Small-conductance Cl- channels contribute to volume regulation and phagocytosis in microglia

The shape and volume of microglia (brain immune cells) change when they activate during brain inflammation and become migratory and phagocytic. Swollen rat microglia express a large Cl- current (I-Clswell), whose biophysical properties and functional roles are poorly understood and whose molecular identity is unknown. We constructed a fingerprint of useful biophysical properties for comparison with I-Clswell in other cell types and with cloned Cl- channels. The microglial I-Clswell was rapidly activated by cell swelling but not by voltage, and showed no time-dependence during voltage-clamp steps. Like I-Clswell in many cell types, the halide selectivity sequence was I- > Br- > Cl- > F-. However, it differed in lacking inactivation, even at +100 mV with high extracellular Mg2+, and in having a much lower single-channel conductance: 1-3 pS. Based on these fundamental differences, the microglia channel is apparently a different gene product than the more common intermediate-conductance I-Clswell. Microglia express several candidate genes, with relative mRNA expression levels of: CLIC1 > ClC3 > I-Cln >= ClC2 > Best2 > Best1 >= Best3 > Best4. Using a pharmacological toolbox, we show that all drugs that reduced the microglia current (NPPB, IAA-94, flufenamic acid and DIOA) increased the resting cell volume in isotonic solution and inhibited the regulatory volume decrease that followed cell swelling in hypotonic solution. Both channel blockers tested (NPPB and flufenamic acid) dose-dependently inhibited microglia phagocytosis of E. coli bacteria. Because I-Clswell is involved in microglia functions that involve shape and volume changes, it is potentially important for controlling their ability to migrate to damage sites and phagocytose dead cells and debris.