Targeted modulation of cardiac energetics via the creatine kinase system
Keywords: Myocardial energetics
Creatine kinase
Mitochondrial permeability transition pore
Ischemia/reperfusion

There is a large body of clinical and experimental evidence linking heart disease with impairment of myocardial energetics, particularly the creatine kinase (CK) system. The goal of the experiments described in this thesis was to develop and study models of increased CK phosphotransfer, by overexpressing the CK isoenzymes and/or augmenting intracellular creatine stores.
Pilot experiments were performed in cultured cells, which were used to (a) study the effects of CK overexpression in vitro, and (b) validate constructs prior to generation of transgenic mice. Expression was verified at the protein level for all constructs in HL-1 and HEK293 cells, and enzymatic activity was confirmed. Mitochondrial CK (CKmt) was expressed in the mitochondria, as expected, and CKmt overexpression was associated with a significant reduction in cell death in a model of ischemia/reperfusion injury (68.1 ± 7.1% of control, p≤0.05).
Transgenic mice overexpressing CKmt in the heart were generated by a targeted approach, using PhiC31 integration at the ROSA26 locus. Transgene expression was confirmed in vitro in embryonic stem cells, and in vivo at the mRNA and protein levels. There was only a modest increase in CKmt activity; therefore, homozygous transgenic mice were generated to increase expression levels, and had 27% higher CKmt activity than wild-types (p≤0.01). Mitochondrial localization of CKmt was confirmed by electron microscopy. Citrate synthase activity, a marker of mitochondrial volume, was &Tilde;10% lower in transgenic mice (p≤0.05). Baseline phenotyping studies found that CKmt-overexpressing mice have normal cardiac structure and function. These mice are currently being backcrossed onto a pure C57BL/6 background for further studies in models of heart disease.
In addition to CKmt, transgenic mice overexpressing the cytosolic CK isoenzymes, CK-M and CK-B, were generated. Due to the modest level of expression observed at ROSA26, random-integration transgenesis was used, and multiple lines were generated for each construct (carrying 2 or 6 transgene copies in the CK-M line; 2, 3, or &Tilde;30 in CK-B). Transgene expression was validated at the mRNA, protein, and activity levels. These lines are currently being expanded for further validation and phenotyping studies.
Previous experiments in our group have demonstrated that increasing intracellular creatine (Cr) reduces ischemia/reperfusion injury, and a series of in vitro experiments was performed to determine whether this effect may be mediated by inhibition of the mitochondrial permeability transition pore (mPTP). The mPTP plays a significant role in ischemia/reperfusion, and there is evidence linking the CK system to regulation of the mPTP. Therefore, a model was developed to test whether Cr affects mPTP opening in cardiac-derived HL-1 cells, as this mechanism may contribute to the protective effect observed in vivo. Cr incubation conditions were determined empirically, and 24-hour incubation with 5mM or 10mM Cr was found to significantly delay mPTP opening, to a similar degree to the established mPTP inhibitor, cyclosporin A. This provides evidence that Cr may exert protective effects in the heart by a variety of mechanisms, in addition to its traditional role in energy metabolism.
In summary, the experiments conducted in this thesis have produced a range of tools for studying augmentation of the creatine kinase system as a therapeutic target in heart disease. The results of in vitro assays indicate that mitochondrial CK may be a particularly promising target, and that inhibition of the mitochondrial permeability transition pore may contribute to the cardioprotective effect of creatine. Finally, the transgenic models generated and validated over the course of this project will allow for a wide range of future studies into the potential benefits of CK overexpression in the mammalian heart.