Mitochondria channel calcium

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Mitochondria take up Ca²⁺ in response to certain stimuli, which upregulates aerobic metabolism and sensitizes cells to apoptosis, but the identity of the mitochondrial calcium uniporter (MCU) had remained elusive. De Stefani et al. now reveal that this long sought-after Ca²⁺ channel is a 40 kDa protein of the mitochondrial inner membrane.

As a uniporter regulator called mitochondrial calcium uptake 1 (MICU1; also known as CBARA1) was recently identified in the MitoCarta database (an inventory of mammalian mitochondrial genes), the authors searched this database for the actual MCU. They narrowed down the list of candidates to one protein, which they termed MCU, the tissue expression of which is ubiquitous and correlates with that of MICU1. Importantly, cell fractionation experiments and immuno-electron microscopy revealed that MCU localizes to mitochondria, along with mitochondrial markers, and more detailed experiments showed that it is found in the inner mitochondrial membrane.

MCU is the mitochondrial Ca²⁺ channel.

To confirm that MCU has a role in mitochondrial Ca²⁺ uptake, the authors expressed small interfering RNA (siRNA) against MCU in HeLa cells and treated them with histamine, an inositol-1,4,5-trisphosphate-generating agonist that causes Ca²⁺ release from the endoplasmic reticulum. Mitochondrial Ca²⁺ uptake was significantly reduced in cells expressing MCU siRNA. In the converse experiment, overexpression of MCU caused an increase in mitochondrial Ca²⁺ uptake in response to histamine, sensitizing cells to apoptosis. Together, these data suggest that MCU is the mitochondrial Ca²⁺ channel.

Finally, to confirm the channel activity of MCU, the authors generated recombinant MCU protein, inserted it into lipid bilayers and tested its electrophysiological properties in response to Ca²⁺. The results were in agreement with the characteristics previously reported for the unknown uniporter. For example, the MCU was inhibited by ruthenium red, which is known to inhibit Ca²⁺uptake by isolated mitochondria. Furthermore, mutation of two negatively charged residues in the presumed pore-forming region of MCU abolished its channel activity.

As the authors explain, the identification of the mitochondrial Ca²⁺ channel means that “Informative animal models can now be generated and new drugs developed to influence processes regulated by mitochondrial Ca²⁺ signals, such as aerobic metabolism and cell death.”