Selective role of Na+/H+ exchanger in Cx3cr1+ microglial activation, white matter demyelination, and post-stroke function recovery

Na+/H+ exchanger (NHE1) activation is required for multiple microglial functions. We investigated effects of selective deletion of microglial Nhe1 in Cx3cr1-Cre(ER);Nhe1(f/f) mice on neuroinflammation and tissue repair after ischemic stroke. Infarct volume was similar in corn oil or tamoxifen (Tam)-treated mice at 48 hr and 14 days post-stroke. However, the Tam-treated mice showed significantly higher survival rate and faster neurological function recovery during day 1-14 post-stroke. Deletion of microglial Nhe1 prevented the elevation of CD11b(+)/CD45(low-med) microglia in the ischemic hemisphere at day 3 post-stroke, but stimulated expression of Ym1, CD68, TGF-beta, IL-10, decreased expression of CD86 and IL-1 beta, and reduced GFAP(+) reactive astrocytes. Moreover, at day 14 post-stroke, enhanced white matter myelination was detected in the microglial Nhe1 deleted mice. In comparison, neuronal Nhe1-null mice (the CamKII-Cre(+/-);Nhe1(f/f) mice) showed a significant reduction in both acute and subacute infarct volume, along with increased survival rate and moderate neurological function recovery. However, these neuronal Nhe1-null mice did not exhibit reduced activation of CD11b(+)/CD45(low-med) microglia or CD11b(+)/CD45(hi) macrophages in the ischemic brains, and they exhibited no reductions in white matter lesions. Taken together, this study demonstrated that deletion of microglial and neuronal Nhe1 had differential effects on ischemic brain damage. Microglial NHE1 is involved in pro-inflammatory responses during post-stroke brain tissue repair. In contrast, neuronal NHE1 activation is directly associated with the acute ischemic neuronal injury but not inflammation. Our study reveals that NHE1 protein is a potential therapeutic target critical for differential regulation of ischemic neuronal injury, demyelination and tissue repair.