Xiaoyong Chen

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Cell surface components are important for myogenic differentiation. Studies with subconfluent day-2 cultures of rat myoblasts revealed that a cell surface 112 kDa protein was phosphorylated by a Ca{dollar}\sp{lcub}++{rcub}{dollar}, F{dollar}\sp-{dollar}, and Mg{dollar}\sp{lcub}++{rcub}{dollar}-dependent ectoprotein kinase (ecto-PK), and that adequate ATP was present on the cell surface for the efficient functioning of this ecto-PK. The following evidence suggests that both the 112 kDa protein and the ecto-PK may play important role(s) in the initiation of myogenesis. (i) The highest phosphorylation activity was observed in subconfluent culture, i.e. before the onset of myogenesis. (ii) Treatment of cells with myogenesis inhibitors also resulted in a corresponding decrease in the phosphorylated 112 kDa protein (p112). (iii) The level of p112 in a conditional myogenesis-defective mutant corresponded with the cell's eventual ability to differentiate. (iv) A mutant defective in the ecto-PK (F72) was impaired in the phosphorylation of 112 kDa and in myogenesis. (v) A mutant containing only a residual level of 112 kDa protein (D1/S4) was defective in both p112 and myogenesis. (vi) Conditional myogenesis mutant D1 cannot undergo myogenesis and the level of phosphorylation of the 112 kDa protein is very low when grown in 10% horse serum, however when grown in 1% horse serum D1 can fuse and the level of phosphorylation of the 112 kDa protein is normal. (vii) Since the level of p112 was normal in another myogenesis-defective mutant, the phosphorylation of this protein was not likely a consequence of myogenic differentiation.;Myogenic differentiation is comprised of a sequential cascade of multiple steps leading to the formation of multinucleated myotubes. Northern blot analyses with myogenic factor cDNAs and muscle-specific protein cDNAs showed that mutants D1/S4 and F72 had normal Myf5 transcript level, but much lower transcript levels of Myf4, NCAM, MHC, MLC, and TnT than their parental L6 cells. Similar results were observed when L6 cells were treated with myogenesis inhibitors such as phloretin and 5-bromo-2-deoxyuridine (BrdUrd). These findings further suggested that the ecto-PK and 112 kDa protein might be involved in the early stage of myogenesis. When mutants D1/S4 and F72 were transfected with the myf4 cDNA, the transcript levels of MHC, MLC, and TnT were elevated in both transfected cell lines. However the level of p112 and the transcript levels of NCAM and Myf5 were unaltered in these transfectants. These results suggested that the site of action of ecto-PK and 112 kDa protein might occur after Myf5, and before NCAM and Myf4 in the myogenic pathway.



To view the content in your browser, please download Adobe Reader or, alternately,
you may Download the file to your hard drive.

NOTE: The latest versions of Adobe Reader do not support viewing PDF files within Firefox on Mac OS and if you are using a modern (Intel) Mac, there is no official plugin for viewing PDF files within the browser window.