Hypothermia Suppresses Uncoupling of Oxidative-Phosphorylation after Neonatal Cerebral Hypoxia-Ischemia

Abstract

Hypothermia is the best available therapy for neonatal hypoxia-ischemia (HI) brain injury, but its primary mechanisms remain uncertain1. We hypothesize that HI induces, whereas hypothermia represses, uncoupling of oxidative-phosphorylation (OXPHOS)—an increase of the cerebral metabolic rate of oxygen (CMRO2) despite reduction of the mitochondrial energy output2,3. We used a multi-parametric photoacoustic microscopy (PAM) system to compare the effects of HI and post-HI hypothermic treatment on CMRO2 in awake 10-day-old (P10) mice4–9. Here we show that hypoxia (10% O2) elevated CMRO2, but the addition of unilateral carotid artery ligation suppressed CMRO2 and sparked a rapid overshoot of post-HI CMRO2 in the ipsilateral cerebral cortex for at least 2 hours. The post-HI surge of CMRO2 was linked to an increase of mitochondrial oxygen consumption and superoxide outburst, despite reduction of the mitochondrial membrane potential. Notably, post-HI hypothermia blocked the surge of superoxide and CMRO2—primarily by limiting oxygen extraction fraction (OEF)—leading to better preservation of adenosine triphosphate (ATP), creatine (Cr) and N-acetylaspartate (NAA) after HI. Mice that did not receive hypothermia exhibited ~80% reduction of CMRO2 at 24 h post-HI, coupled to a large cortical infarction. These results suggest that mitigation of post-HI uncoupling of OXPHOS is an early and/or pivotal effect of hypothermia. Further, optical measurement of CMRO2 may be a sensitive and non-invasive method to monitor brain damage in hypoxic-ischemic encephalopathy (HIE)10–15.