Unchanged mitochondrial organization and compartmentation of high-energy phosphates in creatine-deficient GAMT-/- mouse hearts
Jelena Branovets, Mervi Sepp, Svetlana Kotlyarova, Natalja Jepihhina, Niina Sokolova, Dunja Aksentijevic, Craig A Lygate, Stefan Neubauer, Marko Vendelin, Rikke Birkedal
Am J Physiol Heart Circ Physiol, 2013 Aug 15; 305(4): H506-20.
- PMID: 23792673
- DOI: 10.1152/ajpheart.00919.2012
Significance
Creatine helps heart cells move energy from where it is made in mitochondria to where it is needed. This study tested whether mice unable to make creatine develop changes in mitochondrial arrangement or in the internal barriers that organize energy transfer. Despite the loss of creatine and creatine kinase function, relaxed heart cells from these mice showed the same mitochondrial organization, respiratory control, and energy compartmentation as normal cells. The findings show that a healthy heart can maintain its basic energy organization without creatine-mediated energy transfer at rest, while raising questions about creatine's importance when the heart is stressed.
Abstract
Disruption of the creatine kinase (CK) system in hearts of CK-deficient mice leads to changes in the ultrastructure and regulation of mitochondrial respiration. We expected to see similar changes in creatine-deficient mice, which lack the enzyme guanidinoacetate methyltransferase (GAMT) to produce creatine. The aim of this study was to characterize the changes in cardiomyocyte mitochondrial organization, regulation of respiration, and intracellular compartmentation associated with GAMT deficiency. Three-dimensional mitochondrial organization was assessed by confocal microscopy. On populations of permeabilized cardiomyocytes, we recorded ADP and ATP kinetics of respiration, competition between mitochondria and pyruvate kinase for ADP produced by ATPases, ADP kinetics of endogenous pyruvate kinase, and ATP kinetics of ATPases. These data were analyzed by mathematical models to estimate intracellular compartmentation. Quantitative analysis of morphological and kinetic data as well as derived model fits showed no difference between GAMT-deficient and wild-type mice. We conclude that inactivation of the CK system by GAMT deficiency does not alter mitochondrial organization and intracellular compartmentation in relaxed cardiomyocytes. Thus, our results suggest that the healthy heart is able to preserve cardiac function at a basal level in the absence of CK-facilitated energy transfer without compromising intracellular organization and the regulation of mitochondrial energy homeostasis. This raises questions on the importance of the CK system as a spatial energy buffer in unstressed cardiomyocytes.
Keywords: confocal imaging; creatine kinase shuttle; guanidinoacetate methyltransferase; intracellular diffusion barriers; mitochondrial positioning; respiration and ATPase kinetics.