Spatial patterning of metabolism by mitochondria, oxygen, and energy sinks in a model cytoplasm

Metabolite gradients might guide mitochondrial localization in cells and angiogenesis in tissues [1, 2]. It is unclear whether they can exist in single cells, because the length scale of most cells is small compared to the expected diffusion times of metabolites. For investigation of metabolic gradients, we need experimental systems in which spatial patterns of metabolism can be systematically measured and manipulated. We used concentrated cytoplasmic extracts from Xenopus eggs [3] as a model cytoplasm, and visualized metabolic gradients formed in response to spatial stimuli. Restriction of oxygen supply to the edge of a drop mimicked distance to the surface of a single cell, or distance from a blood vessel in tissue. We imaged a step-like increase of Nicotinamide adenine dinucleotide (NAD) reduction similar to 600 mu m distant from the oxygen source. This oxic-anoxic switch was preceded on the oxic side by a gradual rise of mitochondrial transmembrane potential (At) and reactive oxygen species (ROS) production, extending over similar to 600 mu m and similar to 300 mu m, respectively. Addition of Adenosine triphosphate (ATP)-consuming beads mimicked local energy sinks in the cell. We imaged At gradients with a decay length of similar to 50-300 mu m around these beads, in the first visualization of an energy demand signaling gradient. Our study demonstrates that mitochondria can pattern the cytoplasm over length scales that are suited to convey morphogenetic information in large cells and tissues and provides a versatile model system for probing of the formation and function of metabolic gradients.