Direct monitoring of mitochondrial calcium levels in cultured cardiac myocytes using a novel fluorescent indicator protein, GCaMP2-mt

BackgroundAn opening of the mitochondrial permeability transition pore (MPTP), which leads to loss of mitochondrial membrane potential (ΔΨm), is the earliest event that commits a cell to death. Mitochondrial matrix calcium ([Ca2+]m) is considered to be a critical regulator of MPTP, but direct monitoring of [Ca2+]m is difficult with previously-reported sensors. We developed a novel fluorescent indicator for [Ca2+]m, GCaMP2-mt, by adding a mitochondrial targeting sequence to a high signal-to-noise Ca2+ sensor protein GCaMP2, and monitored dynamic changes in oxidant-induced cardiac myocyte death.Methods and resultsGCaMP2-mt was transduced into neonatal rat cardiac myocytes using a recombinant adenovirus. We confirmed that GCaMP2-mt colocalized with tetramethylrhodamine ethyl-ester, a fluorescent indicator of ΔΨm. We monitored oxidant-induced responses of [Ca2+]m and ΔΨm using time-lapse confocal microscopy. The response of [Ca2+]m was synchronous with that of cytosolic calcium and was divided into three kinetically-distinct phases; the first phase, during which [Ca2+]m maintained its baseline level; the second phase, during which [Ca2+]m showed a rapid and sudden increase; and the third phase, during which [Ca2+]m continued to increase at a slower rate until the collapse of ΔΨm. The third phase was likely to be mediated through a mitochondrial Ca2+ uniporter, because it was modulated by uniporter-acting drugs. Importantly, there was a remarkable cellular heterogeneity in the third phase, and ΔΨm loss occurred in an all-or-none manner depending on the cellular [Ca2+]m level with a clear cut-off value.ConclusionsDirect monitoring of [Ca2+]m using GCaMP2-mt provides deeper insight into the mechanism of cardiac myocyte death.