Analysis of S100A1 expression during skeletal muscle and neuronal cell differentiation.

To better understand the mechanisms that regulate the function of the calcium-binding proteins S100A1 and S100B in developing systems, we have examined the level of, subcellular distribution of, and target proteins for these proteins in skeletal muscle (L6S4) and neuronal (PC12) cell lines. Both undifferentiated and differentiated L6 and PC12 cells express S100A1 and not S100B. Whereas S100A1 protein levels were higher in differentiated cells than in undifferentiated cells, steady-state mRNA levels did not change in differentiated L6 cells and decreased in differentiated PC12 cells when compared with undifferentiated cells. These results suggest that posttranscriptional rather than transcriptional mechanisms are responsible for increased S100A1 protein expression in myotubes and neurons. The colocalization of S100A1 staining with wheat germ agglutinin staining suggests that S100A1 is associated with the Golgi apparatus and secretory vesicles in PC12 and L6 cells. Using a gel overlay technique, S100A1-binding proteins were detected in undifferentiated and differentiated PC12 and L6 cells and the patterns observed were similar to those observed in brain and skeletal muscle, respectively. Although changes in the intensity of some binding proteins were detected, the overall pattern did not change when differentiated and undifferentiated cells were compared. These results suggest that the complement of S100A1-binding proteins does not change during differentiation, only the levels of some binding proteins. Altogether, our data demonstrate that the L6 and PC12 cell lines are excellent in vitro model systems for studying S100A1 expression and mechanisms that regulate S100A1 expression, subcellular distribution, and interaction with target proteins.