Role of sodium/hydrogen exchanger isoform 1 in microglial activation and proinflammatory responses in ischemic brains

Abstract

Our recent study reveals that Na+/H+ exchanger isoform 1 (NHE-1) mediates H+ extrusion during “respiratory bursting”, which is important for microglial activation. In the present study, we further investigated whether NHE-1 plays a role in pro-inflammatory activation of microglia in vivo using a mouse model of transient focal cerebral ischemia and reperfusion (I/R). Activated microglial cells were identified by their expression of two microglial marker proteins (CD11b and Iba1) as well as by their transformation from a “ramified” to an “amoeboid” morphology. An immediate increase in activated microglial numbers was detected in the ipsilateral ischemic core area of NHE-1+/+ brains at 1 hour (h) I/1 h R, which gradually decreased during 6-24 h I/R. This was followed by a sharp rise in microglial activation in the peri-infarct area and an increase in proinflammatory cytokine formation at 3 day after I/R. Interestingly, HOE 642 (a potent NHE-1 inhibitor) -treated or NHE-1 heterozygous (NHE-1+/-) mice exhibited less microglia activation, less NADPH oxidase activation, or a reduced proinflammatory response at 3-7 day after I/R. Blocking NHE-1 activity also significantly decreased microglial phagocytosis in vitro. In contrast, astrogliosis formation in the peri-infarct area was not affected by NHE-1 inhibition. Taken together, our results demonstrate that NHE-1 protein was abundantly expressed in activated microglia and astrocytes. NHE-1 inhibition reduced microglial pro-inflammatory activation following ischemia.

Intracellular pH reduction prevents excitotoxic and ischemic neuronal death by inhibiting NADPH oxidase

“…NOX2 activity in nonneuronal cell types has previously been shown to be sensitive to modest intracellular pH reductions (24). The mechanism of this effect is unlikely to be simple mass action, given its steep pH dependence; instead, evidence suggests that H + concentration influences the phosphorylation status of p47 phox , a cytosolic "organizer" subunit of the NOX2 complex (20,23).…”

“…2E). In microglia and other immune cells, the H + generated by NOX2 activity is released to the extracellular space through both the voltage-sensitive proton channel Hv1 and the Na + /H + exchanger NHE1, and inhibition of either process causes intracellular acidification and resultant cessation of NOX2 activity (21)(22)(23). A distinction between Hv1 and NHE1 is that protons released through Hv1 relieve the plasma membrane depolarization caused by intracellular H + accumulation, whereas Na + /H + exchange is electroneutral.…”

“…Reduced sodium accumulation in NHE1 +/− neurons may promote neuronal survival (29), and reduced sodium accumulation in nonneuronal cells limits brain edema (40). Moreover, suppression of NOX2 activation by NHE1 or Hv1 inhibition in microglia inhibits the innate immune response, and may thereby promote neuronal survival in the postischemic interval (21)(22)(23).…”

NADPH Oxidase-2: Linking Glucose, Acidosis, and Excitotoxicity in Stroke

“…NOX2 activity is highly pH sensitive (68,125,160,182), with a 50% reduction in activity observed with a 0.2 pH unit reduction in intracellular pH. In many cell types, H + generated by NOX2 is released to the extracellular space through voltage-dependent hydrogen ion channels or by Na + /H + exchangers, and inhibition of this release causes intracellular acidification and resultant cessation of NOX2 activity (111,148,159). The mechanism of this effect is unlikely to be a simple mass action, given its steep pH dependence; instead, evidence suggests that the H + concentration influences the phosphorylation status of p47 phox (159).…”

“…In many cell types, H + generated by NOX2 is released to the extracellular space through voltage-dependent hydrogen ion channels or by Na + /H + exchangers, and inhibition of this release causes intracellular acidification and resultant cessation of NOX2 activity (111,148,159). The mechanism of this effect is unlikely to be a simple mass action, given its steep pH dependence; instead, evidence suggests that the H + concentration influences the phosphorylation status of p47 phox (159). Given the pH sensitivity of NOX2 and the role of this enzyme in NMDA receptor-mediated cell death, modest reductions in pH might limit neurotoxicity by dissociating NMDA receptor activation from superoxide production (Fig.…”