Upon reperfusion after oxygen–glucose deprivation (OGD), hepatocytes display a rapid intracellular burst of superoxide anion (O2•−), first visualized dynamically in 2013 with the reversible fluorescent probe TCA; the publicly accessible record of that work establishes the imaging but attributes no enzymatic source. A decade of follow-up has reframed rather than settled the source question: the succinate/reverse-electron-transport (RET) literature implicates complex I as the ROS-generating site fueled by succinate dehydrogenase (SDH)-mediated succinate oxidation; the probe-imaging literature has localized the burst partly to mitochondria and partly to NOX4-derived endoplasmic-reticulum O2•−; and the same group's 2023 work indicates SDH is a downstream victim of the burst rather than its source. The open problem is to directly and causally identify which enzymatic and subcellular source(s) — mitochondrial complex I (via RET), the complex II flavin site, complex III, NADPH oxidases such as NOX4, xanthine oxidase, or others — generate the hepatocyte reperfusion O2•− burst.

Background

Reactive oxygen species (ROS) are partially reduced oxygen metabolites whose overproduction damages lipids, proteins, and DNA. The superoxide anion radical, O2∙−O_2^{\bullet-}, is the single-electron reduction product of molecular oxygen and the precursor of most other ROS; it is short-lived and kept at very low steady-state levels by superoxide dismutases. Ischemia–reperfusion injury (IRI) is the tissue damage that occurs when blood flow is interrupted (ischemia) and then restored (reperfusion): the reintroduction of oxygen triggers a burst of ROS that contributes to cell death. In the liver, hepatic IRI (HIRI) is a major complication of liver resection and transplantation, and hepatocytes — the parenchymal cells of the liver — are a principal target. Because O2∙−O_2^{\bullet-} is fleeting and present at nanomolar-to-micromolar concentrations, most evidence about it historically came from end-point assays; Zhang, Li, Yang, Hu, Sun, Zhang, Chen and Tang (JACS 2013, 135:14956) introduced the reversible two-photon-compatible fluorescent probe TCA (a caffeic-acid–triazine molecule that reports O2∙−O_2^{\bullet-} through reversible catechol–quinone redox cycling), which could track intracellular O2∙−O_2^{\bullet-} fluxes dynamically. In cultured HL-7702 hepatocytes subjected to oxygen–glucose deprivation (chemical hypoxia with sodium hydrosulfite in glucose-free medium, modeling ischemia) followed by reperfusion with high-glucose medium, TCA imaging showed weak signal during OGD and a marked, rapid fluorescence rise upon reperfusion, directly demonstrating a hepatocyte intracellular O2∙−O_2^{\bullet-} burst at reperfusion. The mechanism generating this burst was not established in that work, and its public abstract attributes no enzymatic source. Several candidate sources are biologically plausible. The mitochondrial electron transport chain (ETC) complexes I–IV host recognized O2∙−O_2^{\bullet-}-producing sites: ischemia drives succinate accumulation by stalling the tricarboxylic-acid cycle, and upon reperfusion rapid oxidation of succinate by complex II over-reduces the ubiquinone pool and drives reverse electron transport (RET), the backward flow of electrons into complex I, a mechanism shown by Chouchani and colleagues (Nature 2014) to dominate reperfusion ROS production in the heart and to be accompanied by succinate accumulation in ischemic liver; liver perfusion studies by Schlegel and colleagues (EBioMedicine 2020) found reperfusion ROS associated with over-reduction and flavin-mononucleotide (FMN) release at complex I under high succinate. Complex II itself (succinate dehydrogenase, SDH, the TCA-cycle enzyme that oxidizes succinate to fumarate while reducing ubiquinone) can also generate O2∙−O_2^{\bullet-} at its flavin site under favorable redox states, complex III produces superoxide during hypoxia, and non-mitochondrial enzymes — NADPH oxidases (NOX, membrane enzymes that transfer electrons from NADPH to oxygen), xanthine oxidase, and cytochrome P450 monooxygenases — are additional documented ROS contributors in HIRI. Which of these sources actually accounts for the imaged hepatocyte reperfusion O2∙−O_2^{\bullet-} burst, and in what proportions, has never been conclusively determined.

Problem Statement

Directly and conclusively identify the enzymatic and subcellular source(s) of the rapid intracellular superoxide anion (O2∙−O_2^{\bullet-}) burst that occurs in hepatocytes upon reperfusion following oxygen–glucose deprivation — the burst first visualized by reversible fluorescence imaging (TCA probe, JACS 2013) — and causally validate the attribution. Specifically, determine which candidate source(s) generate the burst — mitochondrial complex I via succinate-driven reverse electron transport, the complex II (succinate dehydrogenase) flavin site, mitochondrial complex III, NADPH oxidases such as NOX4 at the endoplasmic reticulum or plasma membrane, xanthine oxidase, cytochrome P450 enzymes, or others. Quantifying the relative contribution of each identified source to the imaged burst, confirming the attribution for true ischemia–reperfusion in an in vivo hepatic ischemia-reperfusion model, and reconciling the hydrosulfite-based OGD model with true ischemia–reperfusion remain open. The problem is solved when the origin(s) of the hepatocyte reperfusion O2∙−O_2^{\bullet-} signal are established mechanistically and causally, not merely correlated or co-localized.

References

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