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Using ferritin-labeled protein A and colloidal gold-labeled anti-rabbit IgG, the fate of the sheep transferrin receptor has been followed microscopically during reticulocyte maturation in vitro. After a few minutes of incubation at 37°C, the receptor is found on the cell surface or in simple vesicles of 100-200 nm, in which the receptor appears to line the limiting membrane of the vesicles. With time (60 min or longer), large multivesicular elements (MVEs) appear whose diameter may reach 1-1.5/~m. Inside these large MVEs are round bodies of ~50-nm diam that bear the receptor at their external surfaces. The limiting membrane of the large MVEs is relatively free from receptor. When the large MVEs fuse with the plasma membrane, their contents, the 50-nm bodies, are released into the medium. The 50-nrn bodies appear to arise by budding from the limiting membrane of the intracellular vesicles. Removal of surface receptor with pronase does not prevent exocytosis of internalized receptor. It is proposed that the exocytosis of the ~50-nm bodies represents the mechanism by which the transferrin receptor is shed during reticulocyte maturation.It is well known that the transferrin receptor is lost during the maturation of the reticulocyte into the erythrocyte (1-4). Recent studies have shown that the maturation process can be followed in vitro (4, 5-7). The loss of the transferrin receptor can be used as a marker of maturation (4-6), the transferrin receptor being released, undegraded, to the medium in vesicular form (6, 7). The transferrin receptor is known to recycle many times during the course of Fe 3÷ delivery without degradation of either the receptor or the natural ligand, transferrin (8-l 1). It has been shown in several systems (8,(12)(13)(14)(15)(16)(17)(18)(19)(20), using either labeled transferrin or antibody against the transferrin receptor, that the ligand and the receptor are internalized into vesicles (endosomes) during incubation at temperatures above 10*C and that this internalization may constitute part of the iron delivery mechanism.Since the transferrin receptor is largely lost from sheep reticulocytes during 24 h of incubation in vitro (5-7), intermediate stages associated with vesicle externalization during receptor elimination might become apparent during the longterm incubation of reticulocytes. No studies to date have addressed the question of the processing that may occur to prepare the receptor for externalization during reticulocyte maturation. We have previously shown (6) that a polyclonal 942 antibody against the transferrin receptor is internalized (as judged by resistance to acid treatment), and that during the course of long-term incubation (several hours at 37"C), this internalized antibody and the surface-bound antibody are lost from the cells along with the antibody-binding capacity of the cells. Since no degraded antibody was found, both internal and surface antibody appear to be eliminated from the cells without degradation.The rate of loss of internalized 125I-antibody as we...

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A myriad of proteins and peptides can be identified in normal human urine. These are derived from a variety of sources including glomerular filtration of blood plasma, cell sloughing, apoptosis, proteolytic cleavage of cell surface glycosylphosphatidylinositol-linked proteins, and secretion of exosomes by epithelial cells. Mass spectrometry-based approaches to urinary protein and peptide profiling can, in principle, reveal changes in excretion rates of specific proteins/peptides that can have predictive value in the clinical arena, e.g. in the early diagnosis of disease, in classification of disease with regard to likely therapeutic responses, in assessment of prognosis, and in monitoring response to therapy. These approaches have potential value, not only in diseases of the kidney and urinary tract but also in systemic diseases that are associated with circulating small protein and peptide markers that can pass the glomerular filter. Most large scale biomarker discovery studies reported thus far have used one of two approaches to identify proteins and peptides whose excretion in urine changes in specific disease states: 1) two-dimensional electrophoresis with mass spectrometric and/or immunochemical identification of proteins and 2) top-down mass spectrometric methods (SELDI-TOF-MS and capillary electrophoresis-MS). These studies have been chiefly in the areas of nephrology, urology, and oncology. We review these applications, focusing on two areas of progress, viz.

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