Contribution of sodium channels to lamellipodial protrusion and Rac1 and ERK1/2 activation in ATP-stimulated microglia.

Microglia are motile resident immune cells of the central nervous system (CNS) that continuously explore their territories for threats to tissue homeostasis. Following CNS insult (e.g., cellular injury, infection, or ischemia), microglia respond to signals such as ATP, transform into an activated state, and migrate towards the threat. Directed migration is a complex and highly-coordinated process involving multiple intersecting cellular pathways, including signal transduction, membrane adhesion and retraction, cellular polarization, and rearrangement of cytoskeletal elements. We previously demonstrated that the activity of sodium channels contributes to ATP-induced migration of microglia. Here we show that TTX-sensitive sodium channels, specifically NaV 1.6, participate in an initial event in the migratory process, i.e., the formation in ATP-stimulated microglia of polymerized actin-rich membrane protrusions, lamellipodia, containing accumulations of Rac1 and phosphorylated ERK1/2. We also examined Ca(2+) transients in microglia and found that blockade of sodium channels with TTX produced a downward shift in the level of [Ca(2+) ]i during the delayed, slower recovery of [Ca(2+) ]i following ATP stimulation. These observations demonstrate a modulatory role of sodium channels on Ca(2+) transients in microglia that are likely to affect down-stream signaling cascades. Consistent with these observations, we demonstrate that ATP-induced microglial migration is mediated via Rac1 and ERK1/2, but not p38α/β and JNK, dependent pathways, and that activation of both Rac1 and ERK1/2 is modulated by sodium channel activity. Our results provide evidence for a direct link between sodium channel activity and modulation of Rac1 and ERK1/2 activation in ATP-stimulated microglia, possibly by regulating Ca(2+) transients.