Osteopontin is histochemically detected by the AgNOR acid-silver staining.

Silver nitrate staining of decalcified bone sections is known to reveal osteocyte canaliculi and cement lines. Nucleolar Organising Regions (NOR) are part of the nucleolus, containing argyrophilic proteins (nucleoclin/C23, nucleophosmin/B23) that can be identified by silver staining at low pH. The aim of this study was to clarify the mechanism explaining why AgNOR staining also reveals osteocyte canaliculi. Human bone and kidney sections were processed for silver staining at light and electron microscopy with a modified method used to identify AgNOR. Sections were processed in parallel for immunohistochemistry with an antibody direct against osteopontin. Protein extraction was done in the renal cortex and decalcified bone and the proteins were separated by western blotting. Purified hOPN was also used as a control. Proteins were electro-transferred on polyvinylidene difluoride membranes and stained for AgNOR proteins. In bone, Ag staining identified AgNOR in cell nuclei, as well as in osteocyte canaliculi, cement and resting lines. In the distal convoluted tubules of the kidney, silver deposits were also observed in cytoplasmic granules on the apical side of the cells. Immunolocalization of osteopontin closely matched with all these locations in bone and kidney. Ag staining of membranes at low pH revealed bands for NOR proteins and 56 KDa (kidney), 60KDa (purified hOPN) and 75 KDa (bone) bands that corresponded to osteopontin. NOR proteins and osteopontin are proteins containing aspartic acid rich regions that can bind Ag. Staining protocols using silver nitrate at low pH can identify these proteins on histological sections or membranes.