The E3 ubiquitin ligase FBXL6 controls the quality of newly synthesized mitochondrial ribosomal proteins

In mammals, about 99% of mitochondrial proteins are synthesized in the cytosol as precursors that are subsequently imported into the organelle. The mitochondrial health and functions rely on an accurate quality control of these imported proteins. Here, we show that the E3 ubiquitin ligase F box/leucine-rich-repeat protein 6 (FBXL6) regulates the quality of cytosolically translated mitochondrial proteins. Indeed, we found that FBXL6 substrates are newly synthesized mitochondrial ribosomal proteins. This E3 binds to chaperones involved in the folding and trafficking of newly synthesized peptide and to ribosomal-associated quality control proteins. Deletion of these interacting partners is sufficient to hamper interactions between FBXL6 and its substrate. Furthermore, we show that cells lacking FBXL6 fail to degrade specifically mistranslated mitochondrial ribosomal proteins. Finally, showing the role of FBXL6-dependent mechanism, FBXL6-knockout (KO) cells display mitochondrial ribosomal protein aggregations, altered mitochondrial metabolism, and inhibited cell cycle in oxidative conditions.

Ubiquitin-Dependent Degradation of Mitochondrial Proteins Regulates Energy Metabolism

The ubiquitin proteasome system (UPS) regulates many cellular functions by degrading key proteins. Notably, the role of UPS in regulating mitochondrial metabolic functions is unclear. Here, we show that ubiquitination occurs in different mitochondrial compartments, including the inner mitochondrial membrane, and that turnover of several metabolic proteins is UPS dependent. We specifically detailed mitochondrial ubiquitination and subsequent UPS-dependent degradation of succinate dehydrogenase subunit A (SDHA), which occurred when SDHA was minimally involved in mitochondrial energy metabolism. We demonstrate that SDHA ubiquitination occurs inside the organelle. In addition, we show that the specific inhibition of SDHA degradation by UPS promotes SDHA-dependent oxygen consumption and increases ATP, malate, and citrate levels. These findings suggest that the mitochondrial metabolic machinery is also regulated by the UPS.

Mitochondrial morphology and cellular distribution are altered in SPG31 patients and are linked to DRP1 hyperphosphorylation

Hereditary spastic paraplegia, SPG31, is a rare neurological disorder caused by mutations in REEP1 gene encoding the microtubule-interacting protein, REEP1. The mechanism by which REEP1-dependent processes are linked with the disease is unclear. REEP1 regulates the morphology and trafficking of various organelles via interaction with the microtubules. In this study, we collected primary fibroblasts from SPG31 patients to investigate their mitochondrial morphology. We observed that the mitochondrial morphology in patient cells was highly tubular compared with control cells. We provide evidence that these morphological alterations are caused by the inhibition of mitochondrial fission protein, DRP1, due to the hyperphosphorylation of its serine 637 residue. This hyperphosphorylation is caused by impaired interactions between REEP1 and mitochondrial phosphatase PGAM5. Genetically or pharmacologically induced decrease of DRP1-S637 phosphorylation restores mitochondrial morphology in patient cells. Furthermore, ectopic expression of REEP1 carrying pathological mutations in primary neuronal culture targets REEP1 to the mitochondria. Mutated REEP1 proteins sequester mitochondria to the perinuclear region of the neurons and therefore, hamper mitochondrial transport along the axon. Considering the established role of mitochondrial distribution and morphology in neuronal health, our results support the involvement of a mitochondrial dysfunction in SPG31 pathology.

Cannabinoid CB1 Receptors Are Localized in Striated Muscle Mitochondria and Regulate Mitochondrial Respiration

