LETM1-Mediated K+ and Na+ Homeostasis Regulates Mitochondrial Ca2+ Efflux

HIGHLIGHTS \u2022 Monovalent cation homeostasis is dysregulated upon LETM1 depletion \u2022 K+/H+ exchange activity is decreased in LETM1 knockdown cells \u2022 LETM1 depletion results in K+ accumulation in the mitochondrial matrix \u2022 LETM1 knockdown does not affect expression of major mitochondrial Ca2+ transport modulators \u2022 LETM1-regulated mitochondrial Ca2+ fluxes are dependent on Na+ Ca2+ transport across the inner membrane of mitochondria (IMM) is of major importance for their functions in bioenergetics, cell death and signaling. It is therefore tightly regulated. It has been recently proposed that LETM1\u2014an IMM protein with a crucial role in mitochondrial K+/H+ exchange and volume homeostasis\u2014also acts as a Ca2+/H+ exchanger. Here we show for the first time that lowering LETM1 gene expression by shRNA hampers mitochondrial K+/H+ and Na+/H+ exchange. Decreased exchange activity resulted in matrix K+ accumulation in these mitochondria. Furthermore, LETM1 depletion selectively decreased Na+/Ca2+ exchange mediated by NCLX, as observed in the presence of ruthenium red, a blocker of the Mitochondrial Ca2+ Uniporter (MCU). These data confirm a key role of LETM1 in monovalent cation homeostasis, and suggest that the effects of its modulation on mitochondrial transmembrane Ca2+ fluxes may reflect those on Na+/H+ exchange activity

Mdm38 protein depletion causes loss of mitochondrial K+/H+ exchange activity, osmotic swelling and mitophagy

Loss of the MDM38 gene product in yeast mitochondria results in a variety of phenotypic effects including reduced content of respiratory chain complexes, altered mitochondrial morphology and loss of mitochondrial K+/H+ exchange activity resulting in osmotic swelling. By use of doxycycline-regulated shut-off of MDM38 gene expression, we show here that loss of K+/H+ exchange activity and mitochondrial swelling are early events, associated with a reduction in membrane potential and fragmentation of the mitochondrial reticulum. Changes in the pattern of mitochondrially encoded proteins are likely to be secondary to the loss of K+/H+ exchange activity. The use of a novel fluorescent biosensor directed to the mitochondrial matrix revealed that the loss of K+/H+ exchange activity was immediately followed by morphological changes of mitochondria and vacuoles, the close association of these organelles and finally uptake of mitochondrial material by vacuoles. Nigericin, a K+/H+ ionophore, fully prevented these effects of Mdm38p depletion. We conclude that osmotic swelling of mitochondria triggers selective mitochondrial autophagy or mitophagy