CELLULAR AND SUBCELLULAR COMPARTMENTATION OF CREATINE-KINASE IN BRAIN

Creatine kinase (CK) catalyzes a reversible transphosphorylation react ion that is believed to play a crucial role in the maintenance and cha nneling of high energy phosphate in tissues with high energy demands. In rat brain development, cytosolic (nonmitochondrial) CK levels incre ase rapidly during the peak period of myelination and remain high in t he adult. The cellular compartmentation of CK was investigated through the use of primary cultures of neurons, oligodendrocytes and astrocyt es. The CK activity in cultured oligodendrocytes, which expressed both enzymatic and immunocytochemical markers of myelin, was the highest o f the cell types examined and comparable to levels measured in whole a dult brain; these observations suggest a role for CK in myelinogenesis . We found that low density, dividing astrocyte cultures also exhibite d high B-CK (brain isoenzyme of CK) immunoreactivity, with the nuclear CK staining being especially intense. We studied these cultured astro cytes in more detail with respect to their intense nuclear CK immunore activity. Optical sections of astrocyte nuclei taken with a confocal m icroscope show that the high B-CK present is actually contained within the nucleus, with a nucleoplasmic distribution that does not co-local ize with DNA or RNA. To ascertain whether the high nuclear B-CK in pro liferating astrocytes correlated with ongoing cell division, we conduc ted experiments with confluent, nondividing cultures. These results sh ow that both CK enzyme activity and immunoreactivity are high in the n ucleus of proliferating astrocytes, and significantly reduced in the n ucleus of quiescent, nondividing astrocytes. The high level of CK prot ein and activity in the nucleus of proliferating astrocytes suggests a role for CK in cell division/nuclear function. We detail a model for a nuclear creatine-P/CK energy shuttle in astrocytes, which we propose contributes substantially to astrocyte nuclear function and is likely present in oligodendrocytes as well.