Heterogeneity of protein expression within muscle fibers

Skeletal muscle fibers are elongatedmultinucleated cells. Along its length, an individualfibermaycontainthousandsofmyonuclei,eachcontrol-ling protein synthesis within its surrounding cyto-plasm.Therefore,afibercanbeconsideredtobeaseriesof myonuclear domains, each responding to distinct lo-calized signaling mechanisms that may result in differ-ential gene expression within the fiber. This brief re-view examines phenomena that produce distinct sub-sets of proteins within different regions of a musclefiber.Theseincludechangesinproteinexpressionasso-ciated with activity-induced fiber-type transformation,muscle development, and denervation. Myosin heavy-chain(MyHC)proteinsarefundamentalstructuralandfunctional components of the muscle fiber. They arerepresented by different isoforms, each of which is theproduct of a separate gene that may be differentiallyexpressed during the development of distinct musclefiber types. We have found that in mature chicken andpigeonpectoralismuscle,thetaperedendsoffiberscon-taintheneonatalMyHCisoforminadditiontotheadultisoform found throughout the lengths of the fibers. Ex-amination of serial sections along the length of muscleKey Words: Denervation, Development, Muscle, Muscle Fibers, Myosins