Induction of potassium channels in mouse brain microglia: Cells acquire responsiveness to pneumococcal cell wall components during late development

Lipopolysaccharides derived from cell walls of Gram-negative bacteria have proven a useful tool to simulate bacterial infection of the central nervous system. Rapid activation of microglia within the brain parenchyma as well as in vitro has thereby been shown to be an early event upon bacterial or l ipopolysaccharide challenges. Less is known about microglial responses to a contact with Gram-positive bacteria, such as Streptococcus pneumoniae, a l ethal pathogen causing meningitis with a 30% mortality rate. In the present study, we compared lipopolysaccharide-induced microglial activation in vit ro with that induced by preparations of pneumococcal cell walls. As a reado ut of microglial activation, we studied by patch-clamp recording the expres sion of outward rectifying potassium currents (IKOR+), which are known to b e induced by lipopolysaccharide. We found that pneumococcal cell walls and lipopolysaccharide induced a similar type of IKOR+. Stimulation of IKOR+ by pneumococcal cell walls and lipopolysaccharide involved protein synthesis since it was not induced in the presence of cycloheximide. Pharmacological characterization of the pneumococcal cell wall- and lipopolysaccharide-indu ced currents with specific ion channel blockers indicated for both cases ex pression of the charybdotoxin/margatoxin-sensitive Kv1.3 subtype of the Sha ker family of voltage-dependent potassium channels. Activation of the outwa rd currents by pneumococcal cell walls depended on the developmental stage: while lipopolysaccharide triggered IKOR+ in both embryonal and postnatal m icroglial cells, pneumococcal cell walls had only a marginal effect on embr yonal cells. This, however, does not imply that embryonic microglial cells are unresponsive to pneumococcal cell walls. In both embryonic and postnata l cells, (i) the amplitude of the constitutively expressed inward rectifyin g potassium current was significantly reduced, (ii) tumor necrosis factor-a lpha was released and (iii) the cells changed their morphology, similarly a s it was induced by lipopolysaccharide treatment. Thus, embryonic microglial cells are sensitive to pneumococcal cell wall ch allenges, but respond with a distinctly different pattern of physiological reactions. The expression of IKOR+ could thus be a suitable tool to study s ignalling cascades selectively involved in the activation of microglia by G ram-negative and -positive cell wall components and to functionally disting uish between populations of microglial cells. (C) 1999 IBRO. Published by E lsevier Science Ltd.