Unchanged Mitochondrial Organization and Compartmentation in Creatine Deficient GAMT-/- Mouse Heart

Creatine kinase (CK) system plays an important role in the buffering and transport of high energy phosphate bonds from energy-producing mitochondria to energy-consuming ATPases. Previous studies suggest that disruption of the creatine kinase system in hearts of CK-deficient mice leads to cytoarchitectural modifications in the heart and changes in regulation of mitochondrial respiration. In this work, we tried to find out whether similar changes occurred in creatine deficient mouse model, where creatine synthesizing enzyme guanidinoacetate methyltransferase (GAMT) was knocked out. The aim of this study was to characterize the modifications in cardiomyocyte mitochondrial organization, regulation of respiration and intracellular compartmentation in GAMT-deficient mice.Three-dimensional mitochondrial organization at whole cell level was assessed by confocal microscopy. Kinetic measurements on permeabilized mouse cardiomyocytes included the affinity of oxidative phosphorylation to exogenous ADP and ATP, competition between mitochondria and pyruvate kinase for ADP produced by ATPases, ADP-kinetics of endogenous pyruvate kinase and ATP-kinetics of ATPases. using fluorescence microscopy, ADP-kinetics of respiration was measured on a single cell level. The experimental results were analyzed by mathematical models to estimate intracellular diffusion restrictions and communication between cellular compartments. Quantitative analysis of measured morphologic and kinetic data, as well as derived model fits show no difference between GAMT-deficient and wildtype mice. Thus, we conclude that inhibition of the CK-system by GAMT-deficiency does not alter mitochondrial organization, regulation of respiration and intracellular compartmentation in relaxed cardiomyocytes. This raises questions on the importance of the CK system as a spatial energy buffer in the heart.