Blood oxygen levels may determine cardiac muscle regeneration

Cardiomyocytes – cells that make up the heart’s muscular wall – are unable to regenerate after cardiac failure. However, mammalian neonates and Zebrafish have significant cardiomyocyte regeneration following injury. The hearts of both of these organisms experience hypoxemic conditions. Fish have to deal with water’s poor oxygenation. Mammalian neonates experience the mixing of deoxygenated and oxygenated blood in the venous return of the fetal circulation which leads to heart hypoxemia. However, in mammals, cardiomyocytes lose their regeneration capability after postnatal day $7$‍, when they undergo cell-cycle arrest and mixing of blood ceases.

Researchers trying to determine the causes of this cell-arrest arrest noted that shortly after birth there is an increase in mitochondrial DNA activity indicating that mice switch from the anaerobic glycolysis metabolic pathway, normally used by a fetus, to the oxygen-dependent, mitochondrial oxidative phosphorylation (MOP), metabolic pathway used by adults. MOP is more energy efficient than anaerobic pathways. However, as the mice switch to MOP, researchers found an increase in reactive oxygen species (ROS), which cause cellular oxidative stress and may cause DNA damage. Based upon that, researchers set up to perform three experiments.

Experiment 1: Researchers exposed neonatal mice to hypoxic and hyperoxic conditions and quantified cardiomyocyte cytokinesis and apoptosis rates. Their results show that the cytokinesis rate increases with hypoxic conditions but apoptosis remains unchanged regardless of treatment.

Experiment 2: Researchers injected Diquat, a chemical whose effect is to increase the number of ROS, and observed the effect on cardiomyocyte cytokinesis rate and cell size . Their results shows a significant difference between mice that receive diquat and the control group (Figure 1).

Experiment 3: Researchers injected mice with N-acetyl-cysteine (NAC), a scavenger molecule that binds and neutralizes ROS. Then, they quantified the effects of NAC on cardiomyocyte cytokinesis rate and cell size. Their results also show a significant difference between the treatment and control (Figure 2)

Figure 1. Effect of diquat injection on cardiomyocytes (A) cytokinesis rate, as shown by co-immunostaining with anti-Aurora B+ and (B) cell size using WGA staining. "*" and "**" indicate statistically significant differences of the columns at p < $0.05$‍ and p < $0.01$‍, respectively. Greater pixel/area ratio indicates bigger size.

Figure 2. Effects of N-acetyl-cysteine (NAC) injection on cardiomyocytes on (A) cytokinesis rate as shown by co-immunostaining with anti-Aurora B+ and (B) cell size using WGA staining. "*" and "**" indicate statistically significant differences at p < $0.05$‍ and p < $0.01$‍, respectively.

Adapted from: Puente, B. N. et al. (2014). The oxygen-rich postnatal environment induces cardiomyocyte cell-cycle arrest through DNA damage response. Cell, 157(3), 565–579.