SummaryType 2 diabetes (T2D) develops after years of prediabetes during which high glucose (glucotoxicity) impairs insulin secretion. We report that the ATP-conducting mitochondrial outer membrane voltage-dependent anion channel-1 (VDAC1) is upregulated in islets from T2D and non-diabetic organ donors under glucotoxic conditions. This is caused by a glucotoxicity-induced transcriptional program, triggered during years of prediabetes with suboptimal blood glucose control. Metformin counteracts VDAC1 induction. VDAC1 overexpression causes its mistargeting to the plasma membrane of the insulin-secreting β cells with loss of the crucial metabolic coupling factor ATP. VDAC1 antibodies and inhibitors prevent ATP loss. Through direct inhibition of VDAC1 conductance, metformin, like specific VDAC1 inhibitors and antibodies, restores the impaired generation of ATP and glucose-stimulated insulin secretion in T2D islets. Treatment of db/db mice with VDAC1 inhibitor prevents hyperglycemia, and maintains normal glucose tolerance and physiological regulation of insulin secretion. Thus, β cell function is preserved by targeting the novel diabetes executer protein VDAC1.

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.

Real-time recordings of ER ATP fluxes in single cells using an ER-targeted, genetically encoded ATP sensor within the lumen of the ER reveal a local Ca2+-controlled ATP signal that is restored during energy stress. The data highlight a novel Ca2+-controlled process that supplies the ER with additional energy upon cell stimulation.

In the cell, γ-tubulin establishes a cellular network of threads named the γ-string meshwork. However, the functions of this meshwork remain to be determined. We investigated the traits of the meshwork and show that γ-strings have the ability to connect the cytoplasm and the mitochondrial DNA together. We also show that γ-tubulin has a role in the maintenance of the mitochondrial network and functions as reduced levels of γ-tubulin or impairment of its GTPase domain disrupts the mitochondrial network and alters both their respiratory capacity and the expression of mitochondrial-related genes. By contrast, reduced mitochondrial number or increased protein levels of γ-tubulin DNA-binding domain enhanced the association of γ-tubulin with mitochondria. Our results demonstrate that γ-tubulin is an important mitochondrial structural component that maintains the mitochondrial network, providing mitochondria with a cellular infrastructure. We propose that γ-tubulin provides a cytoskeletal element that gives form to the mitochondrial network.

Background: Endosomal cholesterol storage in Niemann-Pick type C1 deficiency is associated with elevated mitochondrial cholesterol and alterations in energy metabolism. Results: Blocking endosomal cholesterol transport to mitochondria prevented the metabolic alterations in NPC1-deficient cells. Conclusion: Mitochondrial cholesterol accumulation increases oxidative stress and alters energy metabolism. Significance: Mitochondrial cholesterol is a regulator of energy metabolism and mitochondrial function.