IGF-II/man-6-P receptors in rat kidney: Apical localization in proximal tubule cells

Abstract

The present study was performed to determine the local-ization of insulin-like growth factor II (IGF-II) binding in the rat nephron and, furthermore, to determine directly the distribution of the IGF-II/Man-6-P receptor in the proximal tubule. The IGF-II/Man-6-P receptor was visualized by immunohisto- and cytochemical techniques, and the binding of IGF-II was shown by light and electron microscope autoradiography applying 125I-IGF-II directly on cryosections. The tubular uptake of IGF-II was studied by autoradiography after in vivo injection of 125I-IGF-II. The light microscope immunolabeling was confined to a relatively narrow band in the apical part of the proximal tubule cells just below the brush border, and was strongest in the first portion of the proximal tubule, segment 1 (S1), with decreasing intensity in segment 2 (S2) and weaker in segment 3 (S3). Several cytoplasmic bodies were labeled but no label was observed in the basolateral part of the cells. We did not detect any labeling of glomeruli or other segments of the nephron. Electron microscope immunocytochemistry in the proximal tubule revealed a strong labeling in apical endocytic invaginations, small endocytic vacuoles, and large endocytic vacuoles, weaker in lysosomes and dense apical tubules. Light microscope autoradiography after incubation of cryosections with labeled IGF-II demonstrated an intense labeling just beneath the brush border, especially in S1 of the proximal tubule, and, in addition, the corresponding electron microscope autoradiography revealed an intense labeling over large endocytic vacuoles and lysosomes. These results were confirmed and extended by competitive binding experiments and quantitative immunocytochemistry. After i.v. injection of 125I-IGF-II autoradiographic grains were almost exclusively observed in the electronmicroscope over endocytic vacuoles and lysosomes in the S1 of the proximal tubule. The present study shows that the IGF-II/Man-6-P receptor is located in components of the vacuolar system in the apical cytoplasm of rat renal proximal convoluted tubule cells, mainly in S1. This localization was seen to be consistent using various immunolabeling and autoradiographic techniques in vivo and in vitro.