Mitochondrial ultrastructure, function, and calcium buffering by massive calcium loading observed using advanced cryo‐electron microscopy, high‐resolution respirometry and spectrofluorimetry

Abstract

Excessive calcium accumulation is the main cause of cardiac tissue and cell death after a myocardial infarction. As a result of calcium overload, mitochondria become dysfunctional, produce excessive reactive oxygen species (ROS), cannot support the cellular energetic demand, and activate the cyclosporine A‐sensitive mitochondrial permeability transition pore. To discern the molecular, bioenergetic, and structural events associated with this phenomenon, we collected and analyzed a time‐course series of images obtained by utilizing recent advances in electron cryo‐microscopy methods. Energized mitochondria exposed to high calcium boluses, sufficient to elicit mitochondrial permeability transition, resulted in the formation of calcium phosphate granules. Under high calcium loading, the size of the calcium‐phosphate granules ranged from 50–120 nm causing OMM/IMM rupture. CsA treatment resulted in mitochondria with aberrant morphologies, increased cristae density and number, and with larger calcium‐phosphate granules ranging from 40–200 nm. The OMM was partially or completely lost; however, the mitochondria were fully functional and capable of synthesizing ATP. Our tomographic reconstruction placed these granules in close vicinity to the IMM and cristae. In addition, CsA treatment increases the calcium buffering power at greater calcium concentrations, whereas untreated mitochondria had much lower calcium buffering. Our results demonstrate that these granules form an intricate part of the mitochondrial calcium sequestration system. Overall, these results suggest CsA may interact with IMM components to: 1) modulate calcium buffering by facilitating more abundant and larger calcium phosphate granules and 2) preserve the cristae shape and number which avoids cytochrome c loss and mitochondrial permeability transition. Our data reveal that drugs capable of modulating cristae shape and number may be an effective strategy to prevent mitochondrial dysfunction stemming from calcium overload.Support or Funding InformationThis work was supported by NIH grant R00‐HL121160.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.