Silencing of cardiac mitochondrial NHE1 prevents mitochondrial permeability transition pore opening

Inhibition of Na ⁺/H ⁺ exchanger 1 (NHE1) reduces cardiac ischemia-reperfusion (I/R) injury and also cardiac hypertrophy and failure. Although the mechanisms underlying these NHE1-mediated effects suggest delay of mitochondrial permeability transition pore (MPTP) opening, and reduction of mitochon-drial-derived superoxide production, the possibility of NHE1 blockade targeting mitochondria has been incompletely explored. A short-hairpin RNA sequence mediating specific knock down of NHE1 expression was incorporated into a lentiviral vector (shRNA-NHE1) and transduced in the rat myocardium. NHE1 expression of mitochondrial lysates revealed that shRNA-NHE1 transductions reduced mitochondrial NHE1 (mNHE1) by ~60%, supporting the expression of NHE1 in mitochondria membranes. Electron microscopy studies corroborate the presence of NHE1 in heart mitochondria. Immunostain-ing of rat cardiomyocytes also suggests colocalization of NHE1 with the mitochondrial marker cytochrome c oxidase. To examine the functional role of mNHE1, mitochondrial suspensions were exposed to increasing concentrations of CaCl ₂ to induce MPTP opening and consequently mitochondrial swelling. shRNA-NHE1 transduction reduced CaCl ₂-induced mitochondrial swelling by 64 ± 4%. Whereas the NHE1 inhibitor HOE-642 (10 (xM) decreased mitochondrial Ca ²⁺-induced swelling in rats transduced with nonsilencing RNAi (37 ± 6%), no additional HOE-642 effects were detected in mitochondria from rats transduced with shRNA-NHE1. We have characterized the expression and function of NHE1 in rat heart mitochondria. Because mitochondria from rats injected with shRNA-NHE1 present a high threshold for MPTP formation, the beneficial effects of NHE1 inhibition in I/R resulting from mitochondrial targeting should be considered.