Heterogeneous flux capacity and oxygen sensitivity lead to subcellular ETC flux gradients in mouse oocytes

Mitochondria are metabolic hubs of the cell that provide energy and metabolites to meet the energetic, biosynthetic and signaling demands of the cell. Mitochondrial activities are characterized by the metabolic fluxes through their internal metabolic pathways. One of the most important mitochondrial metabolic pathways is the electron transport chain (ETC), where electron carriers such as NADH donate their electrons to oxygen to power mitochondrial respiration. Mitochondrial activities are dynamically and spatially regulated during organism development to ensure robust development. Recent work has revealed the existence of a subcellular ETC flux gradient within a single mouse oocyte, where mitochondria closer to the cell membrane display a higher ETC flux, but the mechanism underlying the formation of this gradient is unknown. In this work, we study the origin of the ETC flux gradients by modulating them through perturbations of external oxygen concentration and temperature. Interpreting the data with spatial kinetic modeling of mitochondrial respiration, we discover that the subcellular ETC flux gradient cannot be explained by reaction-diffusion of oxygen alone, but is a result of mitochondrial heterogeneity where mitochondria closer to the cell membrane display larger ETC flux capacity and lower oxygen sensitivity. Our work suggests that kinetically distinct subpopulations of mitochondria are spatially sorted according to their metabolic activities to form intracellular metabolic gradients.