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Architectural and functional remodeling of cardiac and skeletal muscle cells in mice lacking specific isoenzymes of creatine kinase.

Muscle is the major consumer of fuels and ATP in the body. Mitochondria and glycolytic complexes serve as the main energy production locations, while the highest energy demands are associated with the sarcoplasmic reticulum, myofibrillar compartments and plasma membrane. Creatine kinase (CK) is a dimeric protein, which is deeply involved in the production of high energy storage compounds. This enzyme reversibly phosphorylates creatine (Cr) to phosphocreatine (PCr), and it is also highly adapted to specialized muscle function. To date, four major isoenzymes of CK have been identified, two of which occur in the cytosol and two in mitochondria. Disruption of the phosphotransfer system induced by an absence of either the sarcomeric mitochondrial CK or cytosolic CK or both isoenzymes of CK (CK(-/-)) in muscle cells leads to morphological and functional adaptations towards preservation of muscle contractile abilities. Remodeling of the cell ultrastructure observed in CK(-/-) cardiomyocytes and glycolytic fibers was associated with direct transfer of energy from places of energy production to locations of energy utilization. This direct interaction among the organelles can maintain a high ATP/ADP ratio near the cellular ATPases when CK is not functionally active. This review summarizes the function and role of CK across different muscle cells in knockout mice.