Biology subjects

Baierna, B.

Publications and source records attributed to Baierna, B..

4 recordsLinked to original sources

bioRxiv2026

A respiratory microdomain links ubiquinone biosynthesis to succinate dehydrogenase in an apicomplexan mitochondrion lacking Complex I

Mitochondrial complex I (CI) is a major entry point for NADH-derived electrons and an important site of redox regulation in most eukaryotes. However, apicomplexan parasites, including Toxoplasma gondii, lack CI, raising a fundamental question of how mitochondrial respiration is organized in these parasites. Here, we identify a parasite-specific mitochondrial microdomain that physically and functionally couples ubiquinone (UQ) biosynthesis to succinate dehydrogenase (SDH). Using proximity labeling, membrane-based yeast two-hybrid analyses, reciprocal co-immunoprecipitations, and blue native PAGE, we show that UQ biosynthesis proteins associate with parasite-specific SDH subunits and co-migrate as a high-molecular-weight assembly. Knockdown of TgCoq4, a component of the UQ biosynthesis pathway, reduced SDH activity, and adding exogenous UQ to the assay did not restore it, indicating that UQ availability is not sufficient for efficient succinate oxidation. FCCP stimulates malate- and TMPD/ascorbate-supported respiration to maximal capacity but fails to do so with succinate as the substrate. This result, combined with reactive oxygen species (ROS) data, suggests that succinate causes over-reduction of the UQ pool, limiting further electron flow. We also find that CIII associates with the UQ synthesis machinery, suggesting a larger protein assembly. Together, these findings reveal that UQ biosynthesis and electron transport are physically and functionally integrated in T. gondii, defining a previously unrecognized organization of mitochondrial respiration in an organism lacking CI.

biochemistry↗

bioRxiv2026

Legacy 4(1H)-quinolone scaffolds activity against acute and chronic Toxoplasma gondii infection

Toxoplasma gondii is a protozoan parasite capable of infecting most warm-blooded animals, including humans, and can cause severe disease in immunocompromised individuals and the developing fetus. Current treatments for toxoplasmosis are effective only against the acute stage of infection and have limited or no activity against the latent bradyzoite stage found within tissue cysts. The mitochondrion of T. gondii is a validated drug target, and the clinically used drug atovaquone acts by inhibiting the mitochondrial electron transport chain (ETC) at the coenzyme Q:cytochrome c oxidoreductase (bc1 complex). In this study, we evaluate two legacy 4(1H)-quinolones: ICI 56,780 and WR 243246, previously shown to inhibit the Plasmodium falciparum bc1 complex, for their efficacy against T. gondii. Both compounds inhibit tachyzoite growth with low-nanomolar EC values and disrupt parasite mitochondrial function by blocking cytochrome c reduction and collapsing the mitochondrial membrane potential. Notably, ICI 56,780 protects mice from lethal infection with type I RH tachyzoites. Importantly, ICI 56,780 also exhibits potent activity against chronic-stage parasites, reducing cyst size and bradyzoite viability in vitro and showing low-nanomolar EC values against in vivo-derived bradyzoites. In mice chronically infected with T. gondii, treatment with ICI 56,780 significantly decreases brain cyst burden. Although these 4(1H)-quinolones display some pharmacokinetic limitations, our findings highlight their potential as promising chemotypes active against both acute and chronic stages of T. gondii and provide a framework for future medicinal chemistry efforts to improve drug-like properties while preserving or enhancing anti-bradyzoite activity.

microbiology↗

bioRxiv2026

Evolutionary Remodeling of Ubiquinone Biosynthesis in Toxoplasma gondii Reveals an Essential Bi-functional Monooxygenase

Apicomplexan parasites like Toxoplasma gondii harbor a highly divergent mitochondrial proteome, much of which remains uncharacterized despite its essentiality for parasite survival. One such critical pathway is ubiquinone (UQ) biosynthesis. Here, we characterize the UQ synthesis machinery in T. gondii and show that conserved enzymes, TgCoq3 and TgCoq5, are essential for growth and mitochondrial function, forming a multi-protein complex. Using proximity labeling and subcellular fractionation, a strategy suited for detecting proteins of low abundance, we identify TgCoqFAD, a unique FAD-dependent monooxygenase required for UQ synthesis. Unlike canonical eukaryotic systems that employ multiple monooxygenases to modify specific carbons on the UQ aromatic ring, TgCoqFAD catalyzes two distinct hydroxylation steps, an activity not previously reported in eukaryotes. Molecular docking and chemical screening identified TgCoqFAD inhibitors that impair tachyzoite growth and bradyzoite viability. These findings reveal a streamlined and divergent UQ biosynthesis pathway in apicomplexans and establish TgCoqFAD as a promising antiparasitic target.

biochemistry↗

bioRxiv2022

Toxoplasma gondii Coq1 is a heptaprenyl diphosphate synthase and its inhibition by a lipophilic bisphosphonate protects mice against T. gondii infection

Prenyldiphosphate synthases catalyze the reaction of allylic diphosphates with one or more isopentenyl diphosphate molecules to form compounds such as farnesyl diphosphate, used in e.g. sterol biosynthesis and protein prenylation, as well as longer "polyprenyl" diphosphates, used in ubiquinone and menaquinone biosynthesis. Quinones play an essential role in electron transport and are associated with the inner mitochondrial membrane due to the presence of the polyprenyl group. In this work, we investigated the synthesis of the polyprenyl diphosphate that alkylates the ubiquinone ring precursor in Toxoplasma gondii, an opportunistic pathogen that causes serious disease in immunocompromised patients and the unborn fetus. The enzyme that catalyzes this early step of the ubiquinone synthesis is Coq1 (TgCoq1), and we show that it produces the C35 species, heptaprenyl diphosphate. TgCoq1 localizes to the mitochondrion, and is essential for in vitro T. gondii growth. We demonstrate that the growth defect of a T. gondii TgCoq1 mutant is rescued by complementation with a homologous TgCoq1 gene or with a (C45) solanesyl diphosphate synthase from Trypanosoma cruzi (TcSPPS). We find that a lipophilic bisphosphonate (BPH-1218) inhibits T. gondii growth at low nM concentrations, while overexpression of the TgCoq1 enzyme dramatically reduced growth inhibition by the bisphosphonate. Both the severe growth defect of the mutant and the inhibition by BPH-1218 were rescued by supplementation with a long chain (C30) ubiquinone (UQ6). Importantly, BPH-1218 also protected mice against a lethal T. gondii infection. TgCoq1 thus represents a potential drug target that could be exploited for improved chemotherapy of toxoplasmosis. ImportanceMillions of people are infected with Toxoplasma gondii and the available treatment for toxoplasmosis is not ideal. Most of the drugs currently used are only effective for the acute infection and treatment can trigger serious side-effects requiring changes in the therapeutic approach. There is, therefore, a compelling need for safe and effective treatments for toxoplasmosis. In this work, we characterize an enzyme of the mitochondrion of T. gondii that can be inhibited by an isoprenoid pathway inhibitor. We present evidence that demonstrate that inhibition of the enzyme is linked to parasite death. In addition, the drug is able to protect mice against a lethal dose of T. gondii. Our results thus reveal a promising chemotherapeutic target for the development of new medicines for toxoplasmosis.

microbiology↗