UTROPHIN LOCALIZATION IN NORMAL AND DYSTROPHIN-DEFICIENT HEART

Background The localization of dystrophin at the sarcolemma of cardiac skeletal fibers and cardiac Purkinje fibers has been described. Dystr ophin deficiency produces clinical manifestations of disease in skelet al muscles and hearts of patients with Duchenne and Becker muscular dy strophy. Utrophin (or dystrophin-related protein), a dystrophin homolo gous protein, was found to be expressed in fetal muscles and reexpress ed in dystrophin-deficient skeletal muscle fibers. We therefore examin ed utrophin expression in normal and in dystrophin-deficient hearts. M ethods and Results The expression and subcellular distribution of utro phin was examined in cardiac muscle by immunoblot and immunofluorescen ce analysis in normal bovine heart compared with dystrophin. Utrophin expression was also examined in normal and dystrophin-deficient hearts of MDX mice. Three monoclonal antibodies reacting with dystrophin and utrophin solely or reacting with both proteins along with two polyclo nal antibodies reacting with either utrophin or dystrophin and utrophi n were tested. In normal bovine heart, utrophin was not expressed at t he periphery of fibers but was strongly expressed in intercalated disk s and in the cytoplasm of cardiac Purkinje fibers. In cardiocytes, utr ophin was colocalized along transverse T tubules with dystrophin. Dyst rophin was present at the periphery of cardiocytes and cardiac Purkinj e fibers as well as in transverse T tubules but was absent or faintly expressed in intercalated disks. The results with monoclonal and polyc lonal antibodies were identical. Western blot analysis revealed that t he detected molecules corresponded only to a 400-kD protein band and n ot to possible shorter transcripts of utrophin or dystrophin (apo-utro phin or apo-dystrophin), In dystrophin-deficient hearts of MDX mice, u trophin alone was abundant but not organized in the same networklike d istribution. Conclusions This first localization of utrophin in normal heart (in Purkinje fibers, transverse tubules, and intercalated disks ) showed a distinct subcellular localization of this protein with dyst rophin, suggesting an important function of this protein in intercellu lar communication. In dystrophin-deficient hearts of MDX mice, utrophi n alone is overexpressed as in skeletal muscle sarcolemma, an area nor mally occupied by dystrophin but not organized in the same networklike distribution.