Impaired oxygen-sensitive regulation of mitochondrial biogenesis within the von Hippel-Lindau syndrome

Mitochondria are the main consumers of oxygen within the cell. How mitochondria sense oxygen levels remains unknown. Here we show an oxygen-sensitive regulation of TFAM, an activator of mitochondrial transcription and replication, whose alteration is linked to tumours arising in the von Hippel-Lindau syndrome. TFAM is hydroxylated by EGLN3 and subsequently bound by the von Hippel-Lindau tumour-suppressor protein, which stabilizes TFAM by preventing mitochondrial proteolysis. Cells lacking wild-type VHL or in which EGLN3 is inactivated have reduced mitochondrial mass. Tumorigenic VHL variants leading to different clinical manifestations fail to bind hydroxylated TFAM. In contrast, cells harbouring the Chuvash polycythaemia VHL^R200W mutation, involved in hypoxia-sensing disorders without tumour development, are capable of binding hydroxylated TFAM. Accordingly, VHL-related tumours, such as pheochromocytoma and renal cell carcinoma cells, display low mitochondrial content, suggesting that impaired mitochondrial biogenesis is linked to VHL tumorigenesis. Finally, inhibiting proteolysis by targeting LONP1 increases mitochondrial content in VHL-deficient cells and sensitizes therapy-resistant tumours to sorafenib treatment. Our results offer pharmacological avenues to sensitize therapy-resistant VHL tumours by focusing on the mitochondria.

Effects of Tryptophan Supplementation and Exercise on the Fate of Kynurenine Metabolites in Mice and Humans

The kynurenine pathway of tryptophan (TRP) degradation (KP) generates metabolites with effects on metabolism, immunity, and mental health. Endurance exercise training can change KP metabolites by changing the levels of KP enzymes in skeletal muscle. This leads to a metabolite pattern that favors energy expenditure and an anti-inflammatory immune cell profile and reduces neurotoxic metabolites. Here, we aimed to understand if TRP supplementation in untrained vs. trained subjects affects KP metabolite levels and biological effects. Our data show that chronic TRP supplementation in mice increases all KP metabolites in circulation, and that exercise reduces the neurotoxic branch of the pathway. However, in addition to increasing wheel running, we did not observe other effects of TRP supplementation on training adaptations, energy metabolism or behavior in mice. A similar increase in KP metabolites was seen in trained vs. untrained human volunteers that took a TRP drink while performing a bout of aerobic exercise. With this acute TRP administration, TRP and KYN were higher in the trained vs. the untrained group. Considering the many biological effects of the KP, which can lead to beneficial or deleterious effects to health, our data encourage future studies of the crosstalk between TRP supplementation and physical exercise.

LIM and cysteine-rich domains 1 (LMCD1) regulates skeletal muscle hypertrophy, calcium handling, and force

BACKGROUND

Skeletal muscle mass and strength are crucial determinants of health. Muscle mass loss is associated with weakness, fatigue, and insulin resistance. In fact, it is predicted that controlling muscle atrophy can reduce morbidity and mortality associated with diseases such as cancer cachexia and sarcopenia.

METHODS

We analyzed gene expression data from muscle of mice or human patients with diverse muscle pathologies and identified LMCD1 as a gene strongly associated with skeletal muscle function. We transiently expressed or silenced LMCD1 in mouse gastrocnemius muscle or in mouse primary muscle cells and determined muscle/cell size, targeted gene expression, kinase activity with kinase arrays, protein immunoblotting, and protein synthesis levels. To evaluate force, calcium handling, and fatigue, we transduced the flexor digitorum brevis muscle with a LMCD1-expressing adenovirus and measured specific force and sarcoplasmic reticulum Ca²⁺ release in individual fibers. Finally, to explore the relationship between LMCD1 and calcineurin, we ectopically expressed Lmcd1 in the gastrocnemius muscle and treated those mice with cyclosporine A (calcineurin inhibitor). In addition, we used a luciferase reporter construct containing the myoregulin gene promoter to confirm the role of a LMCD1-calcineurin-myoregulin axis in skeletal muscle mass control and calcium handling.

RESULTS

Here, we identify LIM and cysteine-rich domains 1 (LMCD1) as a positive regulator of muscle mass, that increases muscle protein synthesis and fiber size. LMCD1 expression in vivo was sufficient to increase specific force with lower requirement for calcium handling and to reduce muscle fatigue. Conversely, silencing LMCD1 expression impairs calcium handling and force, and induces muscle fatigue without overt atrophy. The actions of LMCD1 were dependent on calcineurin, as its inhibition using cyclosporine A reverted the observed hypertrophic phenotype. Finally, we determined that LMCD1 represses the expression of myoregulin, a known negative regulator of muscle performance. Interestingly, we observed that skeletal muscle LMCD1 expression is reduced in patients with skeletal muscle disease.

