Conference Agenda

A common misconception in mitochondrial sciences is to use the terms ‘bioenergetics’ and ‘OXPHOS’ interchangeably. OXPHOS is a process dependent on the concerted action of several individual molecular entities but each of them may operate as stand-alone and participate in non-OXPHOS pathways. It is therefore not surprising that targeted inhibition of the electron transport chain will abolish OXPHOS but the remaining complexes may still engage in bioenergetic activities. In this context, pathways generating NAD⁺ during hypoxia will be addressed that fuel the α-ketoglutarate dehydrogenase complex (KGDHC), a critical component in the oxidative decarboxylation branch of glutaminolysis. These are: i) the residual complex I activity in relation to fumarate being a terminal electron acceptor for complex II reversal, ii) the reversal of the mitochondrial malate dehydrogenase (isoform 2), and iii) mitochondrial diaphorases.

The cardiopulmonary system adapts to acute alveolar hypoxia and systemic hypoxemia by hypoxic pulmonary vasoconstriction and hyperventilation, thereby optimizing systemic oxygenation. Pulmonary arterial smooth muscle cells (PASMCs) and glomus cells in the carotid body sense acute, mild hypoxia and trigger these physiologic responses on a breath to breath basis by increased release of mitochondrial superoxide. We previously showed that presence of the cytochrome c oxidase subunit 4 isoform 2 (Cox4i2) is essential for hypoxia-induced mitochondrial superoxide release due to sensitization of the cytochrome c oxidase (COX) to hypoxia [1]. We now describe in more detail the underlying mechanism of Cox4i2-induced superoxide release in acute hypoxia including regulation of the redox state of the electron transport chain, assembly of Cox4i2 in COX and potential functional relevance of Cox4i2-specific cysteines. Moreover, we provide a perspective on other cellular response that are regulated by Cox4i2-dependent superoxide release.

[1] O. Pak, A. Nolte, F. Knoepp, L. Giordano, P. Pecina, M. Hüttemann M, L.I. Grossman, N. Weissmann, N. Sommer, Mitochondrial oxygen sensing of acute hypoxia in specialized cells - Is there a unifying mechanism? BBA Bioenergetics, 1863 (2022) 148911.

In combination with the substrate-uncoupler-inhibitor-titration protocol (SUIT), high-resolution respirometry (HRR) allows for a nuanced analysis of mitochondrial function, particularly in the LEAK, oxidative phosphorylation-OXPHOS, and electron transfer-ET states; which are essential for understanding the bioenergetics of skeletal muscle. Mitochondrial function assessment has been enhanced by coupling HRR with fluorescence and potentiometric sensors. This combination has enabled precise measurements of key parameters like membrane potential, hydrogen peroxide (H2O2), and adenosine triphosphate (ATP) production across various biological samples. Recent technical developments mark a leap forward, allowing simultaneous real-time monitoring of oxygen consumption and the coenzyme Q-redox state.

Previous findings from our group indicate that CoQ2 (used as a Q-mimetic), when properly optimized, does not interfere with the respirometry measurements in permeabilized fibers (pfi). The observed changes in the reduction-oxidation state of the electron transfer system (ETS)-reactive Q-pool upon the addition of various substrates and ADP highlight the dynamic nature of mitochondrial bioenergetics. The current study aims to combine these technologies (HRR, membrane potential, and Q-redox state) to yield a more refined comprehension of how exercise-induced alterations in mitochondrial structure and function contribute to the overall bioenergetic profile of the muscle.

The study revealed that both acute and chronic aerobic exercise induced slight enhancements in mitochondrial respiration in the extensor digitorum longus muscle. Notably, the stability of mitochondrial membrane potential and ETS-reactive CoQ redox state was maintained across various respiratory conditions, unaffected by either acute or chronic exercise. This stability contributes to a deeper comprehension of the physiological changes in skeletal muscle following diverse exercise routines. Further research is ongoing to assess how mitochondrial dynamics and mitochondrial supercomplex formation could add to these findings and reach more conclusive insights.

Prospective human cohort studies are among our best tools for understanding the factors that influence trajectories of aging across multiple domains. Cohort studies that incorporate human mitochondrial bioenergetic profiling strategies are underway and are revealing how mitochondria can drive differences in physical, cognitive, and sensory abilities. This presentation in discuss baseline findings from ongoing human cohort studies, including the Study of Muscle, Mobility and Aging (SOMMA), a multi-center observational study of 879 men and women 70+ yrs of age.