Isshin Shiiba

Mitochondria are dynamic organelles that remodel their shape to regulate cell fate. Mitochondrial division involves interactions with the endoplasmic reticulum (ER), lysosomes, and trans-Golgi network–derived vesicles to facilitate membrane scission. How interorganelle contacts regulate mitochondrial membrane fusion remains largely unknown. Here, we identified a role for Golgi-derived vesicles enriched in phosphatidylinositol 3,4-bisphosphate [PI(3,4)P ₂ ] in regulating mitochondrial fusion. We found that these vesicles were recruited to ER- and mitofusin-marked fusion sites. Accordingly, loss of class II PI3-kinase isoforms α and β (PI3K-C2α and PI3K-C2β), which generate PI(3,4)P ₂ , led to mitochondrial fragmentation resulting from impaired fusion. Furthermore, cardiomyocyte-specific PI3K-C2α and PI3K-C2β double-deletion mice exhibited mitochondrial fragmentation and heart failure. Thus, subpopulations of Golgi-derived vesicles carrying different phosphoinositides control mitochondrial membrane remodeling and homeostasis.

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.

Mitochondria form contact sites with multiple organelles to coordinate diverse cellular processes. Melanosomes, lysosome-related organelles, undergo stepwise maturation to synthesize and store melanin, but how they interact with mitochondria remains unclear. Here we show that mitochondria-melanosome contacts dynamically increase during melanosome maturation and are mediated by STIM1-MFN2 interactions. Using a NanoBiT-based reporter system, MiMSBiT (Mitochondria-Melanosome contact reporter applying NanoBiT), to monitor reversible mitochondria-melanosome contacts in living cells, we demonstrate that STIM1 localizes to melanosomes and promotes their contact with mitochondrial MFN2. A transient decrease in melanosomal lumen calcium induces STIM1 clustering and enhances its association with MFN2. These contacts locally increase mitochondrial ATP availability, leading to melanosome lumen acidification via proton channel activation. This acidification facilitates PMEL fibrillation, a key step in melanosome maturation. Together, our findings reveal a mechanism by which mitochondria-melanosome contacts regulate melanosome maturation.

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.

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In mitochondrial disorders, short stature and growth failure are common symptoms, but their underlying mechanism remains unknown. In this study, we examined the cause of growth failure of mice induced by nestin promoter-driven knockout of the mitochondrial ubiquitin ligase MITOL (MARCH5), a key regulator of mitochondrial function. MITOL-knockout mice have congenital hypoplasia of the anterior pituitary caused by decreased expression of pituitary transcript factor 1 (Pit1). Consistently, both mRNA levels of growth hormone (GH) and prolactin levels were markedly decreased in the anterior pituitary of mutant mice. Growth failure of mutant mice was partly rescued by hypodermic injection of recombinant GH. To clarify whether this abnormality was induced by the primary effect of MITOL knockdown in the anterior pituitary or a secondary effect of other lesions, we performed lentiviral-mediated knockdown of MITOL on cultured rat pituitary GH3 cells, which secrete GH. GH production was severely compromised in MITOL-knockdown GH3 cells. In conclusion, MITOL plays a critical role in the development of the anterior pituitary; therefore, mice with MITOL dysfunction exhibited pituitary dwarfism caused by anterior pituitary hypoplasia. Our findings suggest that mitochondrial dysfunction is commonly involved in the unknown pathogenesis of pituitary dwarfism.

The molecular pathology of diseases seen from the mitochondrial axis has become more complex with the progression of research. A variety of factors, including the failure of mitochondrial dynamics and quality control, have made it extremely difficult to narrow down drug discovery targets. We have identified MITOL (mitochondrial ubiquitin ligase: also known as MARCH5) localized on the mitochondrial outer membrane and previously reported that it is an important regulator of mitochondrial dynamics and mitochondrial quality control. In this review, we describe the pathological aspects of MITOL revealed through functional analysis and its potential as a drug discovery target.

Mitochondria are highly dynamic organelles that constantly fuse, divide, and move, and their function is regulated and maintained by their morphologic changes. Mitochondrial disease (MD) comprises a group of disorders involving mitochondrial dysfunction. However, it is not clear whether changes in mitochondrial morphology are related to MD. In this study, we examined mitochondrial morphology in fibroblasts from patients with MD (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) and Leigh syndrome). We observed that MD fibroblasts exhibited significant mitochondrial fragmentation by upregulation of Drp1, which is responsible for mitochondrial fission. Interestingly, the inhibition of mitochondrial fragmentation by Drp1 knockdown enhanced cellular toxicity and led to cell death in MD fibroblasts. These results suggest that mitochondrial fission plays a critical role in the attenuation of mitochondrial damage in MD fibroblasts.