The Pitfalls of in vivo Cardiac Physiology in Genetically Modified Mice – Lessons Learnt the Hard Way in the Creatine Kinase System

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  Figure 1. Creatine biosynthesis and the myocardial creatine kinase (CK) phosphagen system. Arginine:glycine amidinotransferase (AGAT; EC 2.1.4.1) is predominantly expressed in the kidneys where it combines arginine and glycine to make guanidinoacetate (GAA) or with lysine to make homoarginine (hArg). Circulating GAA is taken up by the liver where GAA N-methyltransferase (GAMT; EC 2.1.1.2) utilises the methyl group from S-adenosyl L-methionine to synthesise creatine. These biosynthetic enzymes are not expressed in cardiomyocytes so creatine must be taken-up via a specific plasma membrane creatine transporter (SLC6A8). Creatine accumulates inside the cell where the sarcomeric mitochondrial isoform of creatine kinase (MtCK; EC:2.7.3.2) catalyses the transfer of a phosphoryl group from ATP to form phosphocreatine (PCr) and ADP. PCr is a high abundance and mobile energy source that can be utilised to rapidly regenerate ATP at times of high demand under the control of the muscle isoform of creatine kinase (the dimer MMCK; EC 2.7.3.2). Free creatine diffuses back to stimulate further oxidative phosphorylation and re-start the cycle. Knockout models exist for all five of these proteins and…

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  Figure 2. The presence or absence of cardiac hypertrophy in CK double knockout mice (CK-dKO) has varied considerably over time and between laboratories, in large part, due to a mixed and changing genetic background. When finally bred onto a pure C57BL/6 J background, no cardiac hypertrophy was evident (Lygate et al., 2012a,b - pure).