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Title

Requirement for aralar and its Ca2+-binding sites in Ca2+ signal transduction in mitochondria from INS-1 clonal beta-cells

Authors
Mármol, Patricia
Pardo, Beatriz
del Arco, Araceli
Satrústegui, Jorgina
Published in Journal of Biological Chemistry. 2009, vol. 284, no. 1, p. 515-24
Abstract Aralar, the mitochondrial aspartate-glutamate carrier present in beta-cells, is a component of the malate-aspartate NADH shuttle (MAS). MAS is activated by Ca2+ in mitochondria from tissues with aralar as the only AGC isoform with an S0.5 of approximately 300 nm. We have studied the role of aralar and its Ca2+-binding EF-hand motifs in glucose-induced mitochondrial NAD(P)H generation by two-photon microscopy imaging in INS-1 beta-cells lacking aralar or expressing aralar mutants blocked for Ca2+ binding. Aralar knock-down in INS-1 beta-cell lines resulted in undetectable levels of aralar protein and complete loss of MAS activity in isolated mitochondria and in a 25% decrease in glucose-stimulated insulin secretion. MAS activity in mitochondria from INS-1 cells was activated 2-3-fold by extramitochondrial Ca2+, whereas aralar mutants were Ca2+ insensitive. In Ca2+-free medium, glucose-induced increases in mitochondrial NAD(P)H were small (1.3-fold) and unchanged regardless of the lack of aralar. In the presence of 1.5 mm Ca2+, glucose induced robust increases in mitochondrial NAD(P)H (approximately 2-fold) in cell lines with wild-type or mutant aralar. There was a approximately 20% reduction in NAD(P)H response in cells lacking aralar, illustrating the importance of MAS in glucose action. When small Ca2+ signals that resulted in extremely small mitochondrial Ca2+ transients were induced in the presence of glucose, the rise in mitochondrial NAD(P)H was maintained in cells with wild-type aralar but was reduced by approximately 50% in cells lacking or expressing mutant aralar. These results indicate that 1) glucose-induced activation of MAS requires Ca2+ potentiation; 2) Ca2+ activation of MAS represents a larger fraction of glucose-induced mitochondrial NAD(P)H production under conditions where suboptimal Ca2+ signals are associated with a glucose challenge (50 versus 20%, respectively); 3) inactivation of EF-hand motifs in aralar prevents activation of MAS by small Ca2+ signals. The results suggest a possible role for aralar and MAS in priming the beta-cell by Ca2+-mobilizing neurotransmitter or hormones.
Keywords Amino Acid Motifs/physiologyAnimalsBinding Sites/geneticsBiological Transport/physiologyCalcium/metabolismCalcium Signaling/physiologyCells, CulturedGene Knockdown TechniquesGlucose/metabolismInsulin/secretionInsulin-Secreting Cells/cytology/metabolismMembrane Transport Proteins/genetics/metabolismMiceMitochondria/genetics/metabolismMitochondrial Membrane Transport ProteinsMitochondrial Proteins/genetics/metabolismMutationNADP/biosynthesisNeurotransmitter Agents/metabolismProtein Isoforms/genetics/metabolism
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PMID: 18996845
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MÁRMOL, Patricia et al. Requirement for aralar and its Ca2+-binding sites in Ca2+ signal transduction in mitochondria from INS-1 clonal beta-cells. In: Journal of Biological Chemistry, 2009, vol. 284, n° 1, p. 515-24. doi: 10.1074/jbc.M806729200 https://archive-ouverte.unige.ch/unige:33544

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Deposited on : 2014-01-22

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