Expression and Contributions of TRPM7 and KCa2.3/SK3 Channels to the Increased Migration and Invasion of Microglia in Anti-Inflammatory Activation States

Abstract

Microglia rapidly respond to CNS injury and disease and can assume a spectrum of activation states. While changes in gene expression and production of inflammatory mediators have been extensively described after classical (LPS-induced) and alternative (IL4-induced) microglial activation, less is known about acquired de-activation in response to IL10. It is important to understand how microglial activation states affect their migration and invasion; crucial functions after injury and in the developing CNS. We reported that LPS-treated rat microglia migrate very poorly, while IL4-treated cells migrate and invade much better. Having discovered that the lamellum of migrating microglia contains a large ring of podosomes – microscopic structures that are thought to mediate adhesion, migration and invasion – we hypothesized that IL4 and IL10 would differentially affect podosome expression, gene induction, migration and invasion. Further, based on the enrichment of the KCa2.3/SK3 Ca2+-activated potassium channel in microglial podosomes, we predicted that it regulates migration and invasion. We found both similarities and differences in gene induction by IL4 and IL10 and, while both cytokines increased migration and invasion, only IL10 affected podosome expression. KCa2.3 currents were recorded in microglia under all three activation conditions and KCNN3 (KCa2.3) expression was similar. Surprisingly then, of three KCa2.3 inhibitors (apamin, tamapin, NS8593), only NS8593 abrogated the increased migration and invasion of IL4 and IL10-treated microglia (and invasion of unstimulated microglia). This discrepancy was explained by the observed block of TRPM7 currents in microglia by NS8593, which occurred under all three activation conditions. A similar inhibition of both migration and invasion was seen with a TRPM7 inhibitor (AA-861) that does not block KCa2.3 channels. Thus, we conclude that TRPM7 (not KCa2.3) contributes to the enhanced ability of microglia to migrate and invade when in anti-inflammatory states. This will be an important consideration in developing TRPM7 inhibitors for treating CNS injury.

Differential Kv1.3, KCa3.1, and Kir2.1 expression in “classically” and “alternatively” activated microglia

“…A). We also did not observe any apamin‐sensitive small‐conductance Ca 2+ ‐activated K Ca 2 or SK channels which also have been reported in cultured primary rat microglia (Khanna et al, ) or the MSL‐9 rat microglial cell line (Siddiqui et al, ) despite the fact that low levels of K Ca 2.3 message were detectable in our study. With respect to polarizing stimuli, both LPS and IFN‐γ have been reported to increase K V 1.3 expression in cultured neonatal rat microglia as early as 1992 (Norenberg et al, ), while findings with IL‐4 again seem to differ between mice and rats.…”

Quantification of the functional expression of the Ca2+-activated K+channel KCa3.1 on microglia from adult human neocortical tissue

“…A consistent finding across studies suggest expression of K Ca 3.1, K ir 2.1, and K v 1.3 (Chen et al 2015;Ferreira et al 2014Ferreira et al , 2015Kaushal et al 2007) with some reports also being consistent with functional expression of K Ca 2.3 (Schlichter et al 2010;Siddiqui et al 2014). A consistent finding across studies suggest expression of K Ca 3.1, K ir 2.1, and K v 1.3 (Chen et al 2015;Ferreira et al 2014Ferreira et al , 2015Kaushal et al 2007) with some reports also being consistent with functional expression of K Ca 2.3 (Schlichter et al 2010;Siddiqui et al 2014).…”

Strategies for Neuroprotection in Multiple Sclerosis and the Role of Calcium

“…Like TRPC3, TRPM1 is upregulated after stimulation of microglia with Aβ [108], whereas TRPM2 is involved in microglia activation [109] after interleukin stimulation [101]. TRPM7 is important for migration properties [110]. Additionally, the TRPM4 channel was found on microglia, but its function remains unclear [111].…”