active molecules, and total synthetic strategies covering the period from 2013 to 2022. It is expected that this study may open the window to timely illuminate and guide further study and development of gelsemium MIAs and their derivatives in clinical practice.

Koumine ameliorates concanavalin A-induced autoimmune hepatitis in mice: involvement of the Nrf2, NF-κB pathways, and gut microbiota

Gelsemiumelegans(Gardner. & Chapm.) Benth. has long been considered a traditional Chinese medicine effective against rheumatoid pain, cancer, cirrhosis, and skin diseases. Koumine (KM), the most abundant alkaloid in G.elegans Benth., demonstrates a variety of biological effects, including antitumor, analgesic, anxiolytic, anti-inflammatory, antidepressant, antioxidant, immunoregulatory, and hepatoprotective effects. Furthermore, the relatively low toxicity of KM makes it a promising drug candidate. This study aimed to investigate the protective effects of KM and its possible mechanisms using a concanavalin A (Con A)-induced autoimmune hepatitis (AIH) model in mice. Mice were orally administered different doses of KM for 14 d before Con A tail vein injections. The effects of KM on serum biochemical markers and liver histopathology were then evaluated 12 h after Con A exposure. The Nrf2 and NF-κB signaling pathways and alterations in gut microbiota were determined using western blotting, immunohistochemistry, and 16S rRNA sequencing to explore the underlying mechanisms of KM exposure. KM pretreatment dose-dependently decreased serum liver injury markers (Alanine aminotransferase, and aspartate aminotransferase) and cytokine levels (Tumor necrosis factor-α and interleukin-6), as well as the liver pathological damage triggered by Con A. Furthermore, the results of the multi-technique analysis indicated that KM activated the Nrf2 pathway, upregulated the expression of anti-oxidation factors HO-1 and Nrf2, and downregulated the expression of Keap1. Moreover, the NF-κB signaling pathway was inhibited. Interestingly, pre-treatment with KM also significantly improved the composition of the gut microbiota probably because it increases the richness of probiotics. Our findings suggest that KM pretreatment could attenuate Con A-induced AIH, the Nrf2 and NF-κB signaling pathways, and that gut microbiota are involved in the process of the hepatoprotective effect. This study provides a theoretical basis for the development of KM as an effective agent against AIH.

Koumine regulates macrophage M1/M2 polarization via TSPO, alleviating sepsis-associated liver injury in mice

BACKGROUND

Translocator protein (TSPO) is an 18-kDa transmembrane protein found primarily in the mitochondrial outer membrane, and it is implicated in inflammatory responses, such as cytokine release. Koumine (KM) is an indole alkaloid extracted from Gelsemium elegans Benth. It has been reported to be a high-affinity ligand of TSPO and to exert anti-inflammatory and immunomodulatory effects in our recent studies. However, the protective effect of KM on sepsis-associated liver injury (SALI) and its mechanisms are unknown.

PURPOSE

To explore the role of TSPO in SALI and then further explore the protective effect and mechanism of KM on SALI.

METHODS

The effect of KM on the survival rate of septic mice was confirmed in mouse models of caecal ligation and puncture (CLP)-induced and lipopolysaccharide (LPS)-induced sepsis. The protective effect of KM on CLP-induced SALI was comprehensively evaluated by observing the morphology of the mouse liver and measuring liver injury markers. The serum cytokine content was detected in mice by flow cytometry. Macrophage polarization in the liver was examined using western blotting. TSPO knockout mice were used to explore the role of TSPO in sepsis liver injury and verify the protective effect of KM on sepsis liver injury through TSPO.

RESULTS

KM significantly improved the survival rate of both LPS- and CLP-induced sepsis in mice. KM has a significant liver protective effect on CLP-induced sepsis in mice. KM treatment ameliorated liver ischaemia, improved liver pathological injuries, and decreased the levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), lactate dehydrogenase (LDH) and proinflammatory cytokines in serum. Western blotting results showed that KM inhibited M1 polarization of macrophages and promoted M2 polarization. In TSPO knockout mice, we found that TSPO knockout can improve the survival rate of septic mice, ameliorate liver ischaemia, improve liver pathological injuries, and decrease the levels of ALT, AST, and LDH. In addition, TSPO knockout inhibits the M1 polarization of macrophages in the liver of septic mice and promotes M2 polarization and the serum levels of proinflammatory cytokines. Interestingly, in TSPO knockout septic mice, these protective effects of KM were no longer effective.

