Differential modulation of microglia superoxide anion and thromboxane B₂generation by the marine manzamines

Abstract Background Thromboxane B₂(TXB₂) and superoxide anion (O₂^-) are neuroinflammatory mediators that appear to be involved in the pathogenesis of several neurodegenerative diseases. Because activated-microglia are the main source of TXB₂and O₂^-in these disorders, modulation of their synthesis has been hypothesized as a potential therapeutic approach for neuroinflammatory disorders. Marine natural products have become a source of novel agents that modulate eicosanoids and O₂^-generation from activated murine and human leukocytes. With the exception of manzamine C, all other manzamines tested are characterized by a complex pentacyclic diamine linked to C-1 of the &#946;-carboline moiety. These marine-derived alkaloids have been reported to possess a diverse range of bioactivities including anticancer, immunostimulatory, insecticidal, antibacterial, antimalarial and antituberculosis activities. The purpose of this investigation was to conduct a structure-activity relationship study with manzamines (MZ) A, B, C, D, E and F on agonist-stimulated release of TXB₂and O₂^-from E. coli LPS-activated rat neonatal microglia in vitro. Results The manzamines differentially attenuated PMA (phorbol 12-myristate 13-acetate)-stimulated TXB₂generation in the following order of decreasing potency: MZA (IC₅₀<0.016 μM) >MZD (IC₅₀= 0.23 μM) >MZB (IC₅₀= 1.6 μM) >MZC (IC₅₀= 2.98 μM) >MZE and F (IC₅₀>10 μM). In contrast, there was less effect on OPZ (opsonized zymosan)-stimulated TXB₂generation: MZB (IC₅₀= 1.44 μM) >MZA (IC₅₀= 3.16 μM) >MZC (IC₅₀= 3.34 μM) >MZD, MZE and MZF (IC₅₀>10 μM). Similarly, PMA-stimulated O₂^-generation was affected differentially as follows: MZD (apparent IC₅₀<0.1 μM) >MZA (IC₅₀= 0.1 μM) >MZB (IC₅₀= 3.16 μM) >MZC (IC₅₀= 3.43 μM) >MZE and MZF (IC₅₀>10 μM). In contrast, OPZ-stimulated O₂^-generation was minimally affected: MZB (IC₅₀= 4.17 μM) >MZC (IC₅₀= 9.3 μM) >MZA, MZD, MZE and MZF (IC₅₀> 10 μM). From the structure-activity relationship perspective, contributing factors to the observed differential bioactivity on TXB₂and O₂^-generation are the solubility or ionic forms of MZA and D as well as changes such as saturation or oxidation of the &#946; carboline or 8-membered amine ring. In contrast, the fused 13-membered macrocyclic and isoquinoline ring system, and any substitutions in these rings would not appear to be factors contributing to bioactivity. Conclusion To our knowledge, this is the first experimental study that demonstrates that MZA, at in vitro concentrations that are non toxic to E. coli LPS-activated rat neonatal microglia, potently modulates PMA-stimulated TXB₂and O₂^-generation. MZA may thus be a lead candidate for the development of novel therapeutic agents for the modulation of TXB₂and O₂^-release in neuroinflammatory diseases. Marine natural products provide a novel and rich source of chemical diversity that can contribute to the design and development of new and potentially useful anti-inflammatory agents to treat neurodegenerative diseases.