Expression and localization of annexin VII (synexin) in muscle cells

Annexin VII (synexin) is a member of the annexin family of proteins, which are characterized by Ca(2+)-dependent binding to phospholipids. We used PCR to isolate from a lambda gt11-mouse fibroblast library annexin VII cDNA fragments corresponding to the two isoforms found in both humans and Dictyostelium discoideum. The two isoforms of 47 kDa and 51 kDa differed by 22 amino acids inserted into the proximal third of the hydrophobic N terminus. Annexin VII-specific polypeptides expressed in Escherichia coli were used to generate isoform-specific monoclonal antibodies. Expression of the two isoforms during myogenesis was followed in the myogenic cell lines BC3H1 and L6. Only the 47 kDa isoform was present in undifferentiated L6 or BC3H1 myoblasts. The 51 kDa isoform appeared after myogenesis had been induced and in striated muscle only the 51 kDa isoform was observed. Immunofluorescence showed that annexin VII was located in the cytosol of mononucleated and fused polynucleated cultured cells, whereas in striated muscle, annexin VII was located preferentially at the plasma membrane and the transverse tubules. However, there was also some residual cytosolic staining, which was more abundant in type II (fast twitch) than in type I (slow twitch) fibers. Permeabilization of L6 cells with digitonin in the presence of 5 mM EGTA led to a release of annexin VII from the cells, which paralleled the loss of cytosolic lactate dehydrogenase (LDH) at low detergent concentrations (50 microM). In the presence of 100 microM extracellular Ca2+, annexin VII remained bound to the plasma membrane even in the presence of high digitonin concentrations. Incubation with the Ca(2+)-specific ionophore A23187 and 100 microM extracellular Ca2+ led to a redistribution of annexin VII from the cytosol to the plasma membrane after 30 minutes of incubation. The results obtained indicate a developmentally and Ca(2+)-regulated localization and expression of annexin VII and raise the possibility that annexin VII may play a role in excitation-contraction coupling in skeletal muscle.