CONCLUSIONS

We report for the first time that TSPO plays a critical role in sepsis-associated liver injury by regulating the polarization of liver macrophages and reducing the inflammatory response. KM, a TSPO ligand, is a potentially desirable candidate for the treatment of SALI that may regulate macrophage M1/M2 polarization through TSPO in the liver.

Translocator Protein 18 kDa (TSPO) as a Novel Therapeutic Target for Chronic Pain

Chronic pain is an enormous modern public health problem, with significant numbers of people debilitated by chronic pain from a variety of etiologies. Translocator protein 18 kDa (TSPO) was discovered in 1977 as a peripheral benzodiazepine receptor. It is a five transmembrane domain protein, mainly localized in the outer mitochondrial membrane. Recent and increasing studies have found changes in TSPO and its ligands in various chronic pain models. Reversing their expressions has been shown to alleviate chronic pain in these models, illustrating the effects of TSPO and its ligands. Herein, we review recent evidence and the mechanisms of TSPO in the development of chronic pain associated with peripheral nerve injury, spinal cord injury, cancer, and inflammatory responses. The cumulative evidence indicates that TSPO-based therapy may become an alternative strategy for treating chronic pain.

The anxiolytic effect of koumine on a predatory sound stress-induced anxiety model and its associated molecular mechanisms

BACKGROUND

Koumine is the most abundant alkaloid extracted from Gelsemium elegans Benth.. Preliminary studies by our research group have shown that koumine has significant anxiolytic effect, but this needs to be further confirmed.

HYPOTHESIS/PURPOSE

To investigate the potential anxiolytic effect of koumine on predatory sound (PS) stress-induced anxiety models and preliminarily explore its therapeutic targets and molecular mechanisms.

STUDY DESIGN AND METHODS

The anxiolytic effect of koumine in an animal model of acute PS stress-induced anxiety were determined. Then, neurosteroids levels in the main brain regions involved in anxiety disorders, as well as plasma adrenocorticotropic hormone (ACTH) and corticosterone (CORT) levels, were determinated. Finally, to clarify the effect of koumine on translocator protein 18 kDa (TSPO), the affinity between koumine and TSPO was evaluated by surface plasmon resonance (SPR) technology.

RESULTS

Koumine treatment mitigated anxiety-like behavior following acute PS stress in the open field test and elevated plus maze test. PS exposure significantly decreased progesterone and allopregnanolone levels in the PFC, Hip, and Amy and increased ACTH and CORT levels in plasma, and koumine administration significantly reversed these effects. Finally, the reliable SPR results showed that the K_D of koumine with TSPO was 155.33 ± 11.0 μM, indicating that koumine is a human TSPO high-affinity ligand that has an affinity comparable to typical TSPO ligands.

CONCLUSION

Our results show that koumine has obvious anxiolytic effect in the PS-induced anxiety model. Targeting TSPO-neurosteroids-HPA axis may be an important mechanism by which koumine exerts its anxiolytic effect.

Investigation of the Possible Allostery of Koumine Extracted From Gelsemium elegans Benth. And Analgesic Mechanism Associated With Neurosteroids

Translocator protein 18 kDa (TSPO) is an evolutionarily conserved 5-transmembrane domain protein, and has been considered as an important therapeutic target for the treatment of pain. We have recently reported the in vitro and in vivo pharmacological characterization of koumine as a TSPO positive allosteric modulator (PAM), more precisely ago-PAM. However, the probe dependence in the allostery of koumine is an important question to resolve, and the possible analgesic mechanism of koumine remains to be clarified. Here, we report the in vivo evaluation of the allostery of koumine when orthosteric ligand PK11195 was used and preliminarily explore the possible analgesic mechanism of koumine associated with neurosteroids. We find that koumine is an ago-PAM of the PK11195-mediated analgesic effect at TSPO, and the analgesic mechanism of this TSPO ago-PAM may be associated with neurosteroids as the analgesic effects of koumine in the formalin-induced inflammatory pain model and chronic constriction injury-induced neuropathic pain model can be antagonized by neurosteroid synthesis inhibitor aminoglutethimide. Although our results cannot fully clarify the allosteric modulatory effect of koumine, it further prove the allostery in TSPO and provide a solid foundation for koumine to be used as a new clinical candidate drug to treat pain.