We have blocked creatine kinase (CK)-mediated phosphocreatine (PCr) -->/<-- ATP transphosphorylation in skeletal muscle by combining targeted mutations in the genes encoding mitochondrial and cytosolic CK in mice. Contrary to expectation, the PCr level was only marginally affected, but the compound was rendered metabolically inert. Mutant muscles in vivo showed significantly impaired tetanic force output, increased relaxation times, altered mitochondrial volume and location, and conspicuous tubular aggregates of sarcoplasmic reticulum membranes, as seen in myopathies with electrolyte disturbances. In depolarized myotubes cultured in vitro, CK absence influenced both the release and sequestration of Ca2+. Our data point to a direct link between the CK-PCr system and Ca2+-flux regulation during the excitation and relaxation phases of muscle contraction.

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Myotonic dystrophy (DM) is commonly associated with CTG repeat expansions within the gene for DM-protein kinase (DMPK). The effect of altered expression levels of DMPK, which is ubiquitously expressed in all muscle cell lineages during development, was examined by disrupting the endogenous Dmpk gene and overexpressing a normal human DMPK transgene in mice. Nullizygous (-/-) mice showed only inconsistent and minor size changes in head and neck muscle fibres at older age, animals with the highest DMPK transgene expression showed hypertrophic cardiomyopathy and enhanced neonatal mortality. However, both models lack other frequent DM symptoms including the fibre-type dependent atrophy, myotonia, cataract and male-infertility. These results strengthen the contention that simple loss- or gain-of-expression of DMPK is not the only crucial requirement for development of the disease.

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Creatine kinases are important in maintaining cellular-energy homeostasis, and neuroprotective effects have been attributed to the administration of creatine and creatine-like compounds. Herein we examine whether ablation of the cytosolic brain-type creatine kinase (B-CK) in mice has detrimental effects on brain development, physiological integrity or task performance. Mice deficient in B-CK (B-CK-/-) showed no gross abnormalities in brain anatomy or mitochondrial ultrastructure, but had a larger intra- and infrapyramidal mossy fibre area. Nuclear magnetic resonance spectroscopy revealed that adenosine triphosphate (ATP) and phosphocreatine (PCr) levels were unaffected, but demonstrated an apparent reduction of the PCr left arrow over right arrow ATP phosphorus exchange capacity in these mice. When assessing behavioural characteristics B-CK-/- animals showed diminished open-field habituation. In the water maze, adult B-CK-/- mice were slower to learn, but acquired the spatial task. This task performance deficit persisted in 24-month-old, aged B-CK-/- mice, on top of the age-related memory decline normally seen in old animals. Finally, a delayed development of pentylenetetrazole-induced seizures (creating a high-energy demand) was observed in B-CK-/- mice. It is suggested that the persistent expression of the mitochondrial isoform ubiquitous mitochondrial CK (UbCKmit) in the creatine/phospho-creatine shuttle provides compensation for the loss of B-CK in the brain. Our studies indicate a role for the creatine-phosphocreatine/CK circuit in the formation or maintenance of hippocampal mossy fibre connections, and processes that involve habituation, spatial learning and seizure susceptibility. However, for fuelling of basic physiological activities the role of B-CK can be compensated for by other systems in the versatile and robust metabolic-energy network of the brain.