An Ultrastructural Study of Glomerular Permeability Using Catalase and Peroxidase as Tracer Proteins

Abstract

The renal glomerulus effectively restricts plasma proteins and other macromolecules from entering the urinary space (1-6). Even though this barrier is not absolute, minimal amounts of protein normally leak into Bowman's space (7). Clearance of a particular macromolecule varies inversely with its size (1-6), as evidenced by studies on the glomerular clearance of proteins and dextrans (5-9). In general, molecules smaller than tool wt 68,000 can pass into the urine, whereas those larger are effectively restricted.While physiological studies indicate that the glomerular filter behaves as if it were an isoporous membrane (10), pores conforming to the predicted dimensions have not so far been found in morphological studies of the glomerulus. Ultrastructural studies using particulate tracers, such as saccharated iron oxide (11) and ferritin (tool wt 500,000) (12) have indicated the presence of a filtration barrier at the level of the basement membrane. Graham and Karnovsky (13), using enzyme tracers of different molecular weights (horseradish peroxidase, myeloperoxidase), have suggested that a basement membrane barrier alone cannot explain the glomerular mechanism to prevent protein loss in the' filtrate. This was indicated by the behavior of myeloperoxidase (tool wt 170,000) which crossed the basement membrane but was held back at the epithelial slit pore. On the other hand, the smaller molecule horseradish peroxidase (tool wt 40,000) permeated the basement membrane and passed through the slit pore into the urinary space. These observations have tended to confirm the hypothesis proposed by Hall (14) that the epithelial slit pore represents the filtration * This work was presented in part at the meeting of the American Association of Pathologists and Bacteriologists, San Francisco, Calif., March 1969, and the IVth International Congress of Nephrology,

Albumin-associated free fatty acids induce macropinocytosis in podocytes

“…Electron micrographs of the kidney often reveal intracellular vesicles in podocytes (8)(9)(10)(11)(12)(13)(14). Twenty-four hours after injection with fluorescently labeled fluidphase tracers (70 kDa FITC-Ficoll), confocal imaging of the kidney glomerulus showed fluorescence labeling, suggesting endocytic uptake ( Figure 1A).…”

“…Numerous studies using electron microscopy have shown increased numbers of protein-containing vesicles in podocytes under nephrotic conditions in which there is significant leakage of proteins across the GBM (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). Previously, we showed that expression of the neonatal Fc receptor, a transcytosis receptor for Ig and albumin, allows podocytes to handle these proteins by internalization and transcytosis (19).…”

A new role for complement in experimental membranous nephropathy in rats.

“…Although there is no evidence that GBM is composed oflipid bilayers, immune deposits in PHN develop in the subepithelial space adjacent to the epithelial cell membrane and membranous slit-pore diaphragm (17,22), which provide the final barrier to filtration of some proteins (49) and might be susceptible to attack by the C5b-C9 complex. Activation of C5b-C9 has been shown to expose hydrophobic peptides which insert into the lipid bilayer of membranes and result in membrane damage (48).…”

Normal Glomerular Permeability and Its Modification by Minimal Change Nephrotic Syndrome

“…However, an alternate explanation appears more probable in INS. Recent studies utilizing ultrastructural tracers have suggested that the glomerular epithelial slit pores may contribute significantly to glomerular permeability characteristics (27)(28)(29)(30). According to this view, there are two barriers to filtration in the glomerulus, the GBM acts only as a coarse filter for relatively large molecules, and the final filtration barrier for smaller molecules is located at or about the level of the epithelial slit pore or diaphragm (4).…”