Implicacion de aralar/agc1/sic25a12,el transportador mitocondrial de aspartato-glutamato, y su dependencia funcional de ca2+ en el metabolismo de la glucosa responsable de la secreción de insulina en la celula-b

Resumen

Aralar (AGC1) is one of the isoforms of the mitochondrial aspartate-glutamate carrier, and the key component of the malate-aspartate shuttle (MAS), which is involved in the transfer of redox equivalents produced in glycolysis from cytosol to mitochondria. Aralar has Ca 2+ -binding motifs (EF-hands) in its N-terminal portion facing the mitochondrial intermembrane space. Previous studies from our laboratory have shown that MAS activity is activated by cytosolic Ca 2+ with a S 0.5 of ~300 nM in brain, heart and skeletal muscle; below the Km of the Ca 2+ uniporter, and thus, MAS activation by Ca 2+ constitutes a mechanism for transducing Ca 2+ signals to mitochoondria independent of the Ca 2+ uniporter. The importance of aralar in metabolic glucose signalling in ß-cells has been addressed by silencing of aralar in INS-1 ß-cells which resulted in a completely abolition of MAS activity. Under these conditions, glucose-induced insulin secretion (GSIS) and mitochondrial NAD(P)H formation were inhibited by 20-25%. The importance of Ca 2+ -binding by aralar was addressed by introducing mutations in aralar which blocked Ca 2+ -binding. As these mutations abolish Ca 2+ activation of MAS, while preserving its basal activity, MAS activation by Ca 2+ in mitochondria from INS-1 ß-cells is clearly conferred by its functional EF-hand pair composed of EF1-EF2. MAS was found to have a Ca 2+ requirement to be operative in intact INS-1 ß-cells, as glucosestimulated NAD(P)H formation in mitochondria seems to correspond to pyruvate oxidation in the total absence of extracellular Ca 2+ , without any participation of aralar. When small cytosolic Ca 2+ signals are applied, mitochondrial NAD(P)H formation induced by glucose is potentiated in intact INS-1 ß-cells, and this potentiation does not take place when aralar has mutations in its EF1-EF2 Ca 2+ -binding motifs. The hypothesis that a single aralar allele may be a risk factor for the development of type 2 diabetes in mice after a year long high fat diet was tested. The results showed that even after this long lipotoxic treatment, insulin secretion was maintained. However, old Aralar +/- mice had a tendency towards a decreased insulin resistance compared to wild-type mice, possibly involving the muscle.