Transgenic mouse model with deficient mitochondrial polymerase exhibits reduced state IV respiration and enhanced cardiac fibrosis

Abstract

Mitochondria produce the energy required for proper cardiac contractile function, and cardiomyocytes that exhibit reduced mitochondrial electron transport will have reduced energy production and decreased contractility. Mitochondrial DNA (mtDNA) encodes the core subunits for the protein complexes of the electron transport chain (ETC). Reduced mtDNA abundance has been linked to reduced ETC and the development of heart failure in genetically engineered mice and in human diseases. Nucleoside reverse transcriptase inhibitors (NRTIs) for HIV/AIDS are used in antiretroviral regimens, which cause decreased mtDNA abundance by inhibiting the mitochondrial polymerase, pol γ as a limiting side effect. We explored consequences of AZT exposure on mtDNA abundance in an established transgenic mouse model (TG) in which a cardiac-targeted mutant form of pol γ displays a dilated cardiomyopathy (DCM) phenotype with increased left ventricle (LV) mass and increased LV end diastolic dimension. TG and wild-type littermate mice received 0.22mg/day AZT or vehicle for 35 days and subsequently analyzed for physiological, histological, and molecular changes. After 35 days, Y955C TGs exhibited cardiac fibrosis independent of AZT. Reduced mtDNA abundance was observed in the Y955C mouse; AZT treatment had no effect on that depletion suggesting that Y955C was sufficient to reduce mtDNA abundance maximally. Isolated mitochondria from AZT-treated Y955C hearts displayed reduced mitochondrial energetic function by oximetric measurement. AZT treatment of the Y955C mutation further reduced basal mitochondrial respiration and state IVo respiration. Together, these results demonstrate that defective pol γ function promotes cardiomyopathy, cardiac fibrosis, mtDNA depletion, and reduced mitochondrial energy production.

Maintaining Ancient Organelles

“…14,15 For example, POLG mutations can cause a broad clinical spectrum including cardiomyopathy, 16,17 a phenotype confirmed in mouse models. 18–20 Notably, the loss of POLG exonuclease activity in mice results in rapid buildup of mutations and deletions in the heart mitochondrion which occurs concurrently with cardiomyopathy. 21 There is a 90-fold increase in mtDNA deletions in POLG exonuclease deficient mice.…”

Inherited mitochondrial genomic instability and chemical exposures

“…In addition, compared to other mouse models, a known disease mutation in POLG (Y955C) was now shown to be linked with increased oxidative stress. Furthermore, cardiac nuclear DNA methylation changes were seen in this mouse (Koczor et al 2013). These mice have been utilized in several gene-environment studies that will be discussed later in this review.…”

P-selectin-mediated LOX expression promotes insulinoma growth in Rip1-Tag2 mice by increasing tissue stiffness

“…Hydroxyproline, a biochemical index of collagen deposition, was quantified as previously described 21. Angiogenic islets and insulinomas from three mice were hydrolyzed in 6 N HCL overnight at 100 °C.…”