Abstract
Histidine-rich glycoprotein (HRG) is a plasma protein synthesized by the liver. We have given the first evidence of a tissue localization of HRG demonstrating its presence in skeletal muscle, associated with the zinc enzyme AMP deaminase (AMPD1). Moreover, we have shown that muscle cells do not synthesize HRG, but they can internalize it from plasma. We have recently demonstrated by confocal laser scanning microscopy that in human skeletal muscle, HRG is mainly localized in the myofibrils, preferentially at the I-band of the sarcomere, in the sarcoplasm, and in the nuclei. Using transmission electron microscopy and immunogold analysis, we carried out this study on human and rat normal skeletal muscles with the purpose to deepen the ultrastructural localization of HRG in skeletal muscle fibers. The immunogold analysis evidenced the presence of HRG in the sarcomeres, mainly in the I-band and to a less extent in the A-band, in the heterochromatin of nuclei, and in the sarcoplasmic reticulum. The colocalization of HRG and skeletal muscle AMPD1 was also analyzed. A colabeling of HRG and AMPD1 was evident at sarcomeric, sarcoplasmic reticulum, and nuclear levels. The significance of these interesting and new results is discussed in this article.