The cannabinoid type 1 (CB₁) receptor is widely distributed in the brain and peripheral organs where it regulates cellular functions and metabolism. In the brain, CB₁ is mainly localized on presynaptic axon terminals but is also found on mitochondria (mtCB₁), where it regulates cellular respiration and energy production. Likewise, CB₁ is localized on muscle mitochondria, but very little is known about it. The aim of this study was to further investigate in detail the distribution and functional role of mtCB₁ in three different striated muscles. Immunoelectron microscopy for CB₁ was used in skeletal muscles (gastrocnemius and rectus abdominis) and myocardium from wild-type and CB₁-KO mice. Functional assessments were performed in mitochondria purified from the heart of the mice and the mitochondrial oxygen consumption upon application of different acute delta-9-tetrahydrocannabinol (Δ⁹-THC) concentrations (100 nM or 200 nM) was monitored. About 26% of the mitochondrial profiles in gastrocnemius, 22% in the rectus abdominis and 17% in the myocardium expressed CB₁. Furthermore, the proportion of mtCB1 versus total CB₁ immunoparticles was about 60% in the gastrocnemius, 55% in the rectus abdominis and 78% in the myocardium. Importantly, the CB₁ immunolabeling pattern disappeared in muscles of CB₁-KO mice. Functionally, acute 100 nM or 200 nM THC treatment specifically decreased mitochondria coupled respiration between 12 and 15% in wild-type isolated mitochondria of myocardial muscles but no significant difference was noticed between THC treated and vehicle in mitochondria isolated from CB₁-KO heart. Furthermore, gene expression of key enzymes involved in pyruvate synthesis, tricarboxylic acid (TCA) cycle and mitochondrial respiratory chain was evaluated in the striated muscle of CB₁-WT and CB₁-KO. CB₁-KO showed an increase in the gene expression of Eno3, Pkm2, and Pdha1, suggesting an increased production of pyruvate. In contrast, no significant difference was observed in the Sdha and Cox4i1 expression, between CB₁-WT and CB₁-KO. In conclusion, CB₁ receptors in skeletal and myocardial muscles are predominantly localized in mitochondria. The activation of mtCB₁ receptors may participate in the mitochondrial regulation of the oxidative activity probably through the relevant enzymes implicated in the pyruvate metabolism, a main substrate for TCA activity.

TRIM4; a novel mitochondrial interacting RING E3 ligase, sensitizes the cells to hydrogen peroxide (H2O2) induced cell death

The emerging evidences suggest that posttranslational modification of target protein by ubiquitin (Ub) not only regulate its turnover through ubiquitin proteasome system (UPS) but is a critical regulator of various signaling pathways. During ubiquitination, E3 ligase recognizes the target protein and determines the topology of ubiquitin chains. In current study, we studied the role of TRIM4, a member of the TRIM/RBCC protein family of RING E3 ligase, in regulation of hydrogen peroxide (H2O2) induced cell death. TRIM4 is expressed differentially in human tissues and expressed in most of the analyzed human cancer cell lines. The subcellular localization studies showed that TRIM4 forms distinct cytoplasmic speckle like structures which transiently interacts with mitochondria. The expression of TRIM4 induces mitochondrial aggregation and increased level of mitochondrial ROS in the presence of H2O2. It sensitizes the cells to H2O2 induced death whereas knockdown reversed the effect. TRIM4 potentiates the loss of mitochondrial transmembrane potential and cytochrome c release in the presence of H2O2. The analysis of TRIM4 interacting proteins showed its interaction with peroxiredoxin 1 (PRX1), including other proteins involved in regulation of mitochondrial and redox homeostasis. TRIM4 interaction with PRX1 is critical for the regulation of H2O2 induced cell death. Collectively, the evidences in the current study suggest the role of TRIM4 in regulation of oxidative stress induced cell death.

Rheb regulates mitophagy induced by mitochondrial energetic status

Mitophagy has been recently described as a mechanism of elimination of damaged organelles. Although the regulation of the amount of mitochondria is a core issue concerning cellular energy homeostasis, the relationship between mitochondrial degradation and energetic activity has not yet been considered. Here, we report that the stimulation of mitochondrial oxidative phosphorylation enhances mitochondrial renewal by increasing its degradation rate. Upon high oxidative phosphorylation activity, we found that the small GTPase Rheb is recruited to the mitochondrial outer membrane. This mitochondrial localization of Rheb promotes mitophagy through a physical interaction with the mitochondrial autophagic receptor Nix and the autophagosomal protein LC3-II. Thus, Rheb-dependent mitophagy contributes to the maintenance of optimal mitochondrial energy production. Our data suggest that mitochondrial degradation contributes to a bulk renewal of the organelle in order to prevent mitochondrial aging and to maintain the efficiency of oxidative phosphorylation.

