Caracterización de los transportadores mitocondriales de ATP-Mg/Pi y su implicación en la señalización por calcio a la mitocondria

Resumen

The transport of metabolites across the inner mitocondrial membrane is carried out by a familiy of proteins called Mitocondrial Carriers (MC). Studies in our laboratory led to the identification of a novel subfamily of mitochondrial carriers, the SCaMCs (Short Calciumbinding Mitochondrial Carriers), with at least four paralogues in mammals (SCaMC- 1/SLC25A24, SCaMC-2/SLC25A25, SCaMC-3/SLC25A23 and SCaMC-3like/SLC25A41) and only one in the yeast Saccharomyces cerevisiae (SAL1). Most SCaMCs are characterized by, in addition to the MC domain, an N-terminal domain which harbours calcium binding motifs that face the intermembrane space. Thus, the activity of these proteins could be regulated by extramitochondrial calcium. SCaMCs correspond to isoforms of the ATP-Mg/Pi carrier, whose transport activity was functionally identified 20 years ago, and catalyze the net transport of adenine nucleotides across the inner mitochondrial membrane. They mediate a reversible electroneutral exchange between ATP-Mg2- and HP042-. The ATP-Mg/Pi carrier regulates the matrix adenine nucleotide content, contributing to the regulation of mitochondrial activities dependent on adenine nucleotides such as gluconeogesis, urea synthesis, mitochondrial biogenesis or permeability transition pore opening. We have studied the transport activity, calcium regulation and function of the SCaMCs in three different models: yeast mitochondria, where Sal1p is the only SCaMC present; mouse liver and brain mitochondria, where SCaMC-3 is the main paralogue; and cell lines mitochondria, where SCaMC-1 is the main paralogue. The yeast ATP-Mg/Pi carrier is activated by calcium with a S0.5 of 15.0 ¿M, and is recruited as a Ca2+-dependent mechanism to import ATP-Mg from the citosol upon glucose addition to nutrient starved yeast, and is also involved in ATP-Mg uptake from the cytosol in yeast growing exponentially in glucose, a condition in which yeast mitochondria are ATP consumers. SCaMC-1 and SCaMC-3 in mammals are activated by calcium with a S0.5 of 12.7 and 3.4 ¿M, respectively. They are involved in the regulation of the calcium retention capacity, by regulating the matrix free calcium concentration, and thus they regulate the permeability transition pore (PTP), a non-specific pore that is formed in the inner mitochondrial membrane under conditions of calcium overload and/or oxidative stress, which has been implicated in cell death by necrosis, for example in situations of ischemia-reperfusion. These findings have led us to propose a new model for the regulation of the calcium retention capacity and PTP opening in mitochondria.