Gene inactivation of Na⁺H⁺ exchanger isoform 1 attenuates apoptosis and mitochondrial damage following transient focal cerebral ischemia

We investigated mechanisms underlying the Na^+/H⁺ exchanger isoform 1 (NHE1)-mediated neuronal damage in transient focal ischemia. Physiological parameters, body and tympanic temperatures, and regional cerebral blood flow during 30 min of middle cerebral artery occlusion were similar in wild-type NHE1 (NHE1^+/+) and NHE1 heterozygous (NHE1^+/-) mice. NHE1^{+/ +} mice developed infarct volume of 57.3 ± 8.8 mm>s.³ at 24 h reperfusion (Rp), which progressed to 86.1 ± 10.0 mm³ at 72 h Rp. This delayed cell death was preceded by release of mitochondrial cytochrome c (Cyt. C), nuclear translocation of apoptosis-inducing factor (AIF), activation of caspase-3, and TUNEL-positive staining and chromatin condensation in the ipsilateral hemispheres of NHE1 ^{+ /+} brains. In contrast, NHE1^+/- mice had a significantly smaller infarct volume and improved neurological function. A similar neuroprotection was obtained with NHE1 inhibitor HOE 642. The number of apoptotic cells, release of AIF and Cyt. C or levels of active caspase-3 was significantly reduced in NHE1+7" brains. These data imply that NHE1 activity may contribute to ischemic apoptosis. Ischemic brains did not exhibit changes of NHE1 protein expression. In contrast, up-regulation of NHE1 expression was found in NHE1^+/- neurons after in vitro ischemia. These data suggest that NHE1 activation following cerebral ischemia contributes to mitochondrial damage and ischemic apoptosis.