HISTIDINE-RICH GLYCOPROTEIN AND HUMAN SKELETAL MUSCLE: POSSIBLE INVOLVEMENT IN MUSCLE CONTRACTION

Histidine-rich glycoprotein (HRG) is an approximately 75 kDa plasma protein synthesized by parenchymal liver cells. It can bind to a variety of ligands such as fibrinogen, plasminogen, heparin, heparin sulphate and divalent transition metal cations. The existence of a specific zinc binding site in its structure suggests that it could act as metallochaperone (1). We have previously provided the first evidence for the presence of HRG in human skeletal muscle (2) and demonstrated for the first time that muscle cells do not synthesize the protein but instead can actively internalize it from plasma (3). To investigate HRG subcellular distribution, an immunohistochemical study was carried out on three human normal skeletal muscle biopsies as, at present, no information is reported in literature about it. Double immunofluorescence, performed using an anti-human HRG polyclonal antibody and the myosin My-32 or the anti actin (α-sarcomeric) monoclonal antibodies, was observed by both optical fluorescence and confocal laser scanning microscopy. In addition Transmission electron microscopy (TEM) and immunogold analysis were carried out. The immunofluorescence results gave evidence of the presence of HRG both within the muscle fibers and in the connective tissue among the fibers. In the sarcoplasm, HRG was preferentially localized at the myofibril level, mostly at the sarcomeric I-band making contact with the ends of A-band. A positive HRG immunoreaction could also be detected in the nuclei. Immunogold analysis confirmed the results obtained by immunofluorescence showing that, at sarcomeric level, HRG is concentrated mostly in the I-band, although gold particles could also be detectable in the actin portion of the A-band. The above results suggest that HRG could be involved in muscle contraction interacting with proteins that take part to the constitution of the thin filament. The data of the literature support this hypothesis as the binding between HRG and cellular tropomyosin has been demonstrated (4) and an interaction between HRG and troponin T has been hypothesized (5). Moreover, the exceptionally high level of zinc in striated muscle suggests that this tissue contains a specialised system for the uptake of zinc. HRG could have a role in the uptake of zinc from serum or in modulating its intracellular availability, by acting as zinc-chaperone. In the light of the above considerations, further studies will have to be carried out on pathological specimens from subjects affected by neuro muscular diseases, to better understand the role of HRG in the physiopathology of the muscle fiber.