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

Background: An opening of the mitochondrial permeability transition pore (MPTP), which leads to loss of mitochondrial membrane potential (ΔΨ(m)), is the earliest event that commits a cell to death. Mitochondrial matrix calcium ([Ca(2+)](m)) is considered to be a critical regulator of MPTP, but direct monitoring of [Ca(2+)](m) is difficult with previously-reported sensors. We developed a novel fluorescent indicator for [Ca(2+)](m), GCaMP2-mt, by adding a mitochondrial targeting sequence to a high signal-to-noise Ca(2+) sensor protein GCaMP2, and monitored dynamic changes in oxidant-induced cardiac myocyte death.

Methods and results: GCaMP2-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 ΔΨ(m). We monitored oxidant-induced responses of [Ca(2+)](m) and ΔΨ(m) using time-lapse confocal microscopy. The response of [Ca(2+)](m) was synchronous with that of cytosolic calcium and was divided into three kinetically-distinct phases; the first phase, during which [Ca(2+)](m) maintained its baseline level; the second phase, during which [Ca(2+)](m) showed a rapid and sudden increase; and the third phase, during which [Ca(2+)](m) continued to increase at a slower rate until the collapse of ΔΨ(m). The third phase was likely to be mediated through a mitochondrial Ca(2+) uniporter, because it was modulated by uniporter-acting drugs. Importantly, there was a remarkable cellular heterogeneity in the third phase, and ΔΨ(m) loss occurred in an all-or-none manner depending on the cellular [Ca(2+)](m) level with a clear cut-off value.

Conclusions: Direct monitoring of [Ca(2+)](m) using GCaMP2-mt provides deeper insight into the mechanism of cardiac myocyte death.