The human mitochondrial DNA polymerase gamma is a holoenzyme, involved in mitochondrial DNA (mtDNA) replication and maintenance, composed of a catalytic subunit (POLG) and a dimeric accessory subunit (POLG2) conferring processivity. Mutations in POLG or POLG2 cause POLG-related diseases in humans, leading to a subset of Mendelian-inherited mitochondrial disorders characterized by mtDNA depletion (MDD) or accumulation of multiple deletions, presenting multi-organ defects and often leading to premature death at a young age. Considering the paucity of POLG2 models, we have generated a stable zebrafish polg2 mutant line (polg2^ia304) by CRISPR/Cas9 technology, carrying a 10-nucleotide deletion with frameshift mutation and premature stop codon. Zebrafish polg2 homozygous mutants present slower development and decreased viability compared to wild type siblings, dying before the juvenile stage. Mutants display a set of POLG-related phenotypes comparable to the symptoms of human patients affected by POLG-related diseases, including remarkable MDD, altered mitochondrial network and dynamics, and reduced mitochondrial respiration. Histological analyses detected morphological alterations in high-energy demanding tissues, along with a significant disorganization of skeletal muscle fibres. Consistent with the last finding, locomotor assays highlighted a decreased larval motility. Of note, treatment with the Clofilium tosylate drug, previously shown to be effective in POLG models, could partially rescue MDD in Polg2 mutant animals. Altogether, our results point at zebrafish as an effective model to study the etiopathology of human POLG-related disorders linked to POLG2, and a suitable platform to screen the efficacy of POLG-directed drugs in POLG2-associated forms.

Sarcoglycanopathies, also known as limb girdle muscular dystrophy 3-6, are rare muscular dystrophies characterized, although heterogeneous, by high disability, with patients often wheelchair-bound by late adolescence and frequently developing respiratory and cardiac problems. These diseases are currently incurable, emphasizing the importance of effective treatment strategies and the necessity of animal models for drug screening and therapeutic verification. Using the CRISPR/Cas9 genome editing technique, we generated and characterized δ-sarcoglycan and β-sarcoglycan knockout zebrafish lines, which presented a progressive disease phenotype that worsened from a mild larval stage to distinct myopathic features in adulthood. By subjecting the knockout larvae to a viscous swimming medium, we were able to anticipate disease onset. The δ-SG knockout line was further exploited to demonstrate that a δ-SG missense mutant is a substrate for endoplasmic reticulum-associated degradation (ERAD), indicating premature degradation due to protein folding defects. In conclusion, our study underscores the utility of zebrafish in modeling sarcoglycanopathies through either gene knockout or future knock-in techniques. These novel zebrafish lines will not only enhance our understanding of the disease’s pathogenic mechanisms, but will also serve as powerful tools for phenotype-based drug screening, ultimately contributing to the development of a cure for sarcoglycanopathies.

IF1 is the natural inhibitor of the mitochondrial ATP synthase during hydrolytic activity. It has been found to be overexpressed in many tumors, where it acts as a pro-oncogenic protein. During oxidative phosphorylation, IF1 binds to a novel site on the OSCP subunit of ATP synthase and promotes tumorigenesis by protecting cancer cells from permeability transition pore (PTP)-dependent apoptosis. In this work, honokiol, a biphenolic compound, showed binding affinity for two sites on the OSCP subunit, as predicted by molecular docking analysis. It was shown to be effective in disrupting the IF1-OSCP interaction and sensitizing cancer cells to apoptosis. In vivo, xenografts of zebrafish injected with IF1-expressing HeLa cells showed tumor development. The same xenografts, treated with honokiol, showed a significant reduction in tumor mass, similar to untreated fish injected with IF1 KO HeLa cells. In vitro, honokiol inhibits colony formation in soft agar of IF1-expressing HeLa cells by promoting the PTP opening and cell death, without any effect on cell proliferation. Interestingly, honokiol was shown to block metastasis in fish xenografts and migration in a wound healing assay, by promoting mitochondrial swelling in both control and IF1 KO cell lines, when cells are moving to close the scratch area. In conclusion, honokiol appears to be a promising anti-cancer compound, with pro-apoptotic properties through the displacement of IF1 from the OSCP subunit of ATP synthase, and anti-metastatic effects that are due to mitochondrial PTP opening.

Arrhythmogenic cardiomyopathy (AC) is a hereditary cardiac disorder characterized by the gradual replacement of cardiomyocytes with fibrous and adipose tissue, leading to ventricular wall thinning, chamber dilation, arrhythmias, and sudden cardiac death. Despite advances in treatment, disease management remains challenging. Animal models, particularly mice and zebrafish, have become invaluable tools for understanding AC’s pathophysiology and testing potential therapies. Mice models, although useful for scientific research, cannot fully replicate the complexity of the human AC. However, they have provided valuable insights into gene involvement, signalling pathways, and disease progression. Zebrafish offer a promising alternative to mammalian models, despite the phylogenetic distance, due to their economic and genetic advantages. By combining animal models with in vitro studies, researchers can comprehensively understand AC, paving the way for more effective treatments and interventions for patients and improving their quality of life and prognosis.

Mitochondrial genetic diseases are complex disorders that impair cellular energy production, leading to diverse clinical manifestations across multiple organ systems. These diseases arise from mutations in either mitochondrial DNA or nuclear DNA. Among nuclear DNA-related cases, mutations in POLG and POLG2 , which encode subunits of mitochondrial DNA polymerase γ, are particularly significant, causing conditions such as Alpers–Huttenlocher syndrome and progressive external ophthalmoplegia. Model organisms have been instrumental in elucidating POLG-related disease mechanisms and advancing therapeutic strategies. Saccharomyces cerevisiae (budding yeast) provided insights into fundamental mitochondrial functions, while Caenorhabditis elegans (roundworm) helped explore POLG’s roles in multicellular organisms. Drosophila melanogaster (fruit fly) has been pivotal in studying neurological aspects, and Mus musculus (mouse) models contributed to understanding systemic effects in mammals. Recently, Danio rerio (zebrafish) has emerged as a promising vertebrate model for drug screening, due to its optical transparency and genetic tractability. Each model system offers unique advantages, collectively bridging the gap between basic research and clinical applications. This review will examine in vivo models used in POLG disorder research, highlighting their contributions to understanding disease mechanisms and therapeutic advancements.

Doxorubicin remains a mainstay of cancer therapy, yet its clinical use is constrained by severe cardiotoxicity, classically attributed to cardiomyocyte death. Here, we identify endothelial inflammation driven by mitochondrial DNA (mtDNA) turnover and release as a critical mediator of doxorubicin-induced cardiac injury. In vivo , a standard doxorubicin regimen activates the cGAS–STING innate immune pathway in endothelial cells before cardiomyocyte dysfunction or death occurs. Mechanistically, doxorubicin recruits the stress-inducible ribonucleotide reductase subunit RRM2B (p53R2) that normally sustains the mitochondrial deoxyribonucleotide pool, to promote aberrant mtDNA turnover and release, fuelling sterile inflammation. The FDA-approved BAX inhibitor Eltrombopag suppresses mtDNA release, attenuates endothelial inflammation, and prevents doxorubicin cardiotoxicity in vivo without compromising anticancer efficacy. These findings reveal an unrecognized proinflammatory role of RRM2B and establish endothelial inflammation as a central driver of chemotherapy-related cardiac injury, identifying the cardiac endothelium as a tractable target for cardioprotection during cancer treatment.