The principal mitochondrial K+ uniport is associated with respiratory complex I

Mitochondria generate ATP via coupling the negative electrochemical potential (proton motive force, Capital Greek (Deltap), consisting of a proton gradient (Capital Greek DeltapH+) and a membrane potential (Capital Greek Psim) across the respiratory chain, to phosphorylation of adenosine diphosphate nucleotide. In turn, DeltapH+ and Capital Greek Psim, are tightly balanced by the modulation of ionic uniporters and exchange-diffusion systems which preserve integrity of mitochondrial membranes and regulate ATP production. Here, we provide direct electrophysiological, pharmacological and genetic evidence that the main mitochondrial electrophoretic pathway for monovalent cations is associated with respiratory complex I, contrary to the long-held dogma that only H+ gradients are built across proteins of the mammalian electron transport chain. Here we propose a theoretical framework to describe how monovalent metal cations contribute to the buildup of H+ gradients and the proton motive force, extending the classical Mitchellian view on chemiosmosis and vectorial metabolism. Keywords: mitochondrial electrogenic transport, chemiosmotic theory, vectorial metabolism, whole-mitochondria electrophysiology.

Patient-derived iPSCs link elevated mitochondrial respiratory complex I function to osteosarcoma in Rothmund-Thomson syndrome

Rothmund-Thomson syndrome (RTS) is an autosomal recessive genetic disorder characterized by poikiloderma, small stature, skeletal anomalies, sparse brows/lashes, cataracts, and predisposition to cancer. Type 2 RTS patients with biallelic RECQL4 pathogenic variants have multiple skeletal anomalies and a significantly increased incidence of osteosarcoma. Here, we generated RTS patient-derived induced pluripotent stem cells (iPSCs) to dissect the pathological signaling leading to RTS patient-associated osteosarcoma. RTS iPSC-derived osteoblasts showed defective osteogenic differentiation and gain of in vitro tumorigenic ability. Transcriptome analysis of RTS osteoblasts validated decreased bone morphogenesis while revealing aberrantly upregulated mitochondrial respiratory complex I gene expression. RTS osteoblast metabolic assays demonstrated elevated mitochondrial respiratory complex I function, increased oxidative phosphorylation (OXPHOS), and increased ATP production. Inhibition of mitochondrial respiratory complex I activity by IACS-010759 selectively suppressed cellular respiration and cell proliferation of RTS osteoblasts. Furthermore, systems analysis of IACS-010759-induced changes in RTS osteoblasts revealed that chemical inhibition of mitochondrial respiratory complex I impaired cell proliferation, induced senescence, and decreased MAPK signaling and cell cycle associated genes, but increased H19 and ribosomal protein genes. In summary, our study suggests that mitochondrial respiratory complex I is a potential therapeutic target for RTS-associated osteosarcoma and provides future insights for clinical treatment strategies.

Quinone binding in respiratory complex I: Going through the eye of a needle. The squeeze-in mechanism of passing the narrow entrance of the quinone site

At the joint between the membrane and hydrophilic arms of the enzyme, the structure of the respiratory complex I reveals a tunnel-like Q-chamber for ubiquinone binding and reduction. The narrow entrance of the quinone chamber located in ND1 subunit forms a bottleneck (eye of a needle) which in all resolved structures was shown to be too small for a bulky quinone to pass through, and it was suggested that a conformational change is required to open the channel. The closed bottleneck appears to be a well-established feature of all structures reported so-far, both for the so-called open and closed states of the enzyme, with no indication of a stable open state of the bottleneck. We propose a squeeze-in mechanism of the bottleneck passage, where dynamic thermal conformational fluctuations allow quinone to get in and out. Here, using molecular dynamics simulations of the bacterial enzyme, we have identified collective conformational changes that open the quinone chamber bottleneck. The model predicts a significant reduction—due to a need for a rare opening of the bottleneck—of the effective bi-molecular rate constant, in line with the available kinetic data. We discuss possible reasons for such a tight control of the quinone passage into the binding chamber and mechanistic consequences for the quinone two-electron reduction. Graphic abstract

Loss of Functional SCO2 Attenuates Oxidative Stress in Diabetic Kidney Disease

Increased oxidative stress in glomerular endothelial cells (GEnCs) contributes to early diabetic kidney disease (DKD). While mitochondrial respiratory complex IV activity is reduced in DKD, it remains unclear whether this is a driver or a consequence of oxidative stress in GEnCs. Synthesis of cytochrome C oxidase 2 (SCO2), a key metallochaperone in the electron transport chain, is critical to the biogenesis and assembly of subunits required for functional respiratory complex IV activity. Here, we investigated the effects of <i>Sco2</i> hypomorphs (<i>Sco2^KO/KI, Sco2^KI/KI</i>), with a functional loss of SCO2, in the progression of DKD using a murine model of Type II Diabetes Mellitus, <i>db/db</i> mice. Diabetic <i>Sco2^KO/KI </i>and <i>Sco2^KI/KI</i> hypomorphs exhibited a reduction in complex IV activity, but an improvement in albuminuria, serum creatinine, and histomorphometric evidence of early DKD as compared to <i>db/db</i> mice. Single-nucleus RNA sequencing with gene set enrichment analysis of differentially expressed genes in the endothelial cluster of <i>Sco2^KO/KI;db/db</i> mice demonstrated an increase in genes involved in VEGF-VEGFR2 signaling and reduced oxidative stress as compared to <i>db/db</i> mice. These data suggest that reduced complex IV activity due to a loss of functional SCO2 might be protective in GEnCs in early DKD.

The ER protein Creld regulates ER-mitochondria contact dynamics and respiratory complex 1 activity

Dynamic contacts are formed between endoplasmic reticulum (ER) and mitochondria that enable the exchange of calcium and phospholipids. Disturbed contacts between ER and mitochondria impair mitochondrial dynamics and are a molecular hallmark of Parkinson’s disease. Cystein-rich with EGF-like domain (Creld) are ER-proteins associated with atrioventricular septal defects, but human CRELD1 is also a poorly characterized risk gene for Parkinson’s disease. Here we show that Creld is required for ER-mitochondria communication. Loss of Creld leads to mitochondrial hyperfusion and reduced ROS signaling in Drosophila melanogaster, Xenopus tropicalis and human cells. We found that reduced respiratory complex I activity lowers hydrogen peroxide levels, which disturbs neuronal activity and leads to impaired locomotion in Creld mutants. Our study presents a new paradigm of neuron dysfunction as a result of impaired ER-mitochondria communication and a new model for Parkinson’s disease.