Phosphorylation of Creatine Kinase in Myogenic Cells: Effects of Okadaic Acid and other Agents Affecting Cellular Protein Phosphorylation

Abstract

Creatine Kinase (CK) is a key enzyme of eukaryotic energy metabolism. In chicken, the two cytosolic isoforms, brain- (B-CK) and muscle-type (M-CK), are expressed in a tissue specific manner. During muscle contraction, CK catalyzes the regeneration of ATP from phosphorylcreatine (PCr) and ADP (reviewed in Wallimann et al., 1992). In differentiating chicken embryonic skeletal muscle cells, during myotube formation, a CK isoenzyme transition from B-CK to M-CK takes place, which was also observed in cultured embryonic skeletal muscle cells (Perriard et al., 1978). B-CK is expressed at a high level until shortly after M-CK expression has started. This isoenzyme switch is part of the biochemical differentiation, which accompanies the morphological development of myoblasts. After leaving the cell cycle, postmitotic myoblasts align themselves into arrays and subsequently fuse to form multinucleated myotubes (see references in David et al., 1990). The myotubes show sarcomeric striations and are able to contract in culture, suggesting that their cellular metabolism resembles that of the in vivo situation. Essential steps in the process of myoblast fusion depend on the action of various protein kinases, such as the phospholipid/Ca²⁺-dependent protein kinase (PKC) and the Ca²⁺/calmodulin-dependent protein (CaM-) kinase (David et al., 1990). Cyclic AMP-dependent protein kinase (PKA) activity is also present during the time of fusion (Rogers et al., 1985).