Cardiac expression and location of hexokinase changes in a mouse model of pure creatine deficiency
Jelena Branovets, Niina Karro, Karina Barsunova, Martin Laasmaa, Craig A Lygate, Marko Vendelin, Rikke Birkedal
Am J Physiol Heart Circ Physiol, 2021 Feb 01; 320(2): H613-H629.
- PMID: 33337958
- DOI: 10.1152/ajpheart.00188.2020
Significance
Creatine-deficient AGAT-/- and GAMT-/- mice have a limited cardiac creatine kinase system, which normally transfers energy within heart cells. This published paper tested whether hexokinase and adenylate kinase could compensate by supplying ADP to mitochondria. Adenylate kinase did not change, but AGAT-/- heart cells had more hexokinase I and greater hexokinase activity in mitochondria, while hexokinase stimulated respiration less effectively. These changes may alter mitochondrial regulation and reactive oxygen species production, showing how pure creatine deficiency can reshape cardiac energy transfer.
Abstract
Creatine kinase (CK) is considered the main phosphotransfer system in the heart, important for overcoming diffusion restrictions and regulating mitochondrial respiration. It is substrate limited in creatine-deficient mice lacking l-arginine:glycine amidinotransferase (AGAT) or guanidinoacetate N-methyltranferase (GAMT). Our aim was to determine the expression, activity, and mitochondrial coupling of hexokinase (HK) and adenylate kinase (AK), as these represent alternative energy transfer systems. In permeabilized cardiomyocytes, we assessed how much endogenous ADP generated by HK, AK, or CK stimulated mitochondrial respiration and how much was channeled to mitochondria. In whole heart homogenates, and cytosolic and mitochondrial fractions, we measured the activities of AK, CK, and HK. Lastly, we assessed the expression of the major HK, AK, and CK isoforms. Overall, respiration stimulated by HK, AK, and CK was ~25, 90, and 80%, respectively, of the maximal respiration rate, and ~20, 0, and 25%, respectively, was channeled to the mitochondria. The activity, distribution, and expression of HK, AK, and CK did not change in GAMT knockout (KO) mice. In AGAT KO mice, we found no changes in AK, but we found a higher HK activity in the mitochondrial fraction, greater expression of HK I, but a lower stimulation of respiration by HK. Our findings suggest that mouse hearts depend less on phosphotransfer systems to facilitate ADP flux across the mitochondrial membrane. In AGAT KO mice, which are a model of pure creatine deficiency, the changes in HK may reflect changes in metabolism as well as influence mitochondrial regulation and reactive oxygen species production.
Keywords: adenylate kinase; cardiac energetics; creatine kinase; creatine-deficient mice; hexokinase.