Sex, stress and UCP2: mitochondria illuminate microglial identity beyond the dichotomy

Abstract

Balanced and dynamic mitochondrial networks are essential for cell survival and function. Mitochondrial networks remodel their connectivity, content and subcellular localization to support optimized energy production under conditions of increased stress. In vivo, stressors can arise from the environment, such as in neuronal injury, or from mutation-induced cellular dysfunction. Cells programmed to identify and respond to these stress signals, like microglia, rely on optimized mitochondrial function, however we know very little about mitochondrial networks of microglia in vivo or their adaptation to environmental or cellular stressors. Here, we define the mitochondrial networks of retinal microglia in physiological conditions in vivo and evaluate network alterations by taking advantage of a microglia-selective mitochondria-labeled mouse model. First, we demonstrate significant differences in the mitochondrial networks of microglia in vivo and in vitro. Then, we induced neuronal injury in the in vivo environment using optic nerve crush, where responsive microglia exhibit more fragmented mitochondrial networks with increased content and perinuclear localization, supporting a state of increased cellular stress. Surprisingly, when we selectively increase cellular stress by knocking out the mitochondria-associated gene uncoupling protein 2 (UCP2), only male UCP2KO microglia establish a hyperfused mitochondrial network after injury, indicating sex differences in microglial stress mitigation. Ovariectomy in UCP2KO females elicits a shift toward the male hyperfused mitochondrial phenotype suggesting that circulating estrogens are a contributing factor to the differences in microglial stress mitigation.