Reactive gliosis is a prominent feature of CNS injury that involves dramatic changes in glial cell morphology together with increased motility, phagocytic activity, and release of inflammatory mediators. We have recently demonstrated that stimulation of the cGMP-protein kinase G (PKG) pathway by NO or atrial natriuretic peptide (ANP) regulates cytoskeleton dynamics and motility in rat astrocytes in culture. In this work, we show that the cGMP-PKG pathway stimulated by ANP, but not by NO, regulates microglial cell morphology by inducing a dramatic reorganization in the actin cytoskeleton. Both ANP (0.01-1.0 microM) and the permeable cGMP analog, dibutyryl-cGMP (1-100 microM), promote a rapid (maximal at 30 min) and concentration-dependent increase in size, rounding, and lamellipodia and filopodia formation in rat brain cultured microglia. These morphological changes involve an augment and redistribution of F-actin and result in increased phagocytic activity. ANP-induced rearrangements in actin cytoskeleton and inert particle phagocytosis are prevented by the PKG inhibitor, Rp-8-Br-PET-cGMPS (0.5 microM), and involve inhibition of RhoA GTPase and activation of Rac1 and Cdc42. However, ANP does not induce NO synthase Type 2 (NOS-2) or tumor necrosis factor-alpha expression and is able to decrease lipopolysaccharide (LPS)-elicited induction of these inflammatory genes. The morphological changes and the decrease of LPS-induced NOS-2 expression produced by ANP in cultured microglia are also observed by immunostaining in organotypic cultures from rat hippocampus. These results suggest that stimulation of the ANP-cGMP-PKG pathway in microglia could play a beneficial role in the resolution of neuroinflammation by removing dead cells and decreasing levels of proinflammatory mediators.
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