Calcium Signaling in Brain Mitochondria

Laura Contreras,Jorgina Satrústegui

doi:10.1074/jbc.m808066200

Abstract

Ca2+ signaling in mitochondria has been mainly attributed to Ca2+ entry to the matrix through the Ca2+ uniporter and activation of mitochondrial matrix dehydrogenases. However, mitochondria can also sense increases in cytosolic Ca2+ through a mechanism that involves the aspartate-glutamate carriers, extramitochondrial Ca2+ activation of the NADH malate-aspartate shuttle (MAS). Both pathways are linked through the shared substrate alpha-ketoglutarate (alphaKG). Here we have studied the interplay between the two pathways under conditions of Ca2+ activation. We show that alphaKG becomes limiting when Ca2+ enters in brain or heart mitochondria, but not liver mitochondria, resulting in a drop in alphaKG efflux through the oxoglutarate carrier and in a drop in MAS activity. Inhibition of alphaKG efflux and MAS activity by matrix Ca2+ in brain mitochondria was fully reversible upon Ca2+ efflux. Because of their differences in cytosolic calcium concentration requirements, the MAS and Ca2+ uniporter-mitochondrial dehydrogenase pathways are probably sequentially activated during a Ca2+ transient, and the inhibition of MAS at the center of the transient may provide an explanation for part of the increase in lactate observed in the stimulated brain in vivo.

Highlights

Ca2ϩ signaling in mitochondria has been mainly attributed to Ca2ϩ entry to the matrix through the Ca2ϩ uniporter (CaU)2 and activation of mitochondrial dehydrogenases [1]
Regulation of malateaspartate shuttle (MAS) and Glycerol-3-P Dehydrogenase Activity by Extramitochondrial and Intramitochondrial Ca2ϩ—MAS activity was shown to increase in response to extramitochondrial calcium in brain and heart mitochondria [3, 4, 36]. It became inhibited in a calcium-dependent manner when calcium was allowed to enter the mitochondria through the Ca2ϩ uniporter [4]
MAS activity in liver mitochondria is not affected by intramitochondrial Ca2ϩ, as its activity is the same in the absence or presence of Ruthenium Red (RR), it is activated by extramitochondrial calcium to a smaller extent than in brain or heart (Fig. 1C) as reported earlier [36]

Summary

Introduction

Ca2ϩ signaling in mitochondria has been mainly attributed to Ca2ϩ entry to the matrix through the Ca2ϩ uniporter (CaU) and activation of mitochondrial dehydrogenases (mitDH) [1]. Immunocytochemistry and in situ hybridization data and mRNA levels in acutely isolated brain cells indicate that aralar is localized preferentially in neurons (8 –12) This is consistent with a higher MAS activity in neuronal than astrocyte cultures [8] and with aralar being one of the more enriched proteins during differentiation of P19 cells to a neuronal phenotype [13]. Large Ca2ϩ signals that induce robust mitochondrial Ca2ϩ transients fail to activate the pathway [4] This suggested that in neuronal mitochondria the aralar-MAS pathway is inhibited under conditions in which the CaUmitDH pathway is activated. This surprising result indicates that Ca2ϩ activation of the malate aspartate shuttle and tricarboxylic acid cycle activity are somehow mutually exclusive in neurons. Our results suggest that MAS inhibition during Ca2ϩ-induced Krebs cycle activation would drive pyruvate to lactate formation and may play a role in lactate formation during brain activation

Results

Discussion

Conclusion