Bacterial Endotoxin Induces Changes in the Organization and Expression of Actin and Modulation of the Cell Membrane of Microglia

Abstract

Background and Objectives: In situ there are at least three morphologically and functionally different forms of microglia: the resting, the activated, and the phagocytic microglia. The signals promoting the morphological changes which adapt microglia to specific functions are still unknown. In this study the effect of bacterial wall lipopolysaccharide (LPS) on the morphology and organization and expression of actin in microglia was investigated. In addition, the changes in the appearance of the cell membrane were investigated. Methods: Microglia cultures were prepared from neopallia of newborn mice and treated with LPS. Scanning electron microscopy, labeling with phalloidin, and immunoblotting were used. Results: The majority of nontreated microglia were ameboid in shape with many short processes that extended into lamellipodia. When microglia were treated with LPS most of the microglia acquired a large, round, and flat shape. The rest of the ameboid microglia became larger in size. Fluorescent labeling with phalloidin showed that the F-actin network appeared diffusely arranged throughout the cytoplasm of nontreated microglia. In LPS-treated microglia the F-actin network was reorganized into filamentous bundles extending into microspike-like projections. Using scanning electron microscopy, the nontreated microglia had large membrane folds and few large blebs. In LPS-treated microglia most of the membrane folds and blebs at the cell periphery disappeared with the appearance of many microspike-like projections. Immunoblotting showed that LPS-treated microglia upregulated their actin protein. Conclusions: These changes in the organization of F-actin and the cell membrane may reflect adaptation of activated microglia to specific functional activity, such as increases in their phagocytic activity.