Cytosolic and mitochondrial Ca2+ concentrations in primary hepatocytes change with ageing and in consequence of an mtDNA mutation.

Mitochondrial Ca2+ flux is crucial for the regulation of cell metabolism. Ca2+ entry to the mitochondrial matrix is mediated by VDAC1 and MCU with its regulatory molecules. We investigated hepatocytes isolated from conplastic C57BL/6NTac-mtNODLtJ mice (mtNOD) that differ from C57BL/6NTac mice (controls) by a point mutation in mitochondrial-encoded subunit 3 of cytochrome c oxidase, resulting in functional and morphological mitochondrial adaptations. Mice of both strains up to 12 months old were compared using mitochondrial GEM-GECO1 and cytosolic CAR-GECO1 expression to gain knowledge of age-dependent alterations of Ca2+ concentrations. In controls we observed a significant increase in glucose-induced cytosolic Ca2+ concentration with ageing, but only a minor elevation in mitochondrial Ca2+ concentration. Conversely, glucose-induced mitochondrial Ca2+ concentration significantly declined with ageing in mtNOD mice, paralleled by a slight decrease in cytosolic Ca2+ concentration. This was consistent with a significant reduction of the MICU1 to MCU expression ratio and a decline in MCUR1. Our results can best be explained in terms of the adaptation of Ca2+ concentrations to the mitochondrial network structure. In the fragmented mitochondrial network of ageing controls there is a need for high cytosolic Ca2+ influx, because only some of the isolated mitochondria are in direct contact with the endoplasmic reticulum. This is not important in the hyper-fused elongated mitochondrial network found in ageing mtNOD mice which facilitates rapid Ca2+ distribution over a large mitochondrial area.

Mitochondrial complex IV mutation increases reactive oxygen species production and reduces lifespan in aged mice.

AIM: Mitochondrial DNA (mtDNA) mutations can negatively influence lifespan and organ function. More than 250 pathogenic mtDNA mutations are known, often involving neurological symptoms. Major neurodegenerative diseases share key etiopathogenetic components ie mtDNA mutations, mitochondrial dysfunction and oxidative stress. METHODS: Here, we characterized a conplastic mouse strain (C57BL/6 J-mtNOD) carrying an electron transport chain complex IV mutation that leads to an altered cytochrome c oxidase subunit III. Since this mouse also harbours adenine insertions in the mitochondrial tRNA for arginine, we chose the C57BL/6 J-mtMRL as control strain which also carries a heteroplasmic stretch of adenine repetitions in this tRNA isoform. RESULTS: Using MitoSOX fluorescence, we observed an elevated mitochondrial superoxide production and a reduced gene expression of superoxide dismutase 2 in the 24-month-old mtNOD mouse as compared to control. Together with the decreased expression of the fission-relevant gene Fis1, these data confirmed that the ageing mtNOD mouse had a mitochondrial dysfunctional phenotype. On the functional level, we could not detect significant differences in synaptic long-term potentiation, but found a markedly poor physical constitution to perform the Morris water maze task at the age of 24 months. Moreover, the median lifespan of mtNOD mice was significantly shorter than of control animals. CONCLUSION: Our findings demonstrate that a complex IV mutation leads to mitochondrial dysfunction that translates into survival.