Loss of mitochondrial exo/endonuclease EXOG affects mitochondrial respiration and induces ROS-mediated cardiomyocyte hypertrophy

Abstract

Tigchelaar W, Yu H, de Jong AM, van Gilst WH, van der Harst P, Westenbrink BD, de Boer RA, Silljé HH. Loss of mitochondrial exo/endonuclease EXOG affects mitochondrial respiration and induces ROS-mediated cardiomyocyte hypertrophy. Am J Physiol Cell Physiol 308: C155-C163, 2015. First published November 5, 2014 doi:10.1152/ajpcell.00227.2014.-Recently, a locus at the mitochondrial exo/endonuclease EXOG gene, which has been implicated in mitochondrial DNA repair, was associated with cardiac function. The function of EXOG in cardiomyocytes is still elusive. Here we investigated the role of EXOG in mitochondrial function and hypertrophy in cardiomyocytes. Depletion of EXOG in primary neonatal rat ventricular cardiomyocytes (NRVCs) induced a marked increase in cardiomyocyte hypertrophy. Depletion of EXOG, however, did not result in loss of mitochondrial DNA integrity. Although EXOG depletion did not induce fetal gene expression and common hypertrophy pathways were not activated, a clear increase in ribosomal S6 phosphorylation was observed, which readily explains increased protein synthesis. With the use of a Seahorse flux analyzer, it was shown that the mitochondrial oxidative consumption rate (OCR) was increased 2.4-fold in EXOG-depleted NRVCs. Moreover, ATP-linked OCR was 5.2-fold higher. This increase was not explained by mitochondrial biogenesis or alterations in mitochondrial membrane potential. Western blotting confirmed normal levels of the oxidative phosphorylation (OXPHOS) complexes. The increased OCR was accompanied by a 5.4-fold increase in mitochondrial ROS levels. These increased ROS levels could be normalized with specific mitochondrial ROS scavengers (MitoTEMPO, mnSOD). Remarkably, scavenging of excess ROS strongly attenuated the hypertrophic response. In conclusion, loss of EXOG affects normal mitochondrial function resulting in increased mitochondrial respiration, excess ROS production, and cardiomyocyte hypertrophy.cardiomyocytes; hypertrophy; mitochondria; mitochondrial respiration; ROS THE HEART IS ONE OF THE MOST energy-consuming organs in the human body. This energy, in the form of ATP, is used to maintain proper contractile function and is mainly produced by cellular respiration, in particular by oxidative phosphorylation (OXPHOS) in the mitochondria. There is a strict correlation between energy production (supply) and energy utilization (demand). As the heart has limited energy storage capacity, ATP-generating systems must respond proportionally to fluctuations in demand. This energetic status of the heart is a delicate balance and is often disturbed in cardiovascular diseases, including heart failure.As a byproduct of oxidative phosphorylation, the mitochondria produce reactive oxygen species (ROS). Low levels of ROS may be protective (7, 23) and affect signaling pathways that may stimulate growth (5). However, an imbalance between ROS production and the normal cellular antioxidant defense system will lead to oxidative stress and potentially in DNA damage (10). Therefore, cells have develo...

A domain in human EXOG converts apoptotic endonuclease to DNA-repair exonuclease

“…Conversely, depletion of nucleases that are critical for nuclear BER, flap endonuclease 1 (FEN1) and DNA2 has no or limited effect on mtDNA14. Ectopic expression of hEXOG increases resistance to oxidative stress of proliferating myoblasts15, and depletion of EXOG increases oxidative consumption rate in primary neonatal rat ventricular cardiomyocytes16. These studies provide strong support to the idea that hEXOG is involved in mtDNA repair.…”

In EXOG-depleted cardiomyocytes cell death is marked by a decreased mitochondrial reserve capacity of the electron transport chain

“…The decline in maximal respiration and reserve capacity was the first sign that mitochondrial respiration was hampered in EXOG-depleted cells stimulated with PE, which ultimately resulted in cell death. As EXOG depletion-induced ROS production could not be decreased by treatment with MitoTEMPO, we suggest that increased ROS levels are implicated in several other effects on the cell, including cardiac hypertrophy [14]. In endothelial cells, the exhaustion of the reserve capacity by ROS leads to mitochondrial protein modifications, ultimately resulting in cell death [27,28].…”

“…Interestingly, the EXOG-containing locus was previously identified in a genome-wide association study on QRS duration, which is a risk factor for heart failure [15]. These increased ROS levels triggered cell growth, resulting in a hypertrophic phenotype [14]. We reported that in EXOG-depleted cardiomyocytes, mitochondrial respiration is strongly enhanced and a concomitant increase in ROS has been observed.…”

“…We have recently shown that EXOG depletion activated mitochondrial respiration and induced ROS-mediated hypertrophy [14]. We have recently shown that EXOG depletion activated mitochondrial respiration and induced ROS-mediated hypertrophy [14].…”

“…Recently, we demonstrated that mitochondrial endo/ exonuclease G-like (EXOG) modulates mitochondrial respiration and hypertrophy in cardiomyocytes [14]. Interestingly, the EXOG-containing locus was previously identified in a genome-wide association study on QRS duration, which is a risk factor for heart failure [15].…”