Citrate linked brief mitochondrial stress to cardioprotection in mice
Citrate linked brief mitochondrial stress to cardioprotection in mice
On 4 September, Science Advances published a study in mice whose mitochondrial oxidative stress was briefly increased during embryonic development. In adulthood, their hearts were partially protected from doxorubicin-induced injury. The authors traced the pathway from superoxide to citrate and to modifications on proteins that package DNA.
Mitochondria supply cells with energy and continuously produce small amounts of superoxide, a reactive oxygen species. Excess superoxide damages cells, while a brief signal can trigger an adaptive response that prepares them for a later stress. This response is called mitohormesis.
The authors temporarily suppressed Sod2 in mouse embryos. This gene encodes the enzyme that neutralizes superoxide within mitochondria. Sod2 activity recovered rapidly after birth. Permanent loss of Sod2 causes mice to die soon after birth, whereas the reversible model allowed the researchers to distinguish brief stress from persistent damage. In a 2018 study, the same model had already shown a lasting adaptation in the liver and embryonic fibroblasts. The new study tested whether this memory would persist in the heart during injury.
As adults, these mice had more mitochondria and antioxidant defense proteins in cardiac muscle. The researchers then administered doxorubicin, an anticancer drug that can damage the heart through mitochondrial dysfunction and oxidative stress. In the acute model, the protective program partially preserved heart mass, mitochondrial proteins, and antioxidant defenses in female mice. In the chronic model, the animals retained better cardiac pumping capacity and developed less fibrosis and less reduction in cardiac cell size.
To determine how brief stress creates a lasting memory, the researchers turned to mouse embryonic fibroblasts, which allowed them to intervene separately at each step in the pathway. Superoxide reduced the activity of ACO2, the enzyme that converts citrate to isocitrate within mitochondria. This increased the amount of citrate in the cytosol, the fluid portion of the cell. Citrate left the mitochondria through the SLC25A1 transporter. In the cytosol, ACLY converted it to acetyl-CoA, which is used to acetylate histones, the proteins that package DNA. These modifications help the cell regulate which regions of DNA are transcribed.
Deleting SLC25A1 prevented citrate accumulation, the antioxidant response, and resistance to doxorubicin. Temporary suppression of Aco2 or the addition of citrate reproduced part of the response, while simultaneous suppression of ACLY weakened the protection. Acetylation of histone H3 at position 27, designated H3K27Ac, increased in fibroblasts. In mouse hearts, the authors observed a different acetylation mark, H3K9Ac. In the cellular model, the researchers tested each step of the pathway through separate interventions: superoxide reduced ACO2 activity, citrate entered the cytosol, and the resulting acetyl-CoA increased histone acetylation and protected cells from doxorubicin.