Cyanobacterial Microcystis aeruginosa Lipopolysaccharide Elicits Release of Superoxide Anion, Thromboxane B2, Cytokines, Chemokines, and Matrix Metalloproteinase-9 by Rat Microglia

Microcystis aeruginosa (M. aeruginosa) is a cosmopolitan Gram-negative cyanobacterium that may contaminate freshwater by releasing toxins, such as lipopolysaccharide (LPS) during aquatic blooms, affecting environmental and human health. The putative toxic effects of cyanobacterial LPS on brain microglia, a glial cell type that constitutes the main leukocyte-dependent source of reactive oxygen species in the central nervous system, are presently unknown. We tested the hypothesis that in vitro concentration- and time-dependent exposure to M. aeruginosa LPS strain UTCC 299 would activate rat microglia and the concomitant generation of superoxide anion (O-2(-)). After a 17-h exposure of microglia to M.aeruginosa LPS, the following concentration-dependent responses were observed: 0.1-100 ng/ml M. aeruginosa LPS enhanced O-2(-) generation, with limited inflammatory mediator generation; 1000-10,000 ng/ml M. aeruginosa LPS caused thromboxane B-2 (TXB2), matrix metalloproteinase-9 (MMP-9), and macrophage inflammatory protein-2 (MIP-2/CXCL2) release, concurrent with maximal O-2(-) generation; 100,000 ng/mL M. aeruginosa LPS deactivated O-2(-) production but maintained elevated levels of TXB2, MMP-9, tumor necrosis factor-alpha (TNF-alpha), interleukin 1-alpha (IL-1 alpha), and interleukin-6 (IL-6), macrophage inflammatory protein 1 alpha (MIP-1 alpha/CCL3), and MIP-2/CXCL2, with concomitant lactic dehydrogenase release. Although M. aeruginosa LPS was consistently less potent than Escherichia coli LPS, with the exception of O-2(-), TXB2, and MCP-1/CCL2 generation, it was more efficacious because higher levels of MMP-9, TNF-alpha, IL-1 alpha, IL-6, MIP-1 alpha/CCL3, and MIP-2/CXCL2 were produced. Our in vitro studies suggest that one or more of the inflammatory mediators released during M. aeruginosa LPS stimulation of microglia may play a critical role in the subsequent ability of microglia to generate O-2(-). To our knowledge, this is the first experimental evidence that LPS isolated from a M. aeruginosa strain, can activate brain microglia in vitro, as well as the release of O-2(-), and other inflammatory mediators hypothesized to be involved in neuroinflammation and neurodegeneration.