Ventral tegmental area cannabinoid type-1 receptors control voluntary exercise performance

BACKGROUND

We have shown that the endogenous stimulation of cannabinoid type-1 (CB₁) receptors is a prerequisite for voluntary running in mice, but the precise mechanisms through which the endocannabinoid system exerts a tonic control on running performance remain unknown.

METHODS

We analyzed the respective impacts of constitutive/conditional CB₁ receptor mutations and of CB₁ receptor blockade on wheel-running performance. We then assessed the consequences of ventral tegmental area (VTA) CB₁ receptor blockade on the wheel-running performances of wildtype (gamma-aminobutyric acid [GABA]-CB₁⁺/⁺) and mutant (GABA-CB₁⁻/⁻) mice for CB₁ receptors in brain GABA neurons. Using in vivo electrophysiology, the consequences of wheel running on VTA dopamine (DA) neuronal activity were examined in GABA-CB₁⁺/⁺ and GABA-CB₁⁻/⁻ mice.

RESULTS

Conditional deletion of CB₁ receptors from brain GABA neurons, but not from several other neuronal populations or from astrocytes, decreased wheel-running performance in mice. The inhibitory consequences of either the systemic or the intra-VTA administration of CB1 receptor antagonists on running behavior were abolished in GABA-CB₁⁻/⁻ mice. The absence of CB1 receptors from GABAergic neurons led to a depression of VTA DA neuronal activity after acute/repeated wheel running.

CONCLUSIONS

This study provides evidence that CB₁ receptors on VTA GABAergic terminals exert a permissive control on rodent voluntary running performance. Furthermore, it is shown that CB₁ receptors located on GABAergic neurons impede negative consequences of voluntary exercise on VTA DA neuronal activity. These results position the endocannabinoid control of inhibitory transmission as a prerequisite for wheel-running performance in mice.

At environmental doses, dietary methylmercury inhibits mitochondrial energy metabolism in skeletal muscles of the zebra fish (Danio rerio)

The neurotoxic compound methylmercury (MeHg) is a commonly encountered pollutant in the environment, and constitutes a hazard for human health through fish eating. To study the impact of MeHg on mitochondrial structure and function, we contaminated the model fish species Danio rerio with food containing 13 microg of MeHg per gram, an environmentally relevant dose. Mitochondria from contaminated zebrafish muscles presented structural abnormalities under electron microscopy observation. In permeabilized muscle fibers, we observed, a strong inhibition of both state 3 mitochondrial respiration and functionally isolated maximal cytochrome c oxidase (COX) activity after 49 days of MeHg exposure. However, the state 4 respiratory rate remained essentially unchanged. This suggested a defect at the level of ATP synthesis. Accordingly, we measured a dramatic decrease in the rate of ATP release by skinned muscle fibers using either pyruvate and malate or succinate as respiratory substrates. However, the amount and the assembly of the ATP synthase were identical in both control and contaminated muscle mitochondrial fractions. This suggests that MeHg induced a decoupling of mitochondrial oxidative phosphorylation in the skeletal muscle of zebrafish. Western blot analysis showed a 30% decrease of COX subunit IV levels, a 50% increase of ATP synthase subunit alpha, and a 40% increase of the succinate dehydrogenase Fe/S protein subunit in the contaminated muscles. This was confirmed by the analysis of gene expression levels, using RT-PCR. Our study provides a basis for further analysis of the deleterious effect of MeHg on fish health via mitochondrial impairment.

Mobilization of adenine nucleotide translocators as molecular bases of the biochemical threshold effect observed in mitochondrial diseases

The existence of a biochemical threshold effect in the metabolic expression of oxidative phosphorylation deficiencies has considerable implications for the understanding of mitochondrial bioenergetics and the study of mitochondrial diseases. However, the molecular bases of this phenomenon remain unclear. We report here a new mechanism to explain this threshold effect, based on a reserve of enzymes not initially participating in the respiratory rate that can be activated either to respond to a flux increase or to compensate for a defect induced by a mutation. We show that this mobilization occurs through 1) the assembly of inactive adenine nucleotide translocator isoform 1 subunits into oligomeric active carriers or 2) conformational changes in the adenine nucleotide translocator isoform 1 in a permeability transition pore-like structure. We discuss how these transitions are sensitive to the steady state of oxidative phosphorylation functioning or tissue and analyze their consequences on the threshold effect.