Ryoko Inatome

Mitochondrial iron dynamics are essential for cellular respiration and metabolic homeostasis, yet the molecular mechanisms governing iron supply to mitochondria remain poorly understood. Here we identify a pathway in which haem serves as an iron source for mitochondria, maintaining mitochondrial iron homeostasis and mitochondrial supercomplex integrity, regulated at mitochondria-endoplasmic reticulum contact sites (MERCs). We demonstrate that haem oxygenase 2 (HMOX2), an ER-resident enzyme, is also localized to MERCs and facilitates the supply of haem-derived iron to mitochondria. This process is orchestrated by the mitochondrial ubiquitin ligase MITOL (also known as MARCH5/MARCHF5), which ubiquitinates HMOX2 at K68 with K63-linked polyubiquitin chains, enhancing its haem-degrading activity. Notably, loss of HMOX2 or disruption of MITOL-mediated ubiquitination impairs mitochondrial iron homeostasis and mitochondrial respiration. These findings establish a paradigm in which MERCs function as an iron supply hub, integrating haem metabolism with mitochondrial iron utilization.

In mitochondria, the pyruvate dehydrogenase complex (PDHC) serves as a key metabolic regulator by converting glycolysis-derived pyruvate into acetyl-CoA, thereby controlling carbon flux into the tricarboxylic acid (TCA) cycle. PDHC activity is tightly regulated by two post-translational modifications: phosphorylation of the E1 subunit and lipoylation of the E2 subunit. While phosphorylation of E1 reversibly suppresses pyruvate dehydrogenase (PDH) activity, lipoylation of E2 is essential for intracomplex electron transfer reactions, and together these modifications define PDHC enzymatic activity. Mitochondrial respiratory supercomplexes (SCs) play a critical role in efficient electron transfer during mitochondrial respiration, and PDH has been reported to regulate SC organization. However, it remains unclear whether this regulatory mechanism, including subunit phosphorylation, is linked to protein lipoylation. In this study, we examined the impact of protein lipoylation on the phosphorylation status of the PDHC E1 subunit and on mitochondrial respiratory supercomplex formation during C2C12 differentiation. To this end, suppression of lipoic acid synthase (LIAS), a key enzyme responsible for mitochondrial protein lipoylation, in C2C12 cells resulted in dephosphorylation of the PDHC E1 subunit and formation of specific mitochondrial respiratory supercomplexes. These findings suggest that PDHC E1 dephosphorylation and specific mitochondrial respiratory supercomplex assembly can occur under conditions of impaired E2 lipoylation.

Brown adipose tissue (BAT) is a major site of non-shivering thermogenesis, where mitochondria generate heat instead of adenosine triphosphate (ATP). The thermogenesis occurs through the activity of uncoupling protein 1 (UCP1) which specifically resides in the mitochondrial inner membrane and dissipates the mitochondrial proton gradient upon activation by long-chain free fatty acids (FFA). Although UCP1-independent proton leak has been reported, the mechanism underlying UCP1-independent mitochondrial membrane depolarization remains largely unknown. Here, using primary brown adipocytes, we found that cold-mimicking stimulation induces mitochondrial membrane depolarization even under UCP1 knockout and knockdown conditions. Furthermore, during cold-mimicking stimulation, palmitic acid shows the most prominent increase in a lipolysis-dependent manner. Notably, palmitic acid directly decreases mitochondrial membrane potential specifically in mitochondria isolated from BAT, but not in those isolated from liver or brain. These findings suggest that palmitic acid contributes to mitochondrial depolarization in BAT, thereby contributing to UCP1-independent depolarization.

Abstract Mitochondria play a central role in cellular energy metabolism and homeostasis, and their dysfunction is closely linked to the progression of age-related diseases. The mitochondrial ubiquitin ligase MITOL (also known as MARCHF5) is a key regulator of mitochondrial dynamics and function, and reduced MITOL expression in the mouse heart has been implicated in mitochondrial dysfunction and cardiac aging. In this study, we identified berberrubine as a compound that promotes MITOL expression and activates mitochondria. We further assembled a group of berberrubine-based compounds, including its quinoid form and a newly developed water-soluble derivative, and collectively named them “Mitorubin” as mitochondria-activating compounds with therapeutic potential. While conventional berberrubine has poor water solubility, the addition of acetic acid significantly improved its solubility, enabling formulation as a solution. Mitorubin enhanced MITOL expression in cultured cells, increased mitochondrial DNA content and expression of mitochondrial proteins, and promoted mitochondrial respiration. In a model of age-related cardiac dysfunction, oral administration of Mitorubin restored mitochondrial function, improved cardiac performance, suppressed myocardial hypertrophy, and alleviated pulmonary congestion. Moreover, Mitorubin did not shorten lifespan in aged mice and significantly extended lifespan in high-fat diet-fed mice, suggesting both safety and efficacy under chronic administration. These findings suggest that Mitorubin is a promising mitochondrial activator and may represent a novel therapeutic strategy for age-related diseases.

The proximal domains of mitochondria and the endoplasmic reticulum (ER) are linked by tethering factors on each membrane, allowing the efficient transport of substances, including lipids and calcium, between them. However, little is known about the regulation and function of mitochondria-ER contacts (MERCs) dynamics under mitochondrial damage. In this study, we apply NanoBiT technology to develop the MERBiT system, which enables the measurement of reversible MERCs formation in living cells. Analysis using this system suggests that induction of mitochondrial ROS increases MERCs formation via RMDN3 (also known as PTPIP51)-VAPB tethering driven by RMDN3 phosphorylation. Disruption of this tethering caused lipid radical accumulation in mitochondria, leading to cell death. The lipid radical transfer activity of the TPR domain in RMDN3, as revealed by an in vitro liposome assay, suggests that RMDN3 transfers lipid radicals from mitochondria to the ER. Our findings suggest a potential role for MERCs in cell survival strategy by facilitating the removal of mitochondrial lipid radicals under mitochondrial damage.