The metabolic roles of astrocytes and neurons in the diving brain A study of the mitochondrial distribution in the brain of the hooded seal (Cystophora cristata)

Abstract

The brain is highly dependent on oxygen for its metabolism and, in most mammals, major brain dysfunction occurs already within minutes of insufficient oxygen supply (hypoxia). Oxygen is the final electron acceptor in the electron transport system that drives the production of ATP through oxidative phosphorylation in the mitochondria. The rate at which oxidative phosphorylation produces ATP is dependent on the spatial organisation of the mitochondria. Studies on mice and primates have shown that mitochondria are unevenly distributed between the two major cell types of the brain: neurons and astrocytes. Neurons predominantly rely on aerobic metabolism and have higher mitochondrial density, whereas astrocytes produce energy mainly anaerobically and have lower mitochondrial density. There is evidence of a tight metabolic coupling between astrocytes and neurons especially at the brain’s synapses, where the astrocytes aid the neurons in their metabolism through a mechanism called the astrocyte-neuron lactate shuttle (ANLS). Mammals that experience hypoxia regularly, like pinnipeds, show a remarkable brain hypoxia tolerance. Studies done on the visual cortex of the deep-diving hooded seal (Cystophora cristata) suggest that one of the mechanisms at the base of their tolerance is a metabolic shift between astrocytes and neurons. To further investigate the hooded seal brain metabolism, tissue from the visual cortex of adult hooded seals, juvenile hooded seals and mice was sampled and fixed for immunostaining and fluorescence imaging of astrocytes, neurons and mitochondria to analyse the distribution of mitochondrial sizes and densities between the two cell types. The adult hooded seals had significantly lower mitochondrial densities and larger sized mitochondria in the astrocytes compared to the neurons, whereas the opposite was found in juvenile seals and mice. This could indicate differences in the metabolic roles of astrocytes and neurons between these animals. The overall lower mitochondrial density in the adult seals compared to juvenile seals and mice is in accordance with previous findings of lower brain metabolism and supressed synaptic transmission in the adult hooded seal brain. The differences found between juvenile and adult seals indicate that mitochondrial densities and sizes may be traits that depend on age and/or development of diving abilities in the hooded seals. These findings shed new light to understating the cellular mechanisms behind the hypoxia tolerance of pinnipeds.