Preserved respiratory chain capacity and physiology in mice with profoundly reduced levels of mitochondrial respirasomes.

The mammalian respiratory chain complexes I, III2, and IV (CI, CIII2, and CIV) are critical for cellular bioenergetics and form a stable assembly, the respirasome (CI-CIII2-CIV), that is biochemically and structurally well documented. The role of the respirasome in bioenergetics and the regulation of metabolism is subject to intense debate and is difficult to study because the individual respiratory chain complexes coexist together with high levels of respirasomes. To critically investigate the in vivo role of the respirasome, we generated homozygous knockin mice that have normal levels of respiratory chain complexes but profoundly decreased levels of respirasomes. Surprisingly, the mutant mice are healthy, with preserved respiratory chain capacity and normal exercise performance. Our findings show that high levels of respirasomes are dispensable for maintaining bioenergetics and physiology in mice but raise questions about their alternate functions, such as those relating to the regulation of protein stability and prevention of age-associated protein aggregation.

SARM1 deletion delays cerebellar but not spinal cord degeneration in an enhanced mouse model of SPG7 deficiency.

Hereditary spastic paraplegia is a neurological condition characterized by predominant axonal degeneration in long spinal tracts, leading to weakness and spasticity in the lower limbs. The nicotinamide adenine dinucleotide (NAD+)-consuming enzyme SARM1 has emerged as a key executioner of axonal degeneration upon nerve transection and in some neuropathies. An increase in the nicotinamide mononucleotide/NAD+ ratio activates SARM1, causing catastrophic NAD+ depletion and axonal degeneration. However, the role of SARM1 in the pathogenesis of hereditary spastic paraplegia has not been investigated. Here, we report an enhanced mouse model for hereditary spastic paraplegia caused by mutations in SPG7. The eSpg7 knockout mouse carries a deletion in both Spg7 and Afg3l1, a redundant homologue expressed in mice but not in humans. The eSpg7 knockout mice recapitulate the phenotypic features of human patients, showing progressive symptoms of spastic-ataxia and degeneration of axons in the spinal cord as well as the cerebellum. We show that the lack of SPG7 rewires the mitochondrial proteome in both tissues, leading to an early onset decrease in mito-ribosomal subunits and a remodelling of mitochondrial solute carriers and transporters. To interrogate mechanisms leading to axonal degeneration in this mouse model, we explored the involvement of SARM1. Deletion of SARM1 delays the appearance of ataxic signs, rescues mitochondrial swelling and axonal degeneration of cerebellar granule cells and dampens neuroinflammation in the cerebellum. The loss of SARM1 also prevents endoplasmic reticulum abnormalities in long spinal cord axons, but does not halt the degeneration of these axons. Our data thus reveal a neuron-specific interplay between SARM1 and mitochondrial dysfunction caused by lack of SPG7 in hereditary spastic paraplegia.

Lactate dehydrogenases promote glioblastoma growth and invasion via a metabolic symbiosis.

Lactate is a central metabolite in brain physiology but also contributes to tumor development. Glioblastoma (GB) is the most common and malignant primary brain tumor in adults, recognized by angiogenic and invasive growth, in addition to its altered metabolism. We show herein that lactate fuels GB anaplerosis by replenishing the tricarboxylic acid (TCA) cycle in absence of glucose. Lactate dehydrogenases (LDHA and LDHB), which we found spatially expressed in GB tissues, catalyze the interconversion of pyruvate and lactate. However, ablation of both LDH isoforms, but not only one, led to a reduction in tumor growth and an increase in mouse survival. Comparative transcriptomics and metabolomics revealed metabolic rewiring involving high oxidative phosphorylation (OXPHOS) in the LDHA/B KO group which sensitized tumors to cranial irradiation, thus improving mouse survival. When mice were treated with the antiepileptic drug stiripentol, which targets LDH activity, tumor growth decreased. Our findings unveil the complex metabolic network in which both LDHA and LDHB are integrated and show that the combined inhibition of LDHA and LDHB strongly sensitizes GB to therapy.

MDH2 produced OAA is a metabolic switch rewiring the fuelling of respiratory chain and TCA cycle.

The mitochondrial respiratory chain (RC) enables many metabolic processes by regenerating both mitochondrial and cytosolic NAD+ and ATP. The oxidation by the RC of the NADH metabolically produced in the cytosol involves redox shuttles as the malate-aspartate shuttle (MAS) and is of paramount importance for cell fate. However, the specific metabolic regulations allowing mitochondrial respiration to prioritize NADH oxidation in response to high NADH/NAD+ redox stress have not been elucidated. The recent discovery that complex I (NADH dehydrogenase), and not complex II (Succinate dehydrogenase), can assemble with other respiratory chain complexes to form functional entities called respirasomes, led to the assumption that this supramolecular organization would favour NADH oxidation. Unexpectedly, characterization of heart and liver mitochondria demonstrates that the RC systematically favours electrons provided by the 'respirasome free' complex II. Our results demonstrate that the preferential succinate driven respiration is tightly controlled by OAA levels, and that OAA feedback inhibition of complex II rewires RC fuelling increasing NADH oxidation capacity. This new regulatory mechanism synergistically increases RC's NADH oxidative capacity and rewires MDH2 driven anaplerosis of the TCA, preventing malate production from succinate to favour oxidation of cytosolic malate. This regulatory mechanism synergistically adjusts RC and TCA fuelling in response to extramitochondrial malate produced by the MAS.

Cardiolipin content controls mitochondrial coupling and energetic efficiency in muscle.

Unbalanced energy partitioning participates in the rise of obesity, a major public health concern in many countries. Increasing basal energy expenditure has been proposed as a strategy to fight obesity yet raises efficiency and safety concerns. Here, we show that mice deficient for a muscle-specific enzyme of very-long-chain fatty acid synthesis display increased basal energy expenditure and protection against high-fat diet-induced obesity. Mechanistically, muscle-specific modulation of the very-long-chain fatty acid pathway was associated with a reduced content of the inner mitochondrial membrane phospholipid cardiolipin and a blunted coupling efficiency between the respiratory chain and adenosine 5'-triphosphate (ATP) synthase, which was restored by cardiolipin enrichment. Our study reveals that selective increase of lipid oxidative capacities in skeletal muscle, through the cardiolipin-dependent lowering of mitochondrial ATP production, provides an effective option against obesity at the whole-body level.

Human COX7A2L Regulates Complex III Biogenesis and Promotes Supercomplex Organization Remodeling without Affecting Mitochondrial Bioenergetics.

The mitochondrial respiratory chain is organized in a dynamic set of supercomplexes (SCs). The COX7A2L protein is essential for mammalian SC III2+IV assembly. However, its function in respirasome (SCs I+III2+IVn) biogenesis remains controversial. To unambiguously determine the COX7A2L role, we generated COX7A2L-knockout (COX7A2L-KO) HEK293T and U87 cells. COX7A2L-KO cells lack SC III2+IV but have enhanced complex III steady-state levels, activity, and assembly rate, normal de novo complex IV biogenesis, and delayed respirasome formation. Nonetheless, the KOs have normal respirasome steady-state levels, and only larger structures (SCs I1-2+III2+IV2-n or megacomplexes) were undetected. Functional substrate-driven competition assays showed normal mitochondrial respiration in COX7A2L-KO cells in standard and nutritional-, environmental-, and oxidative-stress-challenging conditions. We conclude that COX7A2L establishes a regulatory checkpoint for the biogenesis of CIII2 and specific SCs, but the COX7A2L-dependent MRC remodeling is essential neither to maintain mitochondrial bioenergetics nor to cope with acute cellular stresses.