Leucine Zipper EF Hand-containing Transmembrane Protein 1 (Letm1) and Uncoupling Proteins 2 and 3 (UCP2/3) Contribute to Two Distinct Mitochondrial Ca2+ Uptake Pathways

Abstract

Cytosolic Ca2+ signals are transferred into mitochondria over a huge concentration range. In our recent work we described uncoupling proteins 2 and 3 (UCP2/3) to be fundamental for mitochondrial uptake of high Ca2+ domains in mitochondria-ER junctions. On the other hand, the leucine zipper EF hand-containing transmembrane protein 1 (Letm1) was identified as a mitochondrial Ca2+/H+ antiporter that achieved mitochondrial Ca2+ sequestration at small Ca2+ increases. Thus, the contributions of Letm1 and UCP2/3 to mitochondrial Ca2+ uptake were compared in endothelial cells. Knock-down of Letm1 did not affect the UCP2/3-dependent mitochondrial uptake of intracellularly released Ca2+ but strongly diminished the transfer of entering Ca2+ into mitochondria, subsequently, resulting in a reduction of store-operated Ca2+ entry (SOCE). Knock-down of Letm1 and UCP2/3 did neither impact on cellular ATP levels nor the membrane potential. The enhanced mitochondrial Ca2+ signals in cells overexpressing UCP2/3 rescued SOCE upon Letm1 knock-down. In digitonin-permeabilized cells, Letm1 exclusively contributed to mitochondrial Ca2+ uptake at low Ca2+ conditions. Neither the Letm1- nor the UCP2/3-dependent mitochondrial Ca2+ uptake was affected by a knock-down of mRNA levels of mitochondrial calcium uptake 1 (MICU1), a protein that triggers mitochondrial Ca2+ uptake in HeLa cells. Our data indicate that Letm1 and UCP2/3 independently contribute to two distinct, mitochondrial Ca2+ uptake pathways in intact endothelial cells.

Trypanosome Letm1 Protein Is Essential for Mitochondrial Potassium Homeostasis

“…Another study has implicated Letm1 in Ca 2ϩ translocation across the IM (14,15). Our results suggest otherwise, agreeing with those that have postulated a role in KHE (1, 3, 10).…”

“…In all of these model systems, treatment with the chemical K ϩ /H ϩ exchanger nigericin compensates for the loss of Letm1-mediated KHE. However, Letm1 was also identified as a calcium (Ca 2ϩ )/H ϩ antiporter in the genome-wide RNAi screen in Drosophila S2 cells (14), a finding corroborated in a later report (15).…”

Overexpression of ryanodine receptor type 1 enhances mitochondrial fragmentation and Ca2+-induced ATP production in cardiac H9c2 myoblasts

“…To test whether RyR1 overexpression can affect ATP production, [ATP] mt was measured using the mitochondrial matrix-targeted FRET-based ATP sensor Ateam by confocal microscopy (see MATERIALS AND METHODS; Fig. 10A) (29,61,79). We confirmed that this ATP sensor was expressed in mitochondria in H9c2 cells after transfection (Fig.…”

Update on vascular endothelial Ca2+signalling: A tale of ion channels, pumps and transporters

“…Mitochondrial Ca 2+ uptake from SOCE in ECs is accomplished by a second type of MCU, which mediates Ca 2+ uptake into mitochondria at nanomolar cytosolic Ca 2+ concentrations and has been termed Letm1 [383] . This is a leucine-zipper-EF hand-containing a TM region which catalyzes the 1:1 electrogenic exchange of Ca 2+ for H + [386] . Conversely, the contribution of the so-called mitochondrial calcium uptake 1 (MICU1), which has recently been discovered by working on HeLa and HEK-293 cells [387] , to endothelial Ca 2+ signaling has not been ascertained yet The mitochondrial NCX (mNCX) is the main mechanism responsible for the outward transport of Ca 2+ towards the cytosol with a stoichiometry of 3Na + :1Ca 2+ (as for the plasmalemmal NCX) [36,41] : the Na + that enters down the electrochemical gradient is exchanged for Ca 2+ [383] .…”