Is Energy Metabolism Homogeneous Within A Cell?

Energy metabolism is the essential group of chemical reactions that generates the energy that maintains our cells alive. But that isn’t all energy metabolism does! Recent studies show that it controls many other cellular processes, including inflammation and immunity, stem cell differentiation and cell death.

Most of these studies demonstrating new roles for energy metabolism used an essential technique to follow metabolic changes: measuring mitochondrial oxygen consumption. Since mitochondria act as the cell’s batteries, powering them up, and oxygen consumption is proportional to mitochondrial activity, measuring oxygen consumption is a great way to understand energy metabolism. Oxygen consumption is both a reliable and quantitative way to follow the cell’s metabolic demands, outputs, and capacity.

Credit: Air Force Medical Service

Typically, oxygen consumption is measured in tissues, homogenates or cell cultures using oxygen-sensitive electrochemical electrodes. This works well but gives an idea only about the average oxygen consumption of the whole sample. We decided to ask if energy metabolism could be different from cell to cell in the same culture, or even within different parts of the same cell. To answer that question, we had to develop a very small electrode which can be used to detect small differences in the concentration of oxygen in the vicinity of individual cells during respiration.

To do so, we established a collaboration between a lab that studies energy metabolism (Dr. Kowaltowski) and a lab that develops new electrochemical techniques (Dr. Bertotti). A doctoral student (Carla Santana Santos) bridged the gap between the two very different research lines and constructed microelectrodes adapted to a scanning system: Scanning Electrochemical Microscopy.

In this technique, a microelectrode biased at a suitable potential works as a probe to investigate the electrochemical activity of a surface with high spatial resolution (in the micrometer range). Positioning this probe in close proximity to the studied sample allows us to gather information on electrochemical processes or substance fluxes occurring at the surface. In our case, the investigated surface was a cell and by moving the microelectrode over the cell, a map of oxygen concentration at a single cell level was obtained. The results were stunning, capable of showing how oxygen consumption behaves in the different areas of the cell.

With this technique, we were able to see that the cell’s oxygen consumption pattern is far from homogeneous. Predictably, the nucleus area of the cell has low oxygen consumption, since there are no mitochondria in the nucleus. Surprisingly, however, the tips of the cells, in which cell thickness is lower and we would expect less oxygen consumption, unexpectedly had high oxygen consumption levels. This may indicate that the mitochondria in this region of the cell are different, although we don’t know yet why, and what the consequences of these differences are. The existence of this new technique, useful for measuring oxygen consumption at a subcellular level, will certainly open new possibilities and uncover many more surprising findings about energy metabolism.

These findings are described in the article entitled Single Cell Oxygen Mapping (SCOM) by Scanning Electrochemical Microscopy Uncovers Heterogeneous Intracellular Oxygen Consumption, recently published in the journal Scientific Reports. This work was led by Mauro Bertotti, Carla Santana Santos, and Alicia J. Kowaltowski from Universidade de São Paulo.

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