Differential modulation of microglia superoxide anion and thromboxane B2 generation by the marine manzamines

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 TXB2 and 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 beta-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 TXB2 and O2- from E. coli LPS-activated rat neonatal microglia in vitro.

The manzamines differentially attenuated PMA (phorbol 12-myristate 13-acetate)-stimulated TXB2 generation in the following order of decreasing potency: MZA (IC50 < 0.016 microM) > MZD (IC50 = 0.23 microM) > MZB (IC50 = 1.6 microM) > MZC (IC50 = 2.98 microM) > MZE and F (IC50 > 10 microM). In contrast, there was less effect on OPZ (opsonized zymosan)-stimulated TXB2 generation: MZB (IC50 = 1.44 microM) > MZA (IC50 = 3.16 microM) > MZC (IC50 = 3.34 microM) > MZD, MZE and MZF (IC50 > 10 microM). Similarly, PMA-stimulated O2- generation was affected differentially as follows: MZD (apparent IC50 < 0.1 microM) > MZA (IC50 = 0.1 microM) > MZB (IC50 = 3.16 microM) > MZC (IC50 = 3.43 microM) > MZE and MZF (IC50 > 10 microM). In contrast, OPZ-stimulated O2- generation was minimally affected: MZB (IC50 = 4.17 microM) > MZC (IC50 = 9.3 microM) > MZA, MZD, MZE and MZF (IC50 > 10 microM). From the structure-activity relationship perspective, contributing factors to the observed differential bioactivity on TXB2 and O2- generation are the solubility or ionic forms of MZA and D as well as changes such as saturation or oxidation of the beta 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.

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 TXB2 and O2- generation. MZA may thus be a lead candidate for the development of novel therapeutic agents for the modulation of TXB2 and 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.