Modulation of the matrix redox signaling by mitochondrial Ca2+

Figure 1 Ca²⁺ transport proteins of mitochondria. In mammalian mitochondria, the uptake of Ca²⁺ is mediated by the Ca²⁺-selective channel MCU, which is part of a high molecular weight protein complex called Uniplex. At least 4 additional proteins (MCUb, MICU1, MICU2 and EMRE) regulate MCU activity. Ca²⁺ is then extruded by a sodium/calcium exchange (NCX) or proton/calcium exchange (HCX). If the protein NCLX has been confirmed to be the mitochondrial NCX, which is down-regulated by the protein SLP-2, the molecular nature of the mitochondrial HCX is still debated. Dimers of mitochondrial ATP synthase have been proposed to form the PTP, a mitochondrial channel regulated by CypD, that facilitates PTP opening by desensitizing PTP to Ca²⁺. Besides being activated by Ca²⁺, PTP has also been proposed to act as a reversible fast Ca²⁺ release channel. Other non-MCU mitochondrial proteins with an indirect or debated effect on Ca²⁺ transport are represented in the blue square (MCUR; SLC25A23; ryanodine receptor, RyR; UCP2; UCP3; LETM1). OMM: Outer mitochondrial membrane; IMS: Inter-membrane space; IMM: Inner mitochondrial membrane; VDAC: Mitochondrial porin; PTP: Permeability transition pore; CypD: Cyclophilin D; MCU: Mitochondrial Ca²⁺ uniporter; MCUR1: Mitochondrial calcium uniporter regulator 1; MICU1: Mitochondrial Ca²⁺ uptake protein 1; MICU2: Mitochondrial Ca²⁺ uptake protein 2; EMRE: Essential MCU regulator.

Figure 2 Ratiometric fluorescence intensity response of roGFP1 to histamine stimulation and exogenous H₂O₂ and DL-dithiothreitol in living HeLa cells. HeLa cells were transfected with the mitochondrial targeted redox sensor roGFP1. Cells were excited at 410 and 480 nm and emission was collected at 535 nm. The 410/480 fluorescence ratio (R) was normalized to the minimum of fluorescence (obtained after the addition of 1 mmol/L H₂O₂) and the maximum (after the addition of 10 mmol/L dithiothreitol, DL-dithiothreitol). The effect of intracellular Ca²⁺ was assessed by stimulating the cell with 100 μmol/L histamine. DTT: DL-Dithiothreitol.

Figure 3 Proposed model for the modulation of the matrix redox signaling by mitochondrial Ca²⁺. After cell stimulation, Ca²⁺ released from the endoplasmic reticulum (ER) enters in mitochondria by the mitochondrial uniplex (left), stimulating matrix Ca²⁺-dependent dehydrogenases, which increase NADH levels and promoting a reduction of the mitochondrial glutathione pool. In cells over-expressing the mitochondrial Na⁺/Ca²⁺ exchanger (NCLX, right) Ca²⁺ extrusion is more efficient and therefore Ca²⁺-dependent dehydrogenases less activated. Such lowering the mitochondrial Ca²⁺ response blunts NADH formation and prevents matrix redox changes of the mitochondrial glutathione pool. NADH: Reduced form of nicotinamide-adenine dinucleotide; NCLX: Mitochondrial Na⁺/Ca²⁺ exchanger.