Abstract
Microglia are highly plastic cells that can assume different phenotypes in response to microenvironmental signals. Lipopolysaccharide (LPS) and interferon-γ (IFN-γ) 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-γ microglia exhibited high K(V) 1.3 current densities (∼50 pA/pF at 40 mV) and virtually no K(Ca) 3.1 and K(ir) currents, while microglia differentiated with IL-4 exhibited large K(ir) 2.1 currents (∼ 10 pA/pF at -120 mV). K(Ca) 3.1 currents were generally low but moderately increased following stimulation with IFN-γ or ATP (∼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. GLIA 2016;65:106-121.