Regional Specialization of Mitochondrial Ca2+ Signaling in Cardiac Cells

Excitation-contraction coupling in cardiac cells is controlled mainly by ICa-gated release of Ca2+ from the sarcoplasmic reticulum (SR), but is also thought to be modulated by mitochondrial Ca+ signaling. Using field-stimulated rat ventricular cardiomyocytes, we have previously found that mitochondria that were primed by rapid beating or caffeine-induced SR-Ca2+ release subsequently were capable of rapid release of Ca2+ when stimulated by shear forces (Belmonte and Morad, 2008, J. Physiol. 586: 1379). To elucidate this finding we measured mitochondrial Ca2+ using Mitycam. The adenoviral Mitycam probe was originally created using the inverse pericam mutant M13-EYFP (V68L/Q69K)(145-238) and EYFP(V168L/Q69K)(1-144)-CaM, fused to the subunit VIII of human cytochrome c oxidase mitochondrial-targeting signal (Kettlewell et al., 2009, J Mol Cell Cardiol, 46:891). Short-time cultured adult feline cardiomyocytes were infected with the mitycam adenovirus at MOI of 200 virus particles per cell. After 3-4 days we measured the Mitycam fluorescence under voltage-clamp conditions using whole-cell fluorometry and 2-D confocal imaging. Supporting information was obtained using TMRE to assess mitochondrial depolarization and NCX-currents to assess cytosolic Ca2+ transients. The whole-cell Mitycam measurements suggested transfer of Ca2+ from the SR to the mitochondria during and following voltage-clamp depolarizations and caffeine-induced SR Ca2+ release. In confocal measurements the distribution of Mitycam-fluorescence showed longitudinal streaks with sarcomeric banding resembling the TMRE staining and a characteristic nuclear band (NB) of enhanced fluorescence that typically connected and extended from the two nuclei. Depolarization, caffeine, mechanical stimulation, and FCCP produced changes in Mitycam fluorescence that often resulted in redistribution between NB, general mitochondrial, and subsarcolemmal mitochondrial compartments. Similarly, injection of FCCP caused local decline in TMRE fluorescence (mitochondrial depolarization), but strongly enhanced subsarcolemmal fluorescence. These findings suggest that mitochondria in different regions of cardiac cells play different roles in Ca2+ signaling. NIH HL16152.