Differential Kv1.3, KCa3.1, and Kir2.1 Expression in Classically and Alternatively Activated Microglia

Microglia are highly plastic cells that can assume different phenotypes in response to microenvironmental signals. Lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) promote differentiation into classically activated M1-like microglia, which produce high levels of pro-inflammatory cytokines and nitric oxide and are thought to contribute to neurological damage in ischemic stroke and Alzheimer's disease. IL-4 in contrast induces a phenotype associated with anti-inflammatory effects and tissue repair. We here investigated whether these microglia subsets vary in their K (+)channel expression by differentiating neonatal mouse microglia into M(LPS) and M(IL-4) microglia and studying their K+ channel expression by whole-cell patch-clamp, quantitative PCR and immunohistochemistry. We identified three major types of K (+)channels based on their biophysical and pharmacological fingerprints: a use-dependent, outwardly rectifying current sensitive to the K(V)1.3 blockers PAP-1 and ShK-186, an inwardly rectifying Ba (2+) -sensitive K(ir)2.1 current, and a Ca (2+) -activated, TRAM-34-sensitive K(Ca)3.1 current. Both K(V)1.3 and K(Ca)3.1 blockers inhibited pro-inflammatory cytokine production and iNOS and COX2 expression demonstrating that K(V)1.3 and K(Ca)3.1 play important roles in microglia activation. Following differentiation with LPS or a combination of LPS and IFN-g microglia exhibited high K(V)1.3 current densities (similar to 50 pA/pF at 40 mV) and virtually no K(Ca)3.1 and Kir currents, while microglia differentiated with IL-4 exhibited large K(ir)2.1 currents (similar to 10 pA/pF at -120 mV). K(Ca)3.1 currents were generally low but moderately increased following stimulation with IFN-gamma or ATP (similar to 10 pS/pF). This differential K+ channel expression pattern suggests that K(V)1.3 and K(Ca)3.1 inhibitors could be used to inhibit detrimental neuroinflammatory microglia functions.