CONCLUSIONS

Our gain- and loss-of-function studies show that LMCD1 controls protein synthesis, muscle fiber size, specific force, Ca²⁺ handling, and fatigue resistance. This work uncovers a novel role for LMCD1 in the regulation of skeletal muscle mass and function with potential therapeutic implications.

Ketone bodies effectively compete with glucose for neuronal acetyl-CoA generation in rat hippocampal slices

Ketone bodies can be used for cerebral energy generation in situ, when their availability is increased as during fasting or ingestion of a ketogenic diet. However, it is not known how effectively ketone bodies compete with glucose, lactate, and pyruvate for energy generation in the brain parenchyma. Hence, the contributions of exogenous 5.0 mM [1-(13)C]glucose and 1.0 mM [2-(13)C]lactate + 0.1 mM pyruvate (combined [2-(13)C]lactate + [2-(13)C]pyruvate) to acetyl-CoA production were measured both without and with 5.0 mM [U-(13)C]3-hydroxybutyrate in superfused rat hippocampal slices by (13)C NMR non-steady-state isotopomer analysis of tissue glutamate and GABA. Without [U-(13)C]3-hydroxybutyrate, glucose, combined lactate + pyruvate, and unlabeled endogenous sources contributed (mean ± SEM) 70 ± 7%, 10 ± 2%, and 20 ± 8% of acetyl-CoA, respectively. With [U-(13)C]3-hydroxybutyrate, glucose contributions significantly fell from 70 ± 7% to 21 ± 3% (p < 0.0001), combined lactate + pyruvate and endogenous contributions were unchanged, and [U-(13)C]3-hydroxybutyrate became the major acetyl-CoA contributor (68 ± 3%)--about three-times higher than glucose. A direct analysis of the GABA carbon 2 multiplet revealed that [U-(13)C]3-hydroxybutyrate contributed approximately the same acetyl-CoA fraction as glucose, indicating that it was less avidly oxidized by GABAergic than glutamatergic neurons. The appearance of superfusate lactate derived from glycolysis of [1-(13)C]glucose did not decrease significantly in the presence of 3-hydroxybutyrate, hence total glycolytic flux (Krebs cycle inflow + exogenous lactate formation) was attenuated by 3-hydroxybutyrate. This indicates that, under these conditions, 3-hydroxybutyrate inhibited glycolytic flux upstream of pyruvate kinase.

Demonstration of glucose-6-phosphate hydrogen 5 enrichment from deuterated water by transaldolase-mediated exchange alone

PURPOSE

Enrichment of glucose position 5 (H5) from deuterated water ((2)H2O) is widely used for quantifying gluconeogenesis. Exchanges of hexose and triose phosphates mediated by transaldolase have been postulated to enrich H5 independently of gluconeogenesis, but to date this mechanism has not been proven. We determined the enrichment of glucose-6-phosphate (G6P), the immediate precursor of endogenously produced glucose, from (2)H2O in erythrocyte hemolysate preparations. Here, transaldolase exchange is active but gluconeogenesis is absent.

METHODS

Hemolysates were prepared from human erythrocytes and incubated with a buffer containing 5% [U-(13)C]G6P, unlabeled fructose 1,6-bisphosphate, and 10% (2)H2O. G6P (2)H-enrichment and (13)C-isotopomer distributions were analyzed by (2)H and (13)C NMR following derivatization to monoacetone glucose.

RESULTS

(2)H NMR analysis revealed high (2)H-enrichment of G6P hydrogens 2, 4, and 5; low enrichment of hydrogen 3, and residual enrichments of hydrogens 1, 6R, and 6S. (13)C NMR isotopomer analysis revealed that [U-(13)C]G6P was converted to [1,2,3-(13)C3]G6P, a predicted product of transaldolase-mediated exchange, as well as [1,2-(13)C2]G6P and [3-(13)C]G6P, predicted products of combined transaldolase and transketolase exchanges.

CONCLUSION

Hydrogen 5 of G6P was enriched from (2)H2O through exchanges mediated by transaldolase. These studies prove that G6P can be enriched in hydrogen 5 by (2)H2O independently of gluconeogenesis.