Inactivation of mitochondrial MUL1 E3 ubiquitin ligase deregulates mitophagy and prevents diet-induced obesity in mice

Obesity is a growing epidemic affecting millions of people worldwide and a major risk factor for a multitude of chronic diseases and premature mortality. Accumulating evidence suggests that mitochondria have a profound role in diet-induced obesity and the associated metabolic changes, but the molecular mechanisms linking mitochondria to obesity remain poorly understood. Our studies have identified a new function for mitochondrial MUL1 E3 ubiquitin ligase, a protein known to regulate mitochondrial dynamics and mitophagy, in the control of energy metabolism and lipogenesis. Genetic deletion of Mul1 in mice impedes mitophagy and presents a metabolic phenotype that is resistant to high-fat diet (HFD)-induced obesity and metabolic syndrome. Several metabolic and lipidomic pathways are perturbed in the liver and white adipose tissue (WAT) of Mul1(-/-) animals on HFD, including the one driven by Stearoyl-CoA Desaturase 1 (SCD1), a pivotal regulator of lipid metabolism and obesity. In addition, key enzymes crucial for lipogenesis and fatty acid oxidation such as ACC1, FASN, AMPK, and CPT1 are also modulated in the absence of MUL1. The concerted action of these enzymes, in the absence of MUL1, results in diminished fat storage and heightened fatty acid oxidation. Our findings underscore the significance of MUL1-mediated mitophagy in regulating lipogenesis and adiposity, particularly in the context of HFD. Consequently, our data advocate the potential of MUL1 as a therapeutic target for drug development in the treatment of obesity, insulin resistance, NAFLD, and cardiometabolic diseases.

Regulation of Metabolism by Mitochondrial MUL1 E3 Ubiquitin Ligase

MUL1 is a multifunctional E3 ubiquitin ligase that is involved in various pathophysiological processes including apoptosis, mitophagy, mitochondrial dynamics, and innate immune response. We uncovered a new function for MUL1 in the regulation of mitochondrial metabolism. We characterized the metabolic phenotype of MUL1(-/-) cells using metabolomic, lipidomic, gene expression profiling, metabolic flux, and mitochondrial respiration analyses. In addition, the mechanism by which MUL1 regulates metabolism was investigated, and the transcription factor HIF-1α, as well as the serine/threonine kinase Akt2, were identified as the mediators of the MUL1 function. MUL1 ligase, through K48-specific polyubiquitination, regulates both Akt2 and HIF-1α protein level, and the absence of MUL1 leads to the accumulation and activation of both substrates. We used specific chemical inhibitors and activators of HIF-1α and Akt2 proteins, as well as Akt2(-/-) cells, to investigate the individual contribution of HIF-1α and Akt2 proteins to the MUL1-specific phenotype. This study describes a new function of MUL1 in the regulation of mitochondrial metabolism and reveals how its downregulation/inactivation can affect mitochondrial respiration and cause a shift to a new metabolic and lipidomic state.

Mulan E3 ubiquitin ligase interacts with multiple E2 conjugating enzymes and participates in mitophagy by recruiting GABARAP

Mulan is an E3 ubiquitin ligase embedded in the outer mitochondrial membrane (OMM) with its RING finger facing the cytoplasm and a large domain located in the intermembrane space (IMS). Mulan is known to have an important role in cell growth, cell death, and more recently in mitophagy. The mechanism of its function is poorly understood; but as an E3 ligase it is expected to interact with specific E2 ubiquitin conjugating enzymes and these complexes will bind and ubiquitinate specific substrates. The unique topology of Mulan can provide a direct link of communicating mitochondrial signals to the cytoplasm. Our studies identified four different E2 conjugating enzymes (Ube2E2, Ube2E3, Ube2G2 and Ube2L3) as specific interactors of Mulan. Each of these E2 conjugating enzymes was fused to the RING finger domain of Mulan and used in a modified yeast two-hybrid screen. Several unique interactors for each Mulan-E2 complex were isolated. One such specific interactor of Mulan-Ube2E3 was the GABARAP (GABAA receptor-associated protein). GABARAP is a member of the Atg8 family of proteins that plays a major role in autophagy/mitophagy. The interaction of GABARAP with Mulan-Ube2E3 required an LC3-interacting region (LIR) located in the RING finger domain of Mulan as well as the presence of Ube2E3. The isolation of four different E2 conjugating enzymes, as specific partners of Mulan E3 ligase, suggests that Mulan is involved in multiple biological pathways. In addition, the interaction of GABARAP with Mulan-Ube2E3 supports the role of Mulan as an important regulator of mitophagy and provides a plausible mechanism for its function in this process.

Inactivation of Omi/HtrA2 protease leads to the deregulation of mitochondrial Mulan E3 ubiquitin ligase and increased mitophagy

Omi/HtrA2 is a nuclear encoded mitochondrial serine protease with dual and opposite functions that depend entirely on its subcellular localization. During apoptosis, Omi/HtrA2 is released into the cytoplasm where it participates in cell death. While confined in the inter-membrane space of the mitochondria, Omi/HtrA2 has a pro-survival function that may involve the regulation of protein quality control (PQC) and mitochondrial homeostasis. Loss of Omi/HtrA2's protease activity causes the neuromuscular disorder of the mnd2 (motor neuron degeneration 2) mutant mice. These mice develop multiple defects including neurodegeneration with parkinsonian features. Loss of Omi/HtrA2 in non-neuronal tissues has also been shown to cause premature aging. The normal function of Omi/HtrA2 in the mitochondria and how its deregulation causes neurodegeneration or premature aging are unknown. Here we report that the mitochondrial Mulan E3 ubiquitin ligase is a specific substrate of Omi/HtrA2. During exposure to H(2)O(2), Omi/HtrA2 degrades Mulan, and this regulation is lost in cells that carry the inactive protease. Furthermore, we show accumulation of Mulan protein in various tissues of mnd2 mice as well as in Omi/HtrA2(-/-) mouse embryonic fibroblasts (MEFs). This causes a significant decrease of mitofusin 2 (Mfn2) protein, and increased mitophagy. Our work describes a new stress-signaling pathway that is initiated in the mitochondria and involves the regulation of Mulan by Omi/HtrA2 protease. Deregulation of this pathway, as it occurs in mnd2 mutant mice, causes mitochondrial dysfunction and mitophagy, and could be responsible for the motor neuron disease and the premature aging phenotype observed in these animals.