CaMKII Does not Control Mitochondrial Ca2+ Uptake in Cardiac Myocytes

Key points

Mitochondrial Ca²⁺ uptake stimulates the Krebs cycle to regenerate the reduced forms of pyridine nucleotides (NADH, NADPH and FADH₂) required for ATP production and ROS elimination. It was previously proposed that Ca²⁺/calmodulin‐dependent protein kinase II (CaMKII) regulates mitochondrial Ca²⁺ uptake via MCU phosphorylation. We used two mouse models with either global deletion of CaMKIIδ (CaMKIIδ KO) or cardiomyocyte‐specific deletion of CaMKIIδ and γ (CaMKIIδ/γ DKO) to interrogate whether CaMKII controls mitochondrial Ca²⁺ uptake in isolated mitochondria and during β‐adrenergic stimulation in cardiac myocytes. CaMKIIδ/γ controlled neither Ca²⁺ uptake, respiration nor ROS emission in isolated cardiac mitochondria nor in isolated cardiac myocytes during β‐adrenergic stimulation and pacing. Our results argue against a relevant role of CaMKII for mitochondrial Ca²⁺ uptake in cardiac myocytes under physiological conditions.

Abstract

Mitochondria are the main source of ATP and reactive oxygen species (ROS) in cardiac myocytes. Furthermore, activation of the mitochondrial permeability transition pore (mPTP) induces programmed cell death. These processes are essentially controlled by Ca²⁺, which is taken up into mitochondria via the Ca²⁺ uniporter (MCU). It was recently proposed that Ca²⁺/calmodulin‐dependent protein kinase II (CaMKII) regulates Ca²⁺ uptake by interacting with the MCU, thereby affecting mPTP activation and programmed cell death. Here, we addressed the role of CaMKII under physiological conditions in which mitochondrial Ca²⁺ uptake matches energy supply to demand of cardiac myocytes. To this end, we measured mitochondrial Ca²⁺ uptake in isolated mitochondria and cardiac myocytes harvested from cardiomyocyte‐specific CaMKII δ and γ double KO (CaMKIIδ/γ DKO) and global CaMKIIδ KO mice. To simulate a physiological workload increase, cardiac myocytes were subjected to β‐adrenergic stimulation (by isoproterenol superfusion) and increase in stimulation frequency (from 0.5 to 5 Hz). No differences in mitochondrial Ca²⁺ accumulation were detected in isolated mitochondria or cardiac myocytes from both CaMKII KO models compared with wild‐type (WT) littermates. Mitochondrial redox state and ROS production were unchanged in CaMKIIδ/γ DKO, whereas we observed a mild oxidation of mitochondrial redox state and an increase in H₂O₂ emission from CaMKIIδ KO cardiac myocytes exposed to an increase in workload. In conclusion, our results argue against a relevant regulation of mitochondrial Ca²⁺ uptake via the MCU or mPTP activation by CaMKII in cardiac myocytes under physiological conditions.

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