Funding outcome

Introduction: Neonatal sepsis is the dysregulated host response to infection common in preterm infants and is considered late-onset sepsis (LOS) when it occurs after 72 hours of birth. The inflammatory response during sepsis produces excessive amounts of signaling molecules named reactive oxygen species (ROS). ROS not only impairs mitochondrial protein function, but also damages mitochondrial DNA. Although the impact of sepsis on mitochondrial function in adults has been explored to some extent, the consequences of LOS on mitochondrial function in the newborn remains unknown. Here, we sought to explore the acute effects of LOS on liver and kidney mitochondrial dynamics. Methods: LOS was induced in 3-day-old Sprague Dawley pups by injecting fecal slurry (FS, 1.0 mg/g body weight), intraperitoneally; controls received vehicle (5% dextrose in water). All pups received sustained release buprenorphine (0.5mg/kg, subcutaneous) for pain control. Pups were euthanized at 4h post-FS. The left lateral lobe of the liver and the left kidney were harvested and homogenized for assessment of mitochondrial function (n=3-5) by high-resolution respirometry. The remaining tissues were flash frozen for RT-qPCR (n=8-12). Data were analyzed using the two-way ANOVA followed by Sidak's multiple comparison test. Results: Liver mitochondrial respiration was increased in FS-injected pups compared to controls. Specifically, male and female livers of FS-injected pups showed an 81.9% and 30.8% increase in NADH pathway respiration (P=0.017 and P=0.0003, respectively) as well as a 65.0% and 32.8% increase in succinate pathway respiration (P=0.045 and P=0.027, respectively). However, no change in Complex IV respiration was observed in either sex. In the liver, mRNA expression of H2O2-producing gene (Sod2) was upregulated 10-fold (P<0.0001) while H2O2-removing genes (Cat, Gsr) were downregulated by 50% (P<0.0001) in FS-injected pups, suggesting a severe oxidative defense imbalance. Interestingly, no change in mitochondrial respiration was observed in the kidney of FS-injected pups. mRNA expression of Sod2 was also upregulated 10-fold (P<0.0001) and 16-fold (P<0.0001) in the kidney of male and female FS-injected pups, respectively. Finally, Lipocalin 2 gene expression was upregulated 2.7-fold (P<0.0001) in FS-injected pups, suggesting the development of kidney damage. Conclusion: Organ-specific mitochondrial response and development of kidney injury were observed during the hyperacute phase of LOS. Whether the change in mitochondrial respiration is adaptive or maladaptive is still under investigation, but this work could identify the mitochondrion as a target for intervention in the treatment of LOS.