Osteopontin synthesis and localization along the human nephron

Abstract

In normal human and rat kidneys, osteopontin (OPN) is present at the apical surface of cells in the distal nephron. After ischemic or toxic renal damage in rats, OPN is upregulated in distal tubular cells (DTC) and expressed de novo in perinuclear vesicles in proximal tubular cells (PTC). In the first phase of this study, OPN localization in ischemic human biopsies was compared with that in ischemic rat kidneys. In the second phase, cultures of PTC and DTC were used to investigate human renal OPN synthesis, secretion, and localization. OPN localization in human biopsies after renal ischemia was comparable to that in ischemic rat kidneys. Microscopic and flow cytometric detection of immunofluorescent OPN staining in tubular cell cultures demonstrated strong plasma membrane localization in DTC, whereas mainly perinuclear intracellular expression was observed in PTC. Northern blotting and reverse transcription-PCR demonstrated production of a single OPN mRNA in PTC and DTC. Detection of OPN by Western blotting and enzyme-linked immunosorbent assay demonstrated that PTC and DTC synthesized and secreted the same three molecular mass OPN forms, in comparable amounts. Finally, confocal microscopy demonstrated different staining patterns for endocytotic/lysosomal vesicles and perinuclear OPN; however, perinuclear OPN exhibited colocalization with the Golgi apparatus. In conclusion, human renal OPN localization in cell cultures demonstrated differences between PTC and DTC comparable to those observed after renal ischemia in vivo. Therefore, these cell cultures represented an excellent model for the study of human OPN synthesis, secretion, and localization in PTC versus DTC. It is reported for the first time that intracellular OPN is located in the Golgi apparatus of both PTC and DTC and that PTC and DTC are able to produce and secrete the same OPN isoforms, in comparable amounts.