Stability and secretion of acetylcholinesterase forms in skeletal muscle cells

Muscle cells express a distinct splice variant of acetylcholinesterase (ACh E(T)), but the specific mechanisms governing this restricted expression rem ain unclear. In these cells, a fraction of AChE subunits is associated with a triple helical collagen, ColQ, each strand of which can recruit a tetram er of AChE(T). In the present study, we examined the expression of the vari ous splice variants of AChE by transfection in the mouse C2C12 myogenic cel ls in vitro, as well as in vivo by injecting plasmid DNA directly into tibi alis anterior muscles of mice and rats. Surprisingly, we found that transfe ction with an ACHE(H) cDNA, generating a glycophosphatidylinositol-anchored enzyme species, produced much more activity than transfection with AChE(T) cDNA in both C2C12 cells and in vivo. This indicates that the exclusive ex pression of AChE(T) in mature muscle is governed by specific splicing. Inte raction of AChE(T) subunits with the complete collagen tail ColQ increased enzyme activity in cultured cells, as well as in muscle fibers in vivo. Tru ncated ColQ subunits, presenting more or less extensive C-terminal deletion s, also increased AChE activity and secretion in C2C12 cells, although the triple helix could not form in the case of the larger deletion. This sugges ts that heteromeric associations are stabilized compared with isolated AChE (T) subunits. Coinjections of AChE(T) and ColQ resulted in the production a nd secretion of asymmetric forms, indicating that assembly, processing, and externalization of these molecules can occur outside the junctional region of muscle fibers and hence does not require the specialized junctional Gol gi apparatus.