3D visualization of mitochondrial solid-phase calcium stores in whole cells

Area of Science:

• Cell Biology
• Mitochondrial Biology
• Biophysics

Background:

• Mitochondrial calcium uptake is crucial for cellular metabolism, signaling, and survival.
• Understanding the storage forms and dynamics of mitochondrial calcium is essential for a complete picture of its utilization.
• Recent identification of mitochondrial calcium transporters necessitates a deeper understanding of calcium storage mechanisms.

Purpose of the Study:

• To directly visualize the forms and distribution of calcium within mitochondria in intact mammalian cells.
• To determine the nature and capacity of mitochondrial calcium stores.
• To elucidate the mechanism of calcium buffering within the mitochondrial matrix.

Main Methods:

• Cryo-scanning transmission electron tomography (CSTET) for direct 3D visualization of mitochondrial calcium.
• Quantitative electron scattering analysis to determine the concentration and nature of calcium stores.

Main Results:

• Direct 3D imaging revealed pervasive amorphous solid granules containing calcium and phosphorus within mitochondrial matrices of various mammalian cell types.
• Quantitative analysis indicated these solid-phase granules function as molar concentrations of dissolved ions.
• These findings demonstrate that mitochondrial calcium buffering occurs via phase separation in live cells.

Conclusions:

• Solid-phase calcium stores, in the form of amorphous granules, represent a major reservoir within the mitochondrial matrix.
• These mitochondrial calcium stores can be mobilized for bioenergetic processes and cellular signaling.
• Phase separation is the mechanism by which calcium is buffered in the mitochondrial matrix of